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5.4. Infrared studies of the lattice modes 1655.5. Inelastic neutron scattering studies 169
6. Tec hn ological app lications of inter calated gr ap hite 1716.1. Introduction 1716.2. Battery and electrode materials 1716.3. Chemical catalytic applications 1726.4. Conductivity applications 172
6.5. Carbon bres 1736.6. Other applications 175
Acknowledgements 176
References 176
1. Introduction
Research on the preparation and properties of graphite intercalation compounds
has recently undergone a resurgence of interest and our fundamental knowledge of
the physics of these remarkable materials has increased substantially. It seems
appropriate to review this subject now but, with research activity and interest in
the eld remaining high, this review may well require updating in the near future.This is a broad review (an outgrowth of lecture notes developed for graduate
students in our research group at MIT) of a wide range of topics from the basic
chemistry and physics of intercalated graphite to engineering applications. It has not
been possible therefore to include reference to all of the important papers that have
been written in this eld (and we apologize to authors whose work would otherwise
have merited inclusion).
Graphite intercalation compounds are formed by the insertion of atomic or
molecular layers of a dierent chemical species called the intercalant between layers
in a graphite host material, as shown in gure 1. The intercalation compounds occur
in highly anisotropic layered structures where the intraplanar binding forces are
large in comparison with the interplanar binding forces. The most common examplesof host materials for intercalation compounds are graphite and the transition metal
dichalcogenides. Of the various types of intercalation compounds, the graphite
compounds are of particular physical interest because of their relatively high degree
of structural ordering. The most important and characteristic ordering property of
graphite intercalation compounds is the staging phenomenon, which is characterized
by intercalate layers that are periodically arranged in a matrix of graphite layers.
Graphite intercalation compounds are thus classied by a stage index n denoting the
number of graphite layers between adjacent intercalate layers, as is illustrated in
gure 2. This staging phenomenon is a general phenomenon in graphite intercalation
compounds, even in those samples with very dilute intercalate concentrations
(n 10).
Intercalation provides to the host material a means for controlled variation of
many physical properties over wide ranges. Because the free carrier concentration of
the graphite host is very low (104 free carriers/atom at room temperature),
intercalation with dierent chemical species and concentrations permits wide vari-
ation of the free carrier concentration and thus of the electrical, thermal and
magnetic properties of the host material. Of these properties, the eect of
intercalation on the electrical conductivity has probably attracted the greatest
amount of attention because of the fabrication of an intercalation compound
(CXAsF5) with a reported room temperature conductivity exceeding that of copper
M. S. Dresselhaus and G. Dresselhaus2
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(Foley et al. 1977). Perhaps even more striking is the range of electrical conductivity
behaviour, ranging from almost insulating behaviour for the c-axis conductivity in
certain acceptor compounds to superconducting in-plane behaviour below 1.0 K forthe rst stage alkali metal donor compounds C8K where neither of the parent
chemical species individually exhibit superconductivity (Hanney et al. 1965, Koike et
al. 1978). The large increase in conductivity in intercalated graphite results from a
charge transfer from the intercalate layer where the carriers have a low mobility to
the graphite layers where the mobility is high. Since the most signicant modica-
tions to the graphite involve graphite layers adjacent to the intercalate layer, it is
convenient to distinguish between the graphite bounding layers adjacent to the
intercalant, and the graphite interior layers that have only graphite nearest-neighbour
layers.
The synthesis of a graphite intercalation compound was rst reported by
Schaa utl (1841). However, the rst systematic studies of these compounds began
in the early 1930s with the introduction of X-ray diraction techniques for stage
index determinations (Homan and Frenzel 1931, Schleede and Wellman 1932).
Though the systematic study of their physical properties began in the late 1940s, it is
only in recent years that research on graphite intercalation compounds has become a
eld of intense activity internationally.
A large number (100) of reagents can be intercalated into graphite. These
intercalants are commonly classied according to whether they form donor or
acceptor compounds. The most common and most widely studied of the donor
compounds are the alkali metal compounds with K, Rb, Cs and Li, though other
Intercalation compounds of graphite 3
Figure 1. Model for C8K according to Ru dor and Schulze (1954) showing the stacking ofgraphite layers (networks of small solid balls) and of potassium layers (networks oflarge hollow balls). The graphite and intercalate layers are arranged in anAA-AA stacking sequence, where A refers to the graphite layers and the Greekletters to the intercalate layers.
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donor intercalants are known, such as alkaline earth metals, lanthanides and metal
alloys of these with each other or with alkali metals. Ternary donor intercalationcompounds have also been prepared using alkali metals with hydrogen or polar
molecules, such as ammonia and tetrahydrofuran, and aromatic molecules, such as
benzene. A very large variety of acceptor compounds have also been prepared, and
are often based on Lewis acid intercalants such as the halogen Br2 or halogen
mixtures, metal chlorides, bromides, uorides and oxyhalides, acidic oxides such as
N2O5 and SO3 and strong Bro nsted acids such as H2SO4 and HNO3. In the
intercalation process, the molecular intercalants generally remain molecular in form.
From gures 1 and 2 it is seen that intercalation causes crystal dilatation along the c-
axis: the larger the molecular intercalants, the larger the dilatation for compounds of
comparable stage. In general, both chemical anities and geometric constraints
associated with intercalant size and intercalant bonding distances determine whether
or not a given chemical species will intercalate.
Many of these compounds are unstable in air, with donor compounds being
easily oxidized and acceptors being easily desorbed. For this reason, most intercala-
tion compounds require encapsulation to ensure chemical stability, though some
compounds, such as graphiteFeCl3 and graphiteSbCl5, are relatively stable in air.
In addition to the large number of chemical species that can be intercalated, a
number of dierent types of graphite host materials are used for each of the various
applications. From a structural point of view, the simplest host material is a single
crystal graphite ake, such as those separated from the limestone rocks found in the
M. S. Dresselhaus and G. Dresselhaus4
Figure 2. Schematic diagram illustrating the staging phenomenon in graphitepotassium
compounds for stages 1 n 4. The potassium layers are indicated by dashed linesand the graphite layers by solid lines connecting open circles, and indicatingschematically a projection of the carbon atom positions. The . . . ABAB . . . graphitelayer stacking for stages n 2 is maintained between intercalate layers, although arhombohedral stacking arrangement appears across intercalate layers. The stackingordering is well-conrmed by X-ray diraction (00l) patterns. For each stage, thedistance Ic between adjacent intercalate layers is indicated. For rst stage C8K,the unit cell includes intercalate layers with stacking indices , -, , (see gures 1and 20).
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Ticonderoga mines of New York State. Because ake dimensions are 1mm in
diameter and only several hundredths of a millimetre in thickness, these materials
often cannot be conveniently used for carrying out physical properties meas-
urements. In such cases, samples of large physical dimensions based on highly
oriented pyrolytic graphite (HOPG) are used (Moore 1973). HOPG is a synthetic
graphite formed by cracking a hydrocarbon at high temperature and subsequent
heat treatment, often combined with the application of pressure. The resultingmaterial is highly oriented along the c-axis (orientational deviations less than 18) but
in the layer planes consists of a randomly ordered collection of crystallites of1 mm
average diameter. For many physical measurements the greater exibility in sample
size provided by the HOPG host material is of greater importance than the more
perfect ordering of the single crystal akes. In fact, HOPG has been the most
common host material for graphite intercalation compounds during the recent
period of active research.
Another type of graphite host material is kish graphite, obtained by the
crystrallization of carbon from molten steel during the steel manufacturing process.
Kish graphite samples typically contain several large single crystallites, exhibiting
much higher structural ordering than HOPG, but not quite as ordered or aschemically pure as natural single crystal akes. On the other hand, kish graphite
samples are normally an order of magnitude greater in area and in thickness when
compared with single crystal akes. Though little use has so far been made of
this host material, intercalation compounds based on kish graphite can also be
prepared.
Carbon bres represent yet another class of synthetic graphite with great
mechanical strength, because the bre axis is along the graphite a-axis where the
iteratomic bonding is extremely strong. The intercalation of carbon bres is under
consideration as a method for variation of the electrical, mechanical and adhesive
properties of this commercially important class of bre materials.
In recent years several excellent review articles have appeared on the synthesisand structure of graphite intercalation compounds (Ebert 1986, He rold 1979), as well
as on the electronic properties (Fischer 1979), the lattice properties (Dresselhaus and
Dresselhaus 1979), and on applications areas (Whittingham and Ebert 1979). In
addition, a large collection of research and review articles on these subjects have
appeared in the Proceedings of the Franco-American Conference of La Napoule
(1977, Mater. Sci. Engng, 31, 1).
This review article on graphite intercalation compounds covers the eld broadly
in an eort to interrelate many of the factors which aect the structural, electronic
and lattice properties of these materials. Section 2 is devoted to the preparation and
characterization of materials, as well as related topics such as intercalation kinetics
and staging. Sections 3, 4 and 5 deal respectively with the structural electronic and
lattice properties of graphite intercalation compounds, while some applications areas
for these materials are briey discussed in section 6.
2. Materials preparation and characterizatio n
2.1. Intercalation methods
2.1.1. Introduction
A number of general methods have been developed for the preparation of
graphite intercalation compounds (Ebert 1976, He rold 1977, 1979) including the
Intercalation compounds of graphite 5
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two-zone vapour transport technique, the liquid intercalation method, the electro-
chemical method and cointercalation techniques. For the various techniques
employed, the parameters of signicance are temperature, vapour pressure, the
chemical and physical properties of the intercalant and the characteristics of the
graphite host material. Small, thin samples intercalate more quickly and often yield
better-staged, more homogeneous material than large, thick samples. Also single
crystal graphite akes intercalate more readily than a highly oriented pyrolyticgraphite (HOPG) specimen (Moore 1973), or than a carbon bre (Hooley 1977a).
Although a given intercalation compound can often be prepared by alternative
growth techniques, the physical and chemical properties of the intercalant play an
important role in favouring one intercalation method over another. Excellent
reviews of methods used for the preparation of specic intercalation compounds
are given in the articles by Ebert (1976) and He rold (1977, 1979).
Intercalation can be achieved starting from solid, liquid or gaseous reagents
(Croft 1960, Holey 1977a, He rold 1977, 1979) though preparation using vapour
transport with the two-zone method is the most common for the preparation of well-
staged specimens (Fredenhagen and Cadenbach 1926, He rold 1955). Intercalation
occurs for many types of reagents (more than 100), ranging from simple ionic speciessuch as alkali metals, diatomic molecules such as the halogens, metal chlorides,
bromides, oxides and sulphides to large organic molecules such as benzene (Croft
1956, E bert 1976, Stumpp 1977, He rold 1977, 1979). T he simpler binary and ternary
compounds are usually prepared by direct synthesis, and the more complicated
materials by a variety of stepwise intercalation procedures.
Because of the high reactivity of most graphite intercalation compounds, they are
commonly prepared and stored in ampoules, containing either intercalate vapour, an
overpressure of an inert gas or vacuum, depending on the intercalate species.
Cooling samples (for example, to liquid nitrogen temperatures) greatly increases
their stability.
Certain intercalation compounds (for example, graphiteBr2), when removedfrom their encapsulating ampoules and exposed to the normal room temperature
environment, will desorb for a period of time, after which there is negligible
desorption. The resulting material, called a residue compound, is chemically stable
and contains an intercalate concentration (usually small), dependent on the
desorption temperature and on the intercalate concentration of the compound prior
to desorption (Hennig 1952b). Lamellar compounds, denoting intercalation com-
pounds that have not been desorbed, may be single-staged, multi-staged or have a
random arrangement of the intercalate layers.
2.1.2. The two-zone vapour transport methodIn the two-zone vapour transport method, the intercalant is typically heated to
some temperature Ti and the graphite, which is some distance away, is heated to a
higher temperature Tg as illustrated in gure 3. The stage of the compound, or the
M. S. Dresselhaus and G. Dresselhaus6
Figure 3. Schematic diagram of the t wo-zone vapour transport method where Tg and Tiindicate the temperature of the graphite and intercalant respectively.
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depend on many parameters, including geometrical factors (sample size, such as
thickness and cross-sectional area and ampoule size and shape) and the accuracy of
temperature control (Hooley et al. 1965, Hooley 1977a). For the preparation of
dilute alkali metal compounds, shaping the ampoule to minimize the free volume for
intercalate vapour around the graphite, and necking down the connection between
the graphite and intercalate zones increases the stage index of the sample (Underhill
et al. 1980). The intercalation ofsmall graphite samples promotes the formation of a
single stage over the sample volume. For a given geometrical arrangement, the
preparation of a dilute compound of specied stage is greatly aided by use of a
growth diagram such as gure 4. An upper limit for Tg is imposed in practice by the
softening of the encapsulating glass and by its increased reactivity with the alkali
metal intercalants at high temperatures. A lower limit on Ti is imposed by the
requirement of some minimum acceptable reaction rate and of the condition p > pt,
where pt is the threshold pressure below which intercalation does not occur. It should
be mentioned that although alkali metal compounds are typically grown under lowvapour pressure (less than 1 torr) conditions, the intercalation rates are relatively
rapid; for example, single-staged graphiteRb samples for n 8 can be prepared inless than 24 hours (Underhill et al. 1980).
Preparation of a variety of single-staged acceptor compounds is also carried out
using the two-zone technique, though the detailed growth conditions are quite
dierent from the donor compounds and vary from one class of acceptors to
another. In the case of acceptors, the intercalant is in molecular form, and normally
a much larger c-axis expansion of the graphite host is required to accommodate the
intercalant. For growth of acceptor compounds by the two-zone method, the
graphite temperature Tg is typically held constant, and the intercalant, at a lower
temperature Ti is varied to produce the desired stage (Hooley 1973). The acceptor
compounds typically have a high threshold vapour pressure and therefore are
prepared under high vapour pressure conditions.
The metal chloride acceptor compounds with AlCl3 and FeCl3 are both prepared
in a similar way (Hooley 1973). The metal chloride intercalant is rst produced in situ
by direct reaction of the heated metal wire with Cl2 gas, as shown in gure 5. Prior to
intercalation, the two-zone ampoule is back-lled with Cl2 gas to encourage staging
(Dzurus and Hennig 1957b, Metz and Hohlwein 1975b); without the Cl2 gas, only
weight uptake is achieved and staging is inhibited (Ru dor and Zeller 1955, Hooley
1972, 1973, Underhill et al. 1979). The presence of Cl2 gas is also necessary for the
M. S. Dresselhaus and G. Dresselhaus8
Table 1. Ti and Tg for preparation of alkali metal compoundswith stages 1 n 3.a
K Rb CsTi 2508C Ti 2088C Ti 1948C
Stage Tg8C Tg8C Tg8C
1 225320 215330 2004252 350400 375430 4755303 450480 450480 550
a From Nixon (1966).
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preparation of many other metal chloride intercalation compounds (Stumpp 1977),
generally increasing the amount of intercalant uptake, reducing the threshold
pressure, and encouraging good staging to occur. Whether or not there is a
signicant uptake of excess chloride in such reactions has not been fully resolved
(He rold 1979).
Once intercalation has been completed, the reaction chamber and its contents are
quenched. To prevent exfoliation during this cooling process, the reaction chamber is
rst quenched on the intercalant side, away from the graphite, so that the vapourpressure is suitably reduced and condensation of metal chloride vapour on the
sample is avoided (Hooley 1973). Once cool, the ampoule containing the intercala-
tion compound is sealed o. Reducing Tg Ti in the presence of a high intercalate
vapour pressure can result in very rapid intercalate uptake, the formation of mixed
stages, and the introduction of large strains which are usually relieved by rapid
(explosive) exfoliation of the sample. Exfoliation can also cause problems in
handling samples. In some cases, exposure of intercalated graphite to vacuum causes
exfoliation, which is prevented by use of an inert gas atmosphere in the encapsulating
ampoule. For example, graphiteAsF5 samples are encapsulated in dry nitrogen gas,
rather than in a vacuum (Falardeau et al. 1978).
For the case of halogen growth, typical growth temperatures are close to room
temperature, owing to the high threshold vapour pressure of the halogens. The only
diatomic homopolar halogen molecule that intercalates readily into graphite is Br2,
though intercalation with ICl and IBr can also be carried out. For Br2 intercalation,
the lowest stage that has been reported is a stage 2 compound. GraphiteBr2compounds are readily prepared using the growth conditions Tg 208C and
308C < Ti < 208C, by inserting liquid bromine into a temperature-controlled
(refrigerated) alcohol bath (Sasa et al. 1971). To illustrate the staging conditions
(regions of stability) for stage 2, 3, 4 and 5 compounds, the adsorption isotherm for
graphiteBr2 is shown in gure 6. Also evident in this gure is the threshold vapour
Intercalation compounds of graphite 9
Figure 5. Schematic diagram of a system used for the preparation of graphiteFeCl3.(a) System used to prepare crystalline FeCl3 in situ by passing Cl2 gas over a heated
Fe wire. This closed system is advantageous because FeCl3 is highly hygroscopic.(b) Two-zone ampoule containing a highly oriented pyrolytic graphite sample(HOPG) in one zone and the crystalline FeCl3 to be intercalated in the other zone.
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pressure for intercalation, pt 0:1p0, where p0 is the pressure where the saturationcompound (second stage C16Br2) is formed. The open circles give the experimental
points of Sasa et al. (1971) for bromination (adsorption) and the solid circles for
debromination (desorption). Because of the instability of these compounds (seen as
hysteresis in the adsorptiondesorption curves), they must be encapsulated in
ampoules for storage and properties measurements. We note from this gure that
in the absence of an ambient Br2 vapour pressure (p=p0 0), about 30% of the Br2
uptake at saturation remains in the resulting residue compound.The two-zone vapour transport method is widely applied to the growth of well-
staged intercalation compounds and for many systems is the preferred growth
method. However, for specic systems, other intercalation methods are advanta-
geous as described below.
2.1.3. Other intercalation methods
Isothermal vapour transport is another method used for the preparation of
acceptor compounds. This method is for example applied to the preparation of the
high conductivity graphiteAsF5 compounds (Falardeau et al. 1978). For this
system, the growth takes place at room temperature using an AsF5
overpressure
of 3atm. The growth time is the principal parameter used to control the stage
index, and well-staged compounds have been prepared for 1 n 5. During the
growth process, Falardeau et al. monitored the stage index visually by measurement
of the increase in c-axis sample thickness, which exhibits distinct steps for each of
these low stage compounds that is produced (gure 7).
Another modication of the basic vapour transport technique to control the
stage index is the use of restricted amounts of intercalant in the reaction chamber.
For example, a number of well-staged compounds have been prepared using a two-
zone vapour transport system but with restricted quantities of crystalline AlCl3, the
smaller the quantity, the higher the stage that results (Gualberto et al. 1980).
M. S. Dresselhaus and G. Dresselhaus10
Figure 6. Isotherms of bromine uptake by highly oriented pyrolytic graphite (HOPG) at208C as measured by Sasa et al. (1971) for the bromination cycle (open circles) and
the debromination cycle (closed circles). Below the threshold pressure pt, nointercalation occurs. Note also that when p 0 on the debromination cycle, a non-vanishing bromine uptake remains, forming a `residue compound.
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Post-intercalation methods have also been used successfully to change theintercalate concentration or oxidation state. For example, using the wash method,
the preparation of dilute compounds from a more concentrated material can be
carried out by washing with a solvent, as for example using acetone to wash
graphiteFeCl3 (Hooley and Soniassy 1970). While this method can provide very
dilute compounds, the resulting samples are not well staged.
Chemical reactions with the intercalate species to change the oxidation state is in
some cases also possible after intercalation is completed. For example, FeCl3 is
readily intercalated into graphite by the two-zone method, but FeCl2 has not been
successfully intercalated directly. However, FeCl3 can be reduced to FeCl2 in the
intercalation compound using H2 at 3758C (Hooley et al. 1968). In this case, the
oxidation state of the intercalant was identied using Mo ssbauer spectroscopy,which shows distinctly dierent spectra for FeCl3 and FeCl2. It is also of interest that
the reduction to FeCl2 could be carried out essentially completely, so that no
characteristic Mo ssbauer spectrum for FeCl3 was observable.
It has also been claimed that complete reduction of a metal chloride can be
carried out to obtain two-dimensional metal sheets between graphite layers (Klotz
and Schneider 1962, Novikov et al. 1971, Volpin et al. 1975). Though a number of
workers have found evidence for the presence of-Fe upon reduction of graphiteFeCl3, using X-ray, Mo ssbauer and other techniques, it has not been established that
staged intercalate metal layers can be prepared by reduction methods (Bewer et al.
1977). Studies of the dierences of H2 and CO adsorption isotherms on unactivated
and activated (by heat treatment in H2) stage 1 graphiteFeCl3 have been interpreted
to indicate that Fe metal is formed on the sample surface, but is not intercalated
(Parkash et al. 1978). Another interesting chemical reaction that has been carried out
within an intercalation compound is the reduction of various graphite metal
chlorides with alkali metals to produce a catalyst for NH3 synthesis (Ichikawa et
al. 1972a). The electron diraction patterns taken by Evans and Thomas (1975) of
the graphiteFeCl3K system show evidence for the presence of KCl and free iron, in
agreement with Mo ssbauer studies (Tricker et al. 1974), also showing free iron. It has
been proposed (Evans and Thomas 1975) that the catalytic activity of this system is
due to the presence of dispersed free iron on the graphite surface.
Intercalation compounds of graphite 11
Figure 7. Plot of c-axis expansion ratio, t=t0, versus reaction time for a typicalintercalation of AsF5 into graphite (from Falardeau et al. 1978). Note the stabilityregions oft=t0 corresponding to each of the stages 1 n 5:
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Somewhat related to the use of chemical rections to change the oxidation state of
the intercalant is the state of the intercalant before and after intercalation. For
example in the case of graphiteHNO3 compounds, Forsman et al. (1978) have
concluded that the intercalant is present in the form of neutral HNO 3 molecules
admixed with charged NO3 ions by identifying NO2 as a by-product of the
intercalation process. It is believed that similar phenomenon may occur in other
acceptor compounds such as with the intercalants AsF5 and SbCl5.Another useful technique for the preparation of certain intercalation compounds
is the use of liquid intercalants. For the case of the alkali metal donor lithium, rst
stage C6Li samples can be prepared by degassing the graphite and then immersing it
in molten lithium in a stainless steel crucible in an argon atmosphere containing less
than 1 ppm H2O and O2 (Zanini et al. 1978a, Basu et al. 1979). Using a solution of
molten lithium and sodium (3.8 wt% Li) stage 2 samples can be produced, with only
Li being intercalated. After intercalation, the external sample surfaces containing a
metal lm are removed by cleavage. Graphitelithium compounds with stages
1 n 4 have also been prepared by compressing lithium powder (1020 kbar)
with crushed natural graphite in an argon atmosphere (Gue rard and He rold 1975).
Somewhat related to the method of intercalation from the melt is intercalationfrom solution. This method can for example be applied to the preparation of
graphitebromine compounds using CCl4 as a solvent (Hennig 1952a, Hennig and
McClelland 1955, Saunders et al. 1963). In this case the intercalate concentration in
the compound is controlled by the intercalate concentration in solution, its
temperature and the immersion time. Though intercalation from solution provides
a convenient intercalation method, it is dicult to prepare well-staged compounds
using this technique. Some other examples of intercalant/solvent mixtures are FeCl3in acetone and FeCl3 in nitro-methane (Hooley 1972), and in nitroethane (Ginderow
and Setton 1963), and other metal chlorides in SOCl2, SO2Cl2 and CCl4 (Stumpp
1977). Gas solutions have also been used to intercalate species that do not alone
intercalate readily. For example, metal chloride vapours such as AlCl3 and FeCl3have been used successfully to intercalate other metal chloride species such as CoCl2and NiCl2 into graphite (Stumpp 1977).
In the case of donor intercalants, liquid ammonia has been used by Ru dor et al.
(1955) and other workers as a solvent for the intercalation of metals M such as Li,
Na, K, Rb, Cs, Co, Sr, Ba, though in this solution growth process a ternary
compound results, as for example stage 1 compounds with stoichiometry
C12M(NH3)2. Other workers (Stein et al. 1966, Ginderow and Setton 1970, Beguin
and Setton 1975) have used organic solvents for intercalating metals, and some dilute
donor compounds have been prepared with this technique.
The preparation of ternary intercalation compounds by either cointercalation or
sequential intercalation has also been pursued. Some examples of interest are
cointercalation of alloys of the alkali metals K, Cs, Rb with Na and of mixed
halogen compounds of Br2 with I2 and Cl2. Preparation of such alloys and mixtures
provides a method for the insertion of materials that do not readily intercalate by
themselves. For example, Na has been intercalated into graphite as a binary
compound only at very high stages (Asher 1959), but Na can be readily intercalated
as an alloy with Cs and K (Billaud and He rold 1974, Billaud et al. 1980). Likewise,
chlorine does not intercalate as a binary compound but can be intercalated as a
mixture with bromine (Furdin et al. 1970, Furdin and He rold 1972) and with iodine
(Bach and He rold 1963, 1968). One explanation that has been proposed for the
M. S. Dresselhaus and G. Dresselhaus12
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failure of Na and Cl2 to intercalate in low stage compounds is the lattice mismatch of
layers of graphite and of solid Na (Dresselhaus and Dresselhaus 1979), and of solid
Cl2 (Hooley 1973). In order to prepare a compound by this type of cointercalation
process it is necessary for one of the elements to intercalate by itself and for the two
intercalants to be miscible (He rold 1979). In these cases, cointercalation improves the
lattice match, thereby promoting the intercalation process.
Another example of cointercalation is the introduction of intercalants withdierent chemical properties. For example, graphiteAlBr3 Br2 can be prepared
as a stage 1 compound with stoichiometry C9AlBr3 Br2, containing a higher
bromine density than the saturated stage 2 compound C 16Br2 (Sasa et al. 1972).
In general, the preparation of metal bromide compounds is similar to that of metal
chloride compounds in that the presence of Br2 gas is usually required for
intercalation and the preparation of well-staged samples (Stumpp 1977). However,
for the case of the metal bromides, the presence of Br2 gas results in the growth of
ternary compounds because of Br2 uptake.
Whereas the alloy compounds mentioned above are intercalated at the same
time, sequential intercalation is also commonly used to insert materials that are not
readily intercalated as binary compounds. For example, hydrogen which does notintercalate by itself, can be inserted into an alkali metal compound. The introduction
of hydrogen into a rst stage C8K compound forms an ordered second stage
compound C16K2H4=3 in which the intercalant resides in a triple layer sandwich
formed by two highly electropositive K layers between which is inserted a less
electropositive H layer (Gue rard et al. 1977b, Lagrange and He rold 1978). A model
proposed by Lagrange and He rold (1978) for the intercalation process that takes
2C8K into C16K2H4=3 is shown in gure 8(a), and the corresponding layer
arrangement is shown in gure 8(b). The intercalate sandwich, including a hydrogen
layer between two potassium layers, has a thickness of 5.18 A . Subsequent
intercalation of C16 K2H4=3 with an alkali metal M K, Rb or Cs yields another
stage 2 compound with stoichiometry C8K2H4=3, C8M and a uni t cell of Ic 8:53 A
IM as shown in gure 8(b). The intercalation of higher stage alkali
metal compounds can also be carried out, and use of such compounds for H2 storage
and as molecular sieves for H2 has been discussed (Lagrange and He rold 1978).
Ternary compounds with HgK and HgRb have been similarly prepared by
Lagrange et al. (1980). Large organic molecules can also be intercalated using the
sequential intercalation technique starting with a graphiteK binary compound. For
example, benzene can be intercalated into stage 2 or stage 3 graphitepotassium, but
not into rst stage C8K (Merle et al. 1977, Bonnetain et al. 1977).
The use of cointercalation and sequential intercalation techniques greatly
expands the number of possible intercalation compounds that can be prepared.
These techniques furthermore form a basis for the use of graphite intercalation
compounds as catalysts for carrying out chemical reactions. Such applications have
been made to the preparation of a number of organic compounds (Setton 1977) and
to the use of graphite layers to promote polymerization (Gole 1977).
Electrochemical techniques are also useful for the preparation of graphite
intercalation compounds, particularly for strong acid intercalants such as sulphuric,
perchloric, nitric and triuoroacetic acids. The graphite is oxidized anodically by
placing the graphite specimen in a platinum cap suspended in concentrated acid, and
using a second platinum counterelectrode. In this case, the stage formation is
controlled by the electrode voltage (Ru dor and Siecke 1958, Bottomley et al.
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1963b, Horn and Boehm 1977, McRae et al. 1980a). The electrochemical preparation
method provides an excellent technique for study of the thermodynamic changes
accompanying a change in stage during the growth process. This technique has been
applied to both the system of donor compounds graphiteK (Aronson et al. 1968)and the acceptor compounds based on the intercalant H 2SO4 (Aronson et al. 1971),
as discussed in section 2.3. Because of the colour change associated with dierent
stages of certain graphite intercalation compounds (for example, alkali metal
compounds), it is possible to use electrochemical means to produce a change in
stage (and colour), thereby yielding an electrochromic eect. Such an electrochromic
eect has been proposed for display device applications (Puger et al. 1979).
2.1.4. Some prototype intercalants
For various physical measurements, specic intercalation compounds are of
particular interest from the point of view of chemical stability, the simplicity of the
intercalate structure, and specic properties such as high electrical conductivity or
anisotropy, magnetic ordering and superconductivity. We discuss below some
prototype intercalation compounds that are particularly suitable for specic applica-
tions.
Because of their stability in air (Lazo and Hooley 1956), graphiteFeCl 3 samples
may be safely removed from their encapsulating ampoules for short periods of time,
and therefore provide prototype materials for properties measurements and ex-
ploratory investigations. Curiously, pristine FeCl3 is itself highly reactive and
hygroscopic, in contrast with the behaviour of its intercalation compounds. It has,
however, not yet been demonstrated that one can prepare a stage 1 graphiteFeCl3
M. S. Dresselhaus and G. Dresselhaus14
Figure 8. Model for ternary graphitealkali metal compounds containing hydrogen asproposed by Lagrange et al. (1978). (a) Schematic model for the transformation ofrst stage 2C8K to second stage C16K2H4=3. (b) Layer arrangement for pristinegraphite, rst stage C8K, second stage C16K2H4=3 and second stage C8K2H4=3, C8M
where the thickness of the alkali metal sandwich is IM 1:97, 2.27, 2.58 A dependingon whether M K, Rb or Cs.
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sample without inclusions of pristine graphite, as was long ago noted by Cowley and
Ibers (1956). On the other hand, stage 1 graphiteAlCl3 is readily prepared, although
it is found to be very unstable when removed from its growth ampoule and exposed
to air (Ru dor and Zeller 1955, Dzurus and Hennig 1957a). Because of the chemical
similarity between graphiteFeCl3 and graphiteAlCl3, stage 1 samples of graphite
AlCl3 are sometimes used in conjunction with higher stage graphiteFeCl3 com-
pounds for properties measurements. Although stage 1 graphiteAlCl3 is highlyreactive and requires encapsulation, high stage (n 4) graphiteAlCl3 samples are of
comparable stability to graphiteFeCl3 samples of similar (high) stage. Environ-
mental stability has also been reported for graphiteSbCl5 compounds for n 2 by
Murthy et al. (1980).
Alkali metal compounds are used frequently as prototype materials for many
property measurements. Of all intercalation compounds these materials are most
easily prepared, exhibit the highest degree of order and are best understood from a
structural point of view. Compounds formed with the heavy alkali metals K, Rb and
Cs form one class of compounds with many similar properties, which can be
compared to compounds formed with the alkali metal donor Li. The graphiteLi
compounds dier from the heavy alkali metal compounds with regard to structural
ordering and intercalate ionic size, and therefore exhibit somewhat dierent proper-
ties. Other donor intercalants exhibiting the same in-plane ordering as Li are the
alkaline earth metals Ba and Sr (Gue rard and He rold 1974) and the lanthanides Eu,
Yb, Sm and Tm (Lagrange et al. 1980).
Among the acceptors, the simplest compounds are the graphiteBr2 compounds,
where the intercalant is a simple homopolar diatomic molecule having its molecular
axis aligned within the intercalate layer. Another simplifying feature of the graphite
Br2 system is the formation of commensurate compounds, in which the ordering of
the bromine intercalate layer is in registry with that in the graphite layers (Eeles and
Turnbull 1965). The graphiteBr2 system however does not form stage 1 compounds(Sasa et al. 1971). On the other hand, the halogen ICl does form stage 1 compounds,
and has the further advantage of having a dipole moment unlike Br2, thereby
providing increased coupling of the intercalant to an applied electromagnetic eld
probe. The failure to prepare stage 1 compounds is common for many acceptor
compounds, as for example with certain metal halides (Stumpp 1977), and the reason
for this diculty is not generally understood. There is also much evidence in the
literature for diculties in the preparation of certain low stage compounds with
specic intercalants, as for example PdCl2 which formed only stage 3 compounds
(Novikov et al. 1973).
Intercalation in some cases permits study of a chemical species that is otherwise
unstable. For example, TlBr2 does not exist by itself but must be kept in liquid Br2.However, stable graphite intercalation compounds can be formed with TlBr3 (Niess
and Stumpp 1978).
Low stage acceptor compounds prepared with the highly reactive Lewis acid
AsF5 have been prototype materials for conductivity studies because of reports of a
room temperature in-plane conductivity exceeding that of copper and an anisotropy
between in-plane and c-axis conductivity of106 (Foley et al. 1977, Falardeau et al.
1977). Signicant advances in materials preparation of graphiteAsF5 compounds
(Falardeau et al. 1978) have also contributed to the widespread interest in these
compounds.
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The graphiteFeCl3 compounds are prototype magnetic materials with anti-
ferromagnetic ordering reported below 3.6 K for stage 1 and ferromagnetic ordering
below 8.5 K for stage 2 (Karimov et al. 1971, Ohhashi and Tsujikawa 1974a) while
the stage 1 alkali metal compound C8K is an intercalation compound in which
superconductivity can be studied below 1 K (Hannay et al. 1965, Koike et al. 1978).
As graphite intercalation compounds nd new application areas, it is likely that
other intercalation compounds will become prototypes for these applications.
2.2. Materials characterization
A number of techniques are exploited for sample characterization of graphite
intercalation compounds, including visual inspection, weight uptake, chemical
analysis, c-axis dilatation, diraction measurements and electron microscopy.
Diraction studies yield the stage index and information on staging delity and
on in-plane order, and electron microscopy provides information on the micro-
structure and submicrostructure. In this section we review some of the most
important sample characterization techniques.
Several simple characterization methods are used to provide useful qualitative
information. The sample colour observed by visual inspection gives qualitativeinformation on the stage: for example, for the alkali metal compounds a yellow,
gold or red colour is characteristic of stage 1 compounds, steel blue for stage 2, dark
blue for stage 3 and graphitemetallic for higher stages (Hennig 1959, He rold 1979).
For acceptor compounds, stage 1 is often blue and higher stage compounds
graphitemetallic. Direct chemical analysis gives the chemical formula for the
intercalation compound. Weight uptake gives information on the sample stage if
the chemical formula for the compound is known and stoichiometry is assumed.
However, this information is qualitative because of sample inhomogeneity, the
presence of intercalate vacancies and the preferential accumulation of intercalant
in the vicinity of crystal defects.
Because of sample expansion along the c-axis due to the intercalation process,dilatation along the c-axis, observed using a travelling microscope, also provides
qualitative information on the sample stage. This method is subject to large
systematic errors because of sample inhomogeneities, the formation of microcracks
and the tendency for exfoliation especially near the sample edges where the length
measurements are usually made. However, for the case of graphiteAsF5, c-axis
sample expansion provides a good indicator of stage, as shown in gure 7. For this
system, c-axis expansion is used as a diagnostic for stopping the growth process at a
given stage (Falardeau et al. 1978).
Since many properties measurements are strongly dependent on the stage index,
it is important to characterize samples to be used for property measurements with
regard to stage index and stage delity. This type of characterization of graphite
intercalation compounds is provided by X-ray diraction using (00l) reections.
Sample characterization for stage index n and repeat distance Ic has been carried out
for large numbers of compounds (more than 50) and the results are given in various
review articles (Hennig 1959, Ebert 1976, Stumpp 1977) and in table 6. It has become
common for property measurement studies on graphite intercalation compounds to
include staging information as obtained by (00l) diractograms. Accurate staging
determinations can be made with a system such as is shown in gure 9 (Leung et al.
1981a). Mo K radiation is used in order to minimize the X-ray absorption by the
glass encapsulating the samples. A typical set of (00l) diractrograms is shown in
M. S. Dresselhaus and G. Dresselhaus16
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gure 10 for graphite and graphiteRb samples of stages 1, 3 and 6. From these
diractograms, the diraction angles l corresponding to the various (00l) reections
are determined, and using Braggs law
l 2Ic sin l; 2:1
Ic, the repeat distance (see gure 2), is accurately evaluated. Since the graphite
interlayer separation is essentially unaected by intercalation, the stage index n is
found from the relation
Ic nc0 di n 1c0 ds; 2:2
where c0 is the distance between adjacent graphite layers (c0 3:35 A ) andds c0 di is the distance separating two graphite layers between which an inter-calate layer is sandwiched. Analysis of (00l) diractograms show that for a given
intercalant, ds and c0 are essentially independent of stage (Hennig 1959, Ru dor
1959), as illustrated in table 2. The validity of (2.2) provides strong evidence that the
n graphite layers remain essentially unperturbed by the intercalation process. From
(2.2) it follows that the increase in length along the c-direction t relative to the
length prior to intercalation t0 is given by
Intercalation compounds of graphite 17
Figure 9. X-ray system for (00l) diractometer scans. K radiation from a Mo X-ray sourceis incident on the sample and the diracted beam is detected by a cooled Li-driftedsilicon detector. This detector permits high resolution energy discrimination of thediracted beam. The energy windows of the single channel analyser are set so thatonly signals corresponding to K1 and K2 radiation are processed. The multi-channel analyser is used for data acquisition of (00l) diractograms (from Leung etal. 1981a).
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tt0 di
nc0: 2:3
Since the separation of neighbouring (00l) diraction peaks becomes smaller as
the stage index increases, the energy discrimination technique provides an invaluable
tool for stage determination of dilute compounds (for example, stage n > 6). In the
analysis of staging for dilute compounds, the greatest sensitivity for determining the
stage index is achieved with low index reections (small 2l). However, the intensities
of the (00l) reections for small l due to the superlattice structure in high stage
compounds are usually very low because of the mismatch of these 2l values with the
maximum (at 2 128) in the envelope function (see gure 10) due to the strong
M. S. Dresselhaus and G. Dresselhaus18
Figure 10. X-ray stage characterization using (00l) diractograms for stages 1, 3 and 6graphiteRb compounds and for pristine graphite (from Leung et al. 1980a). Notethe correlation in 2 of the high intensity reections for the high stage intercalationcompounds with the occurrence of (00l) reections in graphite.
Table 2. Identity period or repeat distanceI
c for graphitepotassium compounds.
Stage n 1 2 3 4
Ic (measured) A Ru dora
5.41 8.77 12.12 15.49(measured) A Parry
b5.35 8.72 12.10 15.45
(measured) A Underhillc
5.32 8.74 12.07 15.44Ic (calculated) 5:41 3:35n 1 A
5.41 8.76 12.11 15.46
References: a Rudor and Schulze (1954);b Nixon and Parry (1968); c Private communication.
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graphite (002) reection. However, for high stage compounds (n 6), highintensities are again obtained for 2 < 18, through matching with the envelopeprovided by the strong graphite (000) peak. Stage determination for very dilute
compounds is made possible by the narrow linewidths of the diracted patterns and
the ability to resolve diraction peaks within 0 :58 of the direct beam (Underhill et al.1980). For example, the full widths at half maximum (FWHM) intensity for the
alkali metal intercalation compounds are roughly equivalent to those of pristinegraphite; for 2 128 (the position of the (002) graphite peak which is the mostintense peak), typical FWHM values for graphite are 0.28.
Using the two-zone vapour transport growth technique described in the previous
section, it is possible to prepare single-staged alkali metal compounds as dilute as
n 8 (Underhill et al. 1980), and using a diractometer system such as in gure 9 tocharacterize them for stage index and stage delity. Stage delity is established by the
absence of satellite diraction peaks due to small quantities of admixed stages, and
by accurate distinction between stages n and n 1 in the analysis of the diract-ometer scans. The absence of broadening of the diraction lines at large diraction
angles provides strong evidence that the samples are single staged, and not an
average over a distribution of stages (Metz and Hohlwein 1975b). The X-ray patternobtained from most intercalation compounds during a staging phase transition is of
the type discussed by Metz and Hohlwein and can be explained in terms of the
domain model for staging proposed by Daumas and He rold (1969) and discussed in
section 2.4.
In discussing stage indelities it is useful to distinguish two types of stage
indelity. A mixed stage sample contains macroscopic regions that exhibit stage n
and other macroscopic regions with dierent stages, usually stages n 1 or n 1. Inthis case the X-ray (00l) diractograms show reections that are broadened relative
to those from a single-staged sample, some reections that can be identied with
each constituent stage, and others that are shifted because of unresolved 2lcomponents. In contrast, a randomly staged material will contain a randomarrangement of regions with various stages. If one of these stages is dominant, a
well-dened diraction pattern will result but with reections showing extensive line
broadening (Metz and Hohlwein 1975b). The random stacking of intercalate layers
has been studied in detail by Metz and Hohlwein (1975a, b) for the graphiteFeCl3system, but no systematic study of this phenomenon has been carried out for any
other intercalant.
Further sample characterization is provided by analysis of the intensity of the
(00l) reections, by relating the square root of the observed intensities II00l to the
magnitude of the structure factor F00l
jF00lj fC sin n"lsin "l
1lX
X
fX exp 2i"Xl-----
-----; 2:4
where fC and fX are atomic scattering factors for carbon and for the intercalant on
layer X, " c0=Ic, and "X zX=Ic, where zX is the distance of an intercalate layerrelative to the centre of the intercalate sandwich, and 1= is the intercalate density.Since the structure factor determines the relative intensity of the (00l) reections, the
index ll of the reection with maximum peak intensity (00ll) approximately
determines the stage n according to the relation
ll n mi; 2:5
Intercalation compounds of graphite 19
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wherem
i is the nearest integer to the ratiod
i=c
0 (Leunget al.
1980b). For multilayerintercalants (for example, FeCl3 and AlCl3), structure factor calculations can
provide valuable information for determining the intercalant stacking arrangement
when compared to experimental II00l measurements. An example of such a deter-
mination is the identication of the Cl3Al2Cl3 stacking arrangement of the
intercalate layer sandwich, as shown in gure 11 (Leung et al. 1980b).
Integrated intensity measurements of (00l) reections also provides an important
characterization technique through determination of the intercalate in-plane density
1=. While gravimetric measurements depend on the total weight uptake duringintercalation, the structure factor depends only on the fraction of intercalate
arranged in ordered stages. For compounds where the in-plane structure is known,
the in-plane intercalate density can be used to obtain the fractional site occupation inthe intercalate layer (Leung et al. 1979). Such an analysis applied to a rst stage
graphiteRb sample, which forms a p2 2R08 superlattice (Ru dor and Schulze1954, Nixon and Parry 1968, Kambe et al. 1980a) and assuming the stoichiometry,
C8Rb, shows 15% vacancies at room temperature (Leung et al. 1979). Real spaceelectron micrographs of thin graphiteRb specimens show the presence of more than
one phase at 300 K for compounds with stages n 2 (Kambe et al. 1980a). Theseexperiments suggest that even in single-stage d materials with long-range c-axis
ordering, defects and multiple phases within the intercalate layer are common for
n 2 compounds. As discussed in section 3, diraction measurements also providethe principal technique for structural studies of the in-plane ordering in graphite
intercalation compounds.
The electron microscope has also provided useful information on the micro-
structure and ultramicrostructur e of graphite intercalation compounds. An espe-
cially vivid application of this technique was made by Evans and Thomas (1975) and
by Thomas et al. (1976), who applied high resolution transmission electron
microscopy (T.E.M.) in the lattice imaging mode to `see individual graphite and
intercalant lattice planes directly. Such a bright eld lattice image of a graphite
FeCl3 samples is shown in gure 12(a) and the schematic diagram illustrating the
interpretation given by Thomas et al. (1976) of this photograph is presented in gure
12(b). This work demonstrates the staging phenomenon directly with real image
M. S. Dresselhaus and G. Dresselhaus20
Figure 11. Plot of relative X-ray integrated intensity versus x for stage 2 graphiteAlCl3,where x is the dimensionless quantity x c0l=Ic (see text). The solid and dashedcurves represent Cl3Al2Cl3 and AlCl6Al intercalate stacking respectively. The tto the experimental measurements (closed circles) of Gualberto et al. (1980) is muchbetter for the Cl3Al2Cl3 intercalate stacking arrangement than for the AlCl6Alstacking (Leung et al. 1980b).
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Intercalation compounds of graphite 21
(a)
(b)
Figure 12. High resolution transmission electron microscopy applied to lattice imaging ofindividual graphite and intercalate lattice planes in graphiteFeCl3 by Thomas et al.(1976). (a) Bright-eld lattice image of a graphiteFeCl3 sample (average stage n 2).The light striations on the micrograph delineate the sheets of FeCl 3 intercalant, andfrom their separation the stage indices may be identied. (b) Schematic diagramillustrating the interpretation of (a) in terms of the various stages that are containedin the photograph.
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photographs and gives evidence for regions of material with large repeat distances
(sixth stage). These authors were also able to relate the observed layer thicknesses in
the intercalation compound with those in the parent materials. Furthermore, the
sample region shown in gures 12(a) and (b) indicates a predominance of stage 2
with some admixtures of stage 1 and trace amounts of stages 5 and 6. In view of the
diculty in preparing a-face samples (see section 4.3), the staging delity shown in
gure 12(a) is impressive. Similar latttice image results have also been presented forpristine graphite and for graphiteK compounds (Evans and Thomas 1975). The
T.E.M. technique is dicult to apply in practice because very thin samples (600 A )are required to permit penetration by the electron beam and because the intercalate
tends to desorb after several hours under the high vacuum conditions of the electron
microscope column. Nevertheless, the use of high resolution transmission electron
microscopy under dynamic conditions could provide extremely valuable information
on the intercalation mechanism.
Very high resolution studies using scanning transmission electron microscopy
(S.T.E.M.) have been carried out on the heavy metal atom migration on thin
amorphous carbon lms, allowing the motion of single atoms to be studied (Isaacson
et al. 1979). Such studies could also provide very interesting information onintercalated graphite. Also of interest is the S.T.E.M. study of commercially
produced `graphimet intercalates by Fischer et al. (1979) showing very thin metal
clusters of about 2060 A in diameter.
Transmission electron microscopy has also been applied very successfully by
Heerschap et al. (1964) and Heerschap and Delavignette (1967) to study the
microstructure associated with in-plane imperfections (stacking faults and disloca-
tions) in graphiteBr2, graphiteICl and graphiteFeCl2 compounds. Their real
image micrographs provide evidence for the existence of isolated dislocations
bounding the edge of an intercalated layer (see gure 13) in the case of all three
intercalant systems. The usual hexagonal layer stacking of pristine graphite is ABAB
as shown in gure 14, while rhombohedral graphite exhibits the ABCABC stacking
shown in gure 15. The Burgers vectors for the dislocations in the graphiteBr 2 and
graphiteICl systems are equivalent to the Burgers vector of the normal partial
dislocations in hexagonal pristine graphite. This result is interpreted in terms of an
interlayer shift of the graphite bounding layers to achieve identical crystallographic
positions, shown in gure 14 as an `A over A stacking arrangement of the graphite
layers about the intercalant (AjA), in agreement with X-ray diraction results for
M. S. Dresselhaus and G. Dresselhaus22
Figure 13. Schematic cross-section of a graphite crystal containing a dislocation boundingan intercalated layer of a reactant. Intercalation often causes a shift of the adjacentcarbon layers to yield a fully symmetrical arrangement relative to the intercalant layerX. For clarication of the A, B, C interlayer arrangement of graphite layers seegures 14 and 15.
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these materials (Ru dor 1941). On the other hand, for the graphiteFeCl3compounds the Burgers vector is found to be perpendicular to the graphite planes
so that the graphite preserves its ABAB stacking upon intercalation with FeCl3, in
agreement with X-ray results of Ru dor and Schulz (1940). It is reasonable to expect
commensurate intercalate structures to exhibit a symmetrical AjA stacking, whileincommensurate structures have no symmetry reason for disturbing the normal
graphite stacking.
The pinning of dislocation loops of1mm diameter at crystal imperfections isalso demonstrated in the micrographs obtained by Heerschap and Delavignette
(1967). Based on these observations it is concluded that upon intercalate desorption
to form residue compounds (Hennig 1959), intercalate islands can be trapped in
Intercalation compounds of graphite 23
Figure 14. Structure of hexagonal graphite showing ABAB stacking (from Wycko 1964).
Figure 15. Structure of rhombohedral graphite showing ABCABC stacking (from Wycko1964).
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dislocation loops, as shown in gure 16. Evidence for these trapped intercalate
islands is also provided by in-plane X-ray and electron diraction patterns from
residue compounds (Maire and Mering 1959, Chung et al. 1977). The preferential
trapping of the intercalant at crystal defects is observed in the T.E.M. micrographs
of Heerschap et al. (1964) and has also been identied by X-ray dispersive analysis
using scanning transmission electron microscopy (Chung 1977).
A combination of electron diraction patterns and real image electron micro-
graphs have shown that epitaxial layers form on the graphite surface under certain
growth conditions. For example, when a graphiteBr2 compound is exposed to
potassium vapour, Evans and Thomas (1975) have shown that KBr forms epitaxially
on the surface, and they have interpreted their results in terms of an extraction of
bromine from the intercalation compound. The formation of epitaxial layers of
potassium and caesium on the surface of graphiteK and graphiteCs compounds
when heated (to 2008C) has also been noted by Halpin and Jenkins (1970) and by
Chung et al. (1977), and this surface growth has been related to intercalate
desorption from the bulk.
Scanning electron microscopy (S.E.M.) also provides information on the
microstructures of the intercalation compounds. Such observations on an a-face
(containing both an a-axis and a c-axis) give evidence for microcrack formationresulting from intercalation (Chung 1977). These S.E.M. obervations further show
that microcracks perpendicular to the c-direction are common in the HOPG host
material and in single crystal akes separated by acid treatment from the carbonate
ore-bearing rocks, but such microcracks are almost absent in single crystal akes
that are separated mechanically.
2.3. Kinetics, thermodynamic s and the intercalation process
Important insight into the intercalation process has been provided by studies
made while intercalation is in progress. Such time-dependent studies involve both
kinetic and thermodynamic considerations as described below.
An important set of experiments relevant to intercalation kinetics was carried out
by Hooley et al. (1965) on the graphitebromine system using a cylindrical HOPG
host sample. In the rst of these experiments, it was found that no intercalation
occurred for bromine vapour pressures less than the threshold pressure for
intercalation pt, where pt 170 torr at 208C for Br2 intercalation. As p was increased
above pt, intercalation rst began into the layers near the terminal free surfaces. By
coating the outer cylindrical surfaces of the graphite sample with impervious
material but leaving the end planes uncoated, and then coating the end planes but
leaving the cylindrical surfaces uncoated, Hooley et al. (1965) concluded that
interaction of the intercalant with both the exposed basal plane surfaces and the
M. S. Dresselhaus and G. Dresselhaus24
Figure 16. Model of a residue compound showing intercalate islands trapped betweendislocation loops.
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exposed cylindrical surfaces is necessary for the initiation of the intercalation
process. From these experiments Hooley et al. inferred that intercalation sets up a
tension between the outer surface region and the inner core region. If these tension
forces are small, the reaction front proceeds to the inner core and the samples
become fully intercalated. If, however, these forces are large, then the intercalation
process stops and the tension breaks up the cylinder into discs. The results of the
Hooley experiments have been interpreted to indicate that a strain energy isintroduced by separating two graphite layers through the insertion of an intercalant.
The connection of a strain mechanism with staging is discussed in the following
section.
As shown in gure 6, intercalation does not begin unless the intercalant vapour
pressure exceeds the threshold pressure pt. It is believed that a threshold pressure is
required to unpin lattice dislocations and to relieve lattice strain since the
intercalation process changes the atomic stacking sequence and requires the motion
of dislocations (Ubbelohde 1968a, Dowell 1977). The intercalation threshold
depends sensitively on intercalate species, on temperature and on the characteristics
of the graphite host materials (Hooley et al. 1965). With regard to sample size, pt is
lower for smaller sample thicknesses, so that thin samples intercalate more readilythan thick samples. With regard to sample perfection, pt is lowest for single crystal
akes, higher for HOPG host materials and yet higher for carbon bres (Hooley and
Bartlett 1967, Hooley 1977a). Measurement of intercalation isotherms for graphite
bres by Hooley and Dietz (1978) with the intercalants Br2 and ICl show that the
high initial value for pt is signicantly reduced (for example, by a factor of2) afteran intercalationdeintercalation cycle, and the intercalation rate is correspondingly
increased. This behaviour is attributed to the formation of microscopic cracks in the
interlocking amorphous carbon networks between the graphitized domains of the
bres, and this explanation is consistent with the observed dependence of pt on the
graphitization temperature of the bres.
Values for pt relative to p0, the pressure at maximum intercalate uptake (loweststage that can be prepared), are shown in gure 6. Values for pt=p0 can be varied overmore than three orders of magnitude with pt=p0 < 10
4 for materials that
intercalate easily (K, Rb, Cs, AlCl3) and pt=p0 > 0:1 for materials that are dicultto intercalate (MoCl5, WCl6, HgCl2). Intercalants with high (pt=p0) values tend tohave lower intercalate uptake (higher stages) at saturation. For example, the lowest
stage that has been prepared with WCl6 having pt=p0 0:5 is stage n 5 (Hooley1977a). Furthermore pt can in some cases be reduced by the presence of other
chemical species, as for example in the case of graphiteFeCl3, the presence of Cl2gas signicantly lowers pt. A decrease in pt is generally achieved by raising the
temperature of the graphite host material.
The initiation of intercalation at the edges of the graphite crystal as discussed
above is also supported by a number of other experiments, including studies of
successive sorptiondesorption cycles in a bromine vapour atmosphere by Marchand
et al. (1973) which were explained by a simple two-dimensional diusion process
along the basal planes of the graphitic crystallites. The importance of the diusion
mechanism was previously recognized by Aronson (1963), who studied the exchange
of normal gaseous Br2 gas with radioactive bromine (Br82 with a half-life of 35.7
hours) that has previously been intercalated into natural graphite powders. The
average concentration of radioactive bromine in the intercalation compound and in
the exchange gas was determined from the decay. From measurement of the
Intercalation compounds of graphite 25
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bromine exchange rate at 303 and 321 K, Aronson concluded that self-diusion of
the bromine in the graphite rather than an exchange between adsorbed and gaseous
bromine at the graphite surface was dominant in the intercalation process. Because
of the distribution of particle sizes and shapes, a fully quantitative study of the
diusion process could not be carried out.
Although intercalation is initiated at the edges of a graphite crystal, kinetic
studies on the graphiteFeCl3 system by Metz and Siemsglu ss (1978) usinggravimetric (weight uptake) techniques have indicated that an approximately
constant macroscopic distribution of intercalant is achieved in the intercalate layers
when the total intercalate uptake is only 20 to 30% of its saturation value. This
absence of a concentration gradient in the intercalated layers has been interpreted in
terms of intercalate nucleation near the edge of the graphite crystal, rapid growth of
the nucleus to an island having approximately the nal structure of the intercalate
layer at saturation intercalate uptake. Thus Metz and Siemsglu ss concluded that
even at these low intercalant uptake values, the intercalate islands are uniformly
distributed on a macroscopic scale between two sequential graphite layers. The slope
of an Arrhenius plot of the intercalation rate constant is interpreted as an activation
energy for nucleus formation at the edges of the graphite crystal (25 kcal/mol for thegraphiteFeCl3 system) and this activation process limits diusion into the bulk
graphite (Metz and Siemsglu ss 1978). The activation energy for nucleus formation is
much larger than the activation energy for diusion which was found to be 23 kcal/
mol by Barker and Croft (1953). Aronson (1963) also noted that more energy was
required to initiate intercalation between two graphite host layers than to sustain
subsequent diusion into the host.
The main conclusions of the MetzSiemsglu ss study on the graphiteFeCl3system are in agreement with kinetic diusion studies by Dowell and Badorrek
(1978) on the intercalation of HNO3, Br2 and PdCl2 into an HOPG host material. In
this study the diusion coecient D was determined by measuring the weight gain
Mt=M1 and its time derivative dMt=M1=dt as a function of time, and using theequation for diusion into a cylinder
Mt
M1
4
1=2Dt
r2
1=2
Dt
r2
1
31=2Dt
r2
3=2; 2:6
where r is the radius of the cylinder, and Mt and M1 are respectively the weight gain
at time t and at very long times. These measurements, taken at constant vapour
pressure on samples with dierent radii and as a function of temperature, show that
the results for both Mt=M1 and dMt=M1=dt can be explained by equation (2.6)in terms of a single diusion coecient which is operative over the entire
composition range. The absence of change in diusion coecient as the sample
passes from one stage to another, as for example from stage 4 to stage 3 on the way
to the saturated stage 2 compound, implies that only minor crystal rearrangements
occur upon stage transformation.
Dowell and Badorrek showed that the magnitudes of D vary widely from one
intercalant to another. For example, for Br2 at 303 K and p 258 torr, D 1:47 106 cm2/min, while for HNO3 at the same temperature and p 81 torr,D 203 106 cm2/min. The value for D obtained by Dowell and Badorrek (1978)for bromine diusion into an HOPG host material of specied cylindrical shape is in
qualitative agreement wi th measurements by Aronson (1963) on intercalated natural
M. S. Dresselhaus and G. Dresselhaus26
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graphite powders. The high diusion coecient for HNO3 has been attributed by
Dowell and Badorrek (1978) to the liquid-like HNO3 intercalate phase at room
temperature, whereas the Br2 intercalant at room temperature is arranged in an
ordered phase. An activation energy of 5 kcal/mol for Br2 diusion in a HOPG host
material has been measured by Mukaibo and Takahashi (1963), and 12 kcal/molfor Br2 diusion in natural graphite by Aronson (1963).
The kinetics of the intercalation process depends intimately on thermodynamicconsiderations. A detailed determination of the thermodynamic parameters con-
nected with changes in stage has been carried out for the case of the graphitealkali
metal compounds with the intercalants K, Rb and Cs (Salzano and Aronson 1966a,
b, c and Aronson et al. 1968). Relatively little attention has been given to the
corresponding thermodynami c data for the acceptor compounds, though some work
has been done on the graphiteH2SO4 system (Aronson et al. 1971).
Referring to the isobaric growth diagram for the graphiteK system presented in
gure 4, we identify temperature (Tg Ti) regions of phase stability where a singlestage is dominant (for example, for n 1, 2, 3), and other temperature regions wheretwo stages are in equilibrium. The transition from one stage to another occurs in
these equilbrium regions and the thermodynamic studies to be described measure thechanges in free energy, enthalpy and entropy associated with a stage transformation .
Thermodynamic data can be obtained directly from calorimetric measurements,
calculations based on the temperatur e dependence of equilibrium pressures, and
electrochemical measurements. For the conditions used to prepare most intercala-
tion compounds, energy is absorbed upon the stage transition n ! n 1 and theentropy decreases.
While calorimetric measurements provide the most direct determination of
thermodynamic data, this method is dicult to apply in practice to the study of
stage transitions in intercalated graphite. This diculty arises from the sensitivity of
the intercalation process to crystalline faults as demonstrated by the kinetic studies
of Hooley and Dietz (1978) and Dowell (1977) on graphite host materials of varyingperfection. On the other hand, direct calorimetric methods can be readily applied to
the study of structural phase transitions or orderdisorder transformations in
graphite intercalation compounds as discussed in section 3.4.
Using a solid state electrochemical cell method on the graphiteK system
(Aronson et al. 1968), and a Knudsen eusion method in conjunction with a
radioactive tracer technique for the graphiteRb and graphiteCs systems (Salzano
and Aronson 1966a, b, c), values for the enthalpy and entropy changes associated
with stage transformations were obtained and the results are given in table 3.
In the electrochemical method, Aronson et al. employed the e.m.f. cell shown in
gure 17, which consisted of a liquid potassium Km
anode and an intercalated
graphite cathode. These electrodes were coupled by a solid potassiumglass
electrolyte which provided transport of K ions from anode to cathode. The
electrochemical reaction at the anode was therefore
Km e K
; 2:7
while at the cathode a stage change n ! n 1 occurred. For example, for thetransformation from stage 3 to stage 2, the reaction at the cathode was
2C36L K e 3C24K 2:8
to yield an overall reaction
Intercalation compounds of graphite 27
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2C36K Km 3C24K: 2:9
The free energy of the reaction Gcell is then obtained from the measured e.m.f. of
the cell, E,
Gcell mFE; 2:10
where F is Faradays constant (F 96 450 C) and m is the number of equivalents ofpotassium that are transferred. Using the thermodynami c relation between the
Gibbs free energy, enthalpy and entropy,
Gcell Hcell TScell; 2:11
M. S. Dresselhaus and G. Dresselhaus28
Table 3. Thermodynamic properties of graphitepotassium, graphiterubidium and graphitecaesium compounds.a
Graphite Graphite Graphite
potassium rubidium caesium
H8 S8 H8 S8 H8 S8
(cal/mol (cal/mol (cal/mol (cal/mol (cal/mol (cal/mol
Equilibrium reaction K) K 8K) Rb) Rb 8K) Cs) Cs 8K)
1a 1=3C24Ms Ms C8 Ms 27 400 25.7
1b 4C10Ms Mg 5C8Ms 38 000 44 33 900 42.2 43 800 43.42 5=7C24Ms Mg 12=7C10Ms 24 000 24 25 300 19.2 29 600 19.6
3 2C36Ms Mg 3C24 Ms 27 800 20.6 27 200 17.0 32 700 18.74 3C48Ms Mg 4C36 Ms 30 000 20.7 29 500 17.6 34 200 18.6
5 4C60Ms Mg 5C48 Ms 30 600 20.8 31 100 18.3 34 900 18.66 5C72Ms Mg 6C60 Ms 31 800 18.1 35 800 18.8
7 6C84Ms Mg 7C72 Ms8 7C96Ms Mg 8C84 Ms 31 700 20.9
a
From Aronson et al. (1968).
Figure 17. Apparatus used by Aronson et al. (1968) for e.m.f. measurements on cells of thetype K (molten) jKglass j graphiteK. Based on the temperature dependence ofthese e.m.f. measurements the enthalpy and entropy changes associated with a stagetransformation were determined.
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the entropy term can be found from the temperature dependence ofGcell according
to the thermodynamic relation Scell @Gcell=@T. Using this value ofScell,the enthalpy change is determined by
Hcell
Gcell T
Scell: 2:12
The resulting values for Hcell and Scell are given in table 3 for several stage
transitions that were studied in the graphiteK system.
Using an eusion method, Salzano and Aronson (1966a, b, c) measured the
equilibrium pressure versus temperature for a number of stage transitions in the
graphiteRb and graphiteCs systems and obtained the results shown in gure 18.
From the slopes of these curves, the enthalpy change was calculated and from the
1=T ! 0 intercept, the entropy change was determined. He rold (1979) has pointedout that the stoichiometry of the system changes during the stage transformation so
that improved values for H8 and S8 are obtained by integration over thelimits of the concentration range associated with the change in stage. He rold further
noted that these corrections are especially important for the low stage transitions and
may explain the anomalous H8 and S8 values in table 3 for the low stage
transitions. However, the thermodynamic results obtained by Salzano and Aronson
are believed to be qualitatively correct. The data in table 3 show S8 to beapproximately independent of stage and H8 to decrease in magnitude withdecreasing stage, at least for n 2. It should be noted that at high temperaturesSalzano and Aronson observed an additional rst stage phase C10M which is less
dense than C8M and is purple in colour. This observation illustrated that dierent in-
plane structures and in-plane intercalate densities are possible for a compound of a
given stage.
Intercalation compounds of graphite 29
Figure 18. Phase diagram by Aronson and Salzano (1966a) for several stage transformationsin the graphitecaesium system.
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Listed in table 4 are the overall enthalpies and entropies of formation for each ofthe compounds listed in table 3, based on the starting reactants pristine graphite and
the intercalant in the gas phase. These entropies and enthalpies of formation are
found by summing the corresponding entropies and enthalpies for each previous
stage transition, taking proper account of the number of carbon and metal atoms for
each transition (Aronson et al. 1968, He rold 1979). It is of interest to note that these
values for the enthalpies of formation are in good agreement with the corresponding
values calculated from (1) the measured heats of vaporization for the alkali metals,
and (2) the measured enthalpies of formation of C8K, C8Rb and C8Cs from the
liquid metal using calorimetric techniques (Saehr 1964).
Salzano and Aronson (1966a) also related these thermodynamic studies to a
determination of the energy required to separate two graphite layers to innityagainst the van der Waals interplanar binding forces which hold the graphite layers
together. This separation energy provides an estimate for the potential barrier that
must be overcome when an intercalate layer is introduced into the graphite host. The
value obtained by Salzano and Aronson (1966a) for this separation energy is
1.23 kcal/g mol carbon, which is in rough agreement with a number of other
estimates of this quantity obtained on the basis of totally dierent techniques.
By considering the stage dependence of the thermodynamic data in terms of an
electrostatic model for the binding of the intercalate metal layer to the adjacent
graphite layers, Salzano and Aronson (1966a) concluded that the amount of charge
transfer to the graphite layers is one electron per intercalant atom (f 1) for the K,Rb and Cs systems, at least for stage n 2, and close to f 1 for the rst stagecompounds. This conclusion is consistent with other experiments relevant to the
electronic structure (see section 4). On the other hand, analysis of the thermodyamic
data for the acceptor intercalant H2SO4 (Aronson et al. 1971) indicated that f 0:3,also in agreement with studies of the electronic properties of acceptors (Hennig 1959,
Ubbelohde and Lewis 1960).
Thermodynamic arguments have also been applied to other aspects of the
intercalation process. For example, Dzurus et al. (1960) used thermodynamic
arguments in an attempt to explain why Na does not intercalate into graphite to
form low stage compounds by plotting the free energy of formation for the K, Rb
M. S. Dresselhaus and G. Dresselhaus30
Table 4. Heats and entropies of f ormation of graphitepotassium, graphiterubidium andgraphitecaesium compounds.a
Graphite Graphite Graphitepotassium rubidium caesium
Hf8 Sf8 Hf8 Sf8 Hf8 S8(cal/mol (cal/mol (cal/mol (cal/mol (cal/mol (cal/mol
Reaction K) K 8K) Rb) Rb 8K) Cs) Cs 8K)
8Cs Mg ! C8 Ms 28 500 24.0 28800 23.3 34200 24.010Cs Mg ! C10Ms 27 000 22.5 27 500 18.5 32 200 19.224Cs Mg ! C24Ms 30 400 20.7 30 600 17.6 34 800 18.736Cs Mg ! C36Ms 31 700 20.7 32 200 18.0 35 800 18.748Cs Mg ! C48Ms 32 300 20.7 33 100 18.1 36 400 18.860Cs Mg ! C60Ms 32 800 20.8 33 600 18.1 36 700 18.8
a FromAronson et al. (1968).
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and Cs intercalation compounds against the ionization potential of these metals and
extrapolating to a positive free energy of formation for a rst stage Na compound. It
would be of interest to explore whether there are thermodynamic constraints which
inhibit the formation of certain low stage compounds, as, for example, rst stage
graphiteBr2. For the graphiteBr2 system, the lowest stage compound that has been
prepared is stage 2.
2.4. The staging phenomenon
A remarkable feature of graphite intercalation compounds is the occurrence of
staging, in contrast to other intercalation compounds such as the transition metal
dichalcogenides, which generally do not exhibit accurate staging. The existence of
staging has been well documented by X-ray (00l) diractograms as discussed in
section 2.2, showing that the resulting superlattice unit cell extends over many
atomic layers; for example, in stage 8 alkali metal compounds, Ic 30 A . Further-
more, the measured linewidth of the diractograms indicates that a single stage
sample can be established over macroscopic dimensions (1000 A ). It should further
be noted that although stacking faults in intercalated graphite are separated by as
little as 30 A (see section 3.4), the staging phenomenon is long range.Staging is very general, occurring in both donor and acceptor compounds,
regardless of whether the intercalate in-plane structure is commensurate or
incommensurate with the graphite host lattice. The existence and extent of staging
does not seem sensitive to the amount of charge transfer between the graphite and
intercalate layers, which is large for the alkali metal donor compounds and
considerably smaller for the molecular acceptor compounds (Ubbelohde 1976).
Furthermore, models for the c-axis charge distribution (Pietronero et al. 1978), the
electrical conductivity (Bok 1978), and the interpretation of Raman and infrared
spectra (Nemanich et al. 1977c, Dresselhaus et al. 1977b, Underhill et al. 1979),
provide strong evidence that a single graphite bounding layer eectively screens the
intercalate from the graphite interior layers, thereby inhibiting long-range electro-static interactions in the intercalation compounds. These observations on the
eective screening of the intercalate layer by the adjacent graphite bounding layers
suggest that the staging phenomenon is related to a long-range lattice strain
interaction rather than an electrostatic eect.
The expansion of the lattice constant in graphiteK compounds as a function of
reciprocal stage (1=n) (Nixon and Parry 1969) and the (1=n) dependence of the latticemode upshift in acceptors and downshifts in donors also strongly suggest a strain
mechanism for staging (Underhill et al. 1979). Raman experiments, indicate that
donor intercalants cause the in-plane graphite layers to expand as electrons are
added, and acceptors to contract as electrons are removed. The Raman experiments
also imply that the electron transfer causes a stage-dependent stiening (acceptors)
or softening (donors) of the carboncarbon bonds in the bounding layer plane.
Whether the intercalate bonding is ionic, covalent or metallic, there is strong
evidence that the long-range forces associated with staging are directly related to
the strain energy of the solids.
The kinetic studies of the intercalation process described in section 2.3 are also
consistent with a strain model for intercalation. These kinetic studies show that once
the exposed surface graphite planes have interacted with the intercalate species, the
intercalant is introduced into the bulk host material as layers close to the exposed
end surfaces, and sequentially into layers increasingly distant from the exposed end
Intercalation compounds of graphite 31
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surface. Initiation of intercalation at these extremal layers minimizes the elastic
energy that must be supplied to produce the dilatation of the graphite host
accompanying the intercalation process. The strain model can also account for the
observation that thin samples intercalate more readily than thick samples because of
the lower elastic impedance of thin samples.
Molecular alignment studies in liquid crystals by de Gennes (1974) show that the
elastic force eld yields a decrease in strain energy by clustering and aligningneighbouring rod-like molecules. Application of this concept to graphite intercala-
tion compounds implies that the strain energy is minimized by the clustering of
intercalate atoms to form platelets between a single set of graphite planes, in which
intraplanar forces are extremely strong. Thus if a random distribution of intercalate
atoms were to be introduced into graphite initially, the strain energy of the system
would be lowered by the clustering of all intercalate atoms between a single set of
graphite planes. However, each intercalate l
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