Grain boundary segregation phenomena observed by gold decoration of NaCl bicrystals

Surface Science 65 (1977) 1-12

0 North-Holland Publishing Company

GRAIN BOUNDARY SEGREGATION PHENOMENA OBSERVED BY GOLD

DECORATION OF NaCl BICRYSTALS

L.B. HARRIS * and N.G. CUTMORE School of Physics, University of New South Wales, P.0. Box I, Kensington, N.S. W. 2033, Australia

Received 24 November 1976; manuscript received in final form 23 February 1977

Examination by electron microscope of gold decorated surfaces near grain boundaries in

nominally pure NaCl bicrystals reveals features that derive directly from the presence of the

boundary: a central strip along the boundary, and isolated structures scattered on either side,

within which nuclei density is larger than normal. Two distinct types of structure are identified

as second-phase precipitates and pre-precipitation segregates of the same residual divalent im-

purity. The segregates can account for previously observed enhanced grain boundary diffusion.

1. Introduction

Although it is common practice to interpret grain boundary phenomena in ionic solids in terms of concepts that are successful with metals, such argument by analogy must be used with caution, particularly since solute segregation often has a far more overriding effect on boundary behaviour in ionic solids than in metals [I]. It is usual, for example, to explain enhanced grain boundary diffusion in the alkali halides [2] in terms of dislocation models developed for metals [3] whereas recent observation [4] suggests that grain boundary transport in the alkali halides is influ- enced less by dislocations than by precipitates of residual impurity. Since certain discrepancies in the literature concerning enhanced boundary diffusion of alkali metal cations can be removed if such diffusion is seen to be a consequence of segregated impurity [5], it is important to attempt direct observation of segregation and precipitation in these materials.

Precipitate particles have been observed in grain boundaries in nominally pure NaCl by scanning electron microscope [6] at a magnification limited to about X5000. Much higher magnification is possible by transmission electron microscopy of replicas of gold-decorated surfaces [7], a technique which has been found

capable of revealing the distribution of impurity in doped specimens [a]. The present investigation had two aims: (1) determination of the suitability of the gold

* Present address: Department of Metallurgy and Materials Science, Imperial College of Science and Technology, Prince Consort Road, London, SW7 ZBP, England.

decoration technique for studying segregation at grain boundaries in alkali halides. (2) investigation of the existence (if any) and form of precipitates of residual impurity near grain boundaries in nominally pure NaCl in order to seek confirmation of the proposal that such precipitates are responsible for the observed enhanced

boundary diffusion of sodium [!I].

2. Experimental

Gold decoration was studied on NaCl plate specimens, 2 3 men thick, cleaved

from as-grown single crystal ingots and bicrystal ingots containing 20” tilt bound-

aries that were pulled from the melt under a dry nitrogen atmosphere. The bicrystal specimens contained a single plane grain boundary ~er~el~di~ular to the two large faces, the decorated surface being a (100) plane normal to the (I 00) tilt axis of the boundary. The predominant divalent cation impurity, as determined by atomic ab- sorption spectroscopy, was calcium, at a mean concentration of 5 ppm, with a similar concentration of potassium and lesser amounts of copper. ntagnesium and

~langancsc. Impurity c(~~~~e~~tratio~~s in the grain boundary would have been higher

[lOi. Because the operating vacuum of low4 Torr in the evaporation unit was insuffi-

cient to produce gold nucleation on air-cleaved surfaces at room temperature, prc- sumably on account of the inhibiting effect of adsorbed impurities [ II], it was necessary to use substrate temperatures >lOO”C. A heater was constructed of ni- chrome wire between mica sheets secured to a stainless steel plate, the specimen being held in contact with the mica by a stainless steel clip, so that temperatures up to 400°C could be attained in 2-~3 min. The specimen was kept at temperature ful 15 min prior to decoration in order to ensure thermal equilibrium. Substrate temperature was found to have two effects on nucleation. Firstly, the ratio of the number of nuclei on cleavage steps to the number that formed at random on the ato~ii~ally smooth regions between cieavage steps increased with increase of temperature, resulting in better definition of the steps. However, the overall number- of nuclei tended to decrease as temperature increased, leading to a loss in outline detail on surface features A decoration temperature of 250°C was found to give op- tinum balance between enhancement of surface contrast and retention of surface detail.

Gold equivalent to a mean film thickness of 1 mn, evaporated from a molyb- denum boat at a contact angle of 90*, produced nuclei of a size suited for easy examination. Best results were obtained using an evaporation rate of 1 nm/min and a process of multiple decoration extending over a period of S---IO min, use being made of a shutter between source and specimen that exposed the specimen for ap- propriate intervals of time. A carbon film of thickness IO- IS nm was deposited FIJI-

mediately after the gold ~ondensatior~ without breaking the vacuum. Sections of the carbon film carrying the decorating gold nuclei were placed on electron microscope grids for examination on a JEM-6C transmission electron microscope.

LB. Harris, N.G. Cutmore / Grain boundary segregation phenomena

3. Observations

Gold decoration features on single crystals were of the conventional type reported earlier [ 121, but decoration structures of a new and significantly different kind were observed on bicrystals. The first was a strip of decoration along the grain bounda~, approximately 120-180 pm wide, within which the nuclei density was a factor of IO greater than normal density observed on single crystal surfaces. The transition from high to normal density on either side of this strip took place across a band some 60-80 pm wide. Gold nucleus size in the decorated strip was appreci-

able smaller than that in normal regions, while particles in the transition bands were ~termediate in size. Gold nuclei in the decorated strip, as shown in fig. I, may be compared with those in the normal region N of fig. 2. A notable feature, apparent

in fig. 1, was the almost complete absence of cleavage steps within the decorated strip.

A second feature unique to bicrystals was the existence adjacent to the boundary of a number of structures within which local nuclei density was much higher than normal. There were two distinct types. The first were regions of high nuclei density, typically circular (fig. 3), occasionally elliptical (fig. 2), approximately 50 nm in diameter, while the second were regions of intermediate nuclei density

Fig. 1. Gold nuclei within the central strip along the grain boundary in a NaCl bicrystal decorated at 25O’C.

Fig. 2. Region of normal nuclei density N on bicrystal surface containing a high-density H struc-

ture , roughly elliptical in shape, surrounded by a region of intermediate nuclei density I; deco-

ratic ,n temperature 250°C.

Fig. 3. A circular H structure (arrowed) on a NaCl bicrystal substrate; decoration temperature

250 “C. Note the variation in nuclei density and size within this structure.

Fig. 4. An I structure within which there is an incipient H structure (arrowed); decoration tem-

perature 250°C. N is background surface decoration.

Fig. 5. NaCl bicrystal decorated at 250°C showing H and I structures.

LB. Harris, N.G. Cutmore / Grain boundary segregation phenomena

I’ig. 6. Surface step S diverted round periphery of I structures.

that varied greatly in shape and siLe, but which were often approximately circular- and O.lLO.3 pm in diameter (figs. 4 and 5). These high and intermediate nuclei density regions will be called H and I structures respectively. As seen in figs. 7, 3

and 5, the H structures are bordered by nuclei that are larger than those inside, whereas the edges of 1 structures are undelineated except where these happen to coincide with cleavage steps. Figs. 6 and 7 show that both H and I structures had a direct influence on the step formation produced on cleavage surfaces.

As shown in fig. 8, H and 1 structures were found only in the region extending some 250 to 350 pm from the central decorated strip. The number of structures increased with increasing proximity to the decorated strip, though the proportion of H structures remained approximately constant at about 10% of the total number over all parts of this region. This trend is illustrated by fig. 9, obtained from a sys- tematic count of structures across one particular section of the surface of a bicrystal specimen. There was also an increase in the size of 1 structures as the decorated strip was approached, the I structure in fig. 4 being one of the larger ones lying within the transition band (fig. S), a fact that is apparent from the relatively high nuclei density N in the background.

Normal surface density of gold nuclei was about 1.5 X 10’ ’ n- a, whereas mean nuclei density in the 1 and H structures was of the order of 1016 no* and 1.1 X 1o17 ,x1-* respectively, the latter being much higher than usually reported values.

LB. Harris, N.G. Cutmore / Grain boundary segregation phenomena I

b

Fig. 7. ZX structures acting as i~~~mogeneii~e~ ta produce V-steps on cleavage surface.

.T+ Grain * T *

Boundary

+- 250 - 350 ~ - Plane

T = Tronsltlon Band

Fig. 8. Surface of bicrystal specimen adjacent to grain boundary, as depicted by gold decora-

tion. Numbers are distances in urn.

Nuclei density in the central decorated strip of lOI m-* should, in principle, have permitted the detection of H structures in this region, but, in practice, discrimina- tion proved to be an exacting and uncertain process. Therefore, the fact that such structures were not detected is not unequivocal proof that they were not there. The mean diameters of the gold particles in the various regions of fig. 2 were 6.6 nm for normal surface decoration, 3.9 nm in 1 structures and 2.7 nm within H structures.

4. Discussion

Gold nucleation occurs preferentially at substrate impurity sites [ 131, resulting in an increased nuclei density on doped NaCl [8], so it is possible for the pattern seen in fig. 8 to be attributed to a distribution of impurity segregated to the boundary. This interpretation is supported by related observations on ultramicroscopic speck density and on light scattering intensity, both of which are quantities known to be caused by discrete particles of second-phase precipitate [14]. Experimental traverses of both these quantities taken across grain boundaries in similar NaCl bicrystals produced profiles [S], with a total width of 500 pm and high intensity within the central 100 pm, exactly of the type that would be obtained if fig. 8 represented segregation of precipitates. Analysis of light-scattering intensity from NaCl crystals containing 10 ppm of Ca*+ revealed the presence of long thin cylindrical precipitates each about 70 nm diameter, with the cylindrical axis aligned along ( 100) or (110) directions [ 141. A (100) cleavage through such cylinders, ap- propriately aligned, would give circular or elliptical features of the same size as the H structures of the present work, which gives a geometrical reason for identifying the H structure as precipitate. Since there was no graded transition of nuclei den- sities between H and I structures, only a clearcut discontinuity, the abruptness of

LB. Harris, N.G. Cutmore / Grain boundary segregation phenomena 9

this transition may be taken to represent the phase change. In this case, the I

structure must be regarded as a preprecipitation segregate. The shape of both H and I structures precludes the possibility that either represents the Suzuki phase [ 151, which has cubic morphology. Segregation of calcium could produce impurity levels in the present bicrystals greater than that which produced the precipitate observed by light scattering [14], and the Suzuki phase does not occur in the NaCl/CaClz system [ 161.

Most H structures had a circular cross section, indicating alignment of cylinders along the [loo] tilt axis, which is a precipitation texture common in KBr tilt

grain boundaries [lo]. The cylindrical shape presumably builds up by diffusion to- wards a slip dislocation, lying along the [ 1001 direction, that acts as the nucleation site for precipitation. However, the rate-limiting mechanism at cylindrical interfaces on rod-shaped precipitates is the slow ledge mechanism of growth [ 171, not the rate of arrival of impurity, so a certain interface diffuseness associated with surrounding

clouds of vacancies and segregated impurity atoms is to be expected. H structures are often found to be partly enveloped by I structures (figs. 2, 4 and 7a), which leads logically to the interpretation of the latter as segregates of the same residual impurity that produces the H structures. A natural progression is therefore the formation of an H structure nucleus inside an I structure, which appears to be what is happening in fig. 4. Since the misfit on cylindrical interfaces on rod-shaped precipitates is accommodated by dislocations [ 171, the outlines of H structures on a cleavage surface are expected to be delineated by surface steps or by dislocation core sites that are preferential sites for gold nucleation. This accounts for the observation that H structures are rimmed by gold particles (figs. 2, 3, 5 and 7). The fact that I structures do not have any clearly defined boundary associated with dislocations is compatible with their status as pre-precipitation segregates.

Both H and I structures were present in the specimen at the time of cleavage, as demonstrated by evidence of obstruction to the fracture process. In the case of H structures, the obstruction resulted in the formation of V-steps (fig. 7) which are characteristic surface features produced when a crack is impeded by inhomogene- ities [ 181. The existence of V-steps is confirmation that H structures are second- phase particles. In the case of 1 structures, which may be regarded as somewhat formless aggregates of impurity-vacancy dipoles, a different behaviour is expected. It was found that obstruction usually resulted in I structures being partly embedded in cleavage steps (figs. 4, 5 and 6).

The nuclei observed at the rim of H structures are formed primarily by gold atoms that have moved over the smooth surface of the NaCl matrix, the higher density and smaller particle size within the H structure indicating that gold atoms are far less mobile on the precipitate surface. A microscopically rough non-cleavage fracture of a precipitate of different structure could provide a surface with a far higher density of tight-binding adsorption sites for the gold atoms, leading to the observed very high particle density (fig. 3).

Gold decoration in the central strip (fig. 8) is anomalous. Nuclei density in this

region is equal to that in 1 structures, which suggests that the decorated strip corres- ponds to the merging of segregates into an impurity layer. However. through the number of I structures increased as the central strip was approached. the gold decoration on the I structures did not merge naturally into the decoration on the central strip; the latter appeared to be superimposed on the former. Further there was an almost complete absence of decorated surface features in this central strip: no cleavage steps, no precipitates (i.e. no H structures), no dislocation boundaries round precipitates, and no identifiable feature that could serve to locate the grain boundary plane. Since gold decoration efficiency can be markedly affected by ex- ternal influences, an electric field at the surface of a NaCl crystal. for example, completely eliminating cleavage steps [ 191, the absent features are not necessarily non-existent. It is possible, for example, for decoration characteristics to be changed

by localised surface charge due to the presence of an adventitions impurity separate from that which forms the H and I structures. Support is available for an expiana- tion based on two different impurities. Most observed grain boundaries were sym- metrical with respect to {OOl} planes. Occasionally, however, the grain boundary plane in a bicrystal ingot during growth would deviate by approximately 45” and grow out through the side of the ingot. as has been discussed [lo]. Such deviated boundaries seemed, by optical microscopy, to be relatively free of precipitate. Ob- servations of a limited number of such boundaries by electron microscope could not detect H or I structures, though the central decorated strip was of the same form and size as that in normal (001) boundaries. Such an observation that separates the two decoration modes shows they have different causes.

The nature of the central decorated strip is important for understanding the effect of segregation on enhanced grain boundary transport [ 11. For the model of a grain boundary as a homogeneous high-diffusivity slab, the width. 6, of the slab in NaCl has been estimated to be of the order of 1 ,um [9,20]. From measurements of‘ sodium tracer diffusion in NaCl, it has been shown [5] that 6 cannot be greater than 10 pm. These values are so very much greater than those expected for dislocation pipe diffusion that they are usually attributed to segregation [ 1,201. However, they are so very much less than the width (-150 pm) of the central decorated strip that the segregated impurity represented by this strip cannot make any sizeable contribution to boundary transport. If it is assumed that the H and I structures are due to an impurity, such as calcium, that can contribute significantly to enhanced

transport, then it is possible to associate an equivalent width 6 with these structures. An estimate of this width can be made on the basis of two assumptions. Firstly. enhanced transport in any one plane normal to transport is assumed to take place over an area equal to the total area of structures in that plane. Secondly, the num-

ber of structures contributing to transport will be those given by fig. 9, assuming that the structures within the central strip can be obtained by extrapolating (dashed lines) to the grain boundary plane. In other words, tranport takes place within a number of cylinders equal to the number of structures, and the volume of a slab equivalent to the volume of the cylinders is calculated. The representation of

L.B. Harris, N.G. Cutmore / Grain boundarv segregation phenomena 11

I 0 I structures I’

I/ x H Structures /I

5 /I /

I’

Distance ( j.dm )

Fig. 9. Surface density of structures as a function of distance normal to grain boundary plane

for one particular region of bicrystal surface.

structures as continuous cylinders is not strictly correct, but it accords with the long macroscopic grain boundary channels observed in electrodecoration experiments [lo].

The width equivalent to H structures is found to be 3 nm, while that for I structures is 0.4 pm, which means that a boundary width which agrees with values obtained from grain boundary diffusion experiments is obtained only if the enhanced transport is assumed to be due to pre-precipitation segregates. Such a result is physically reasonable. The small second-phase H structures do not, in fact, provide any extra transport except over their cylindrical interfaces, whereas the I structure segregates, assuming that they are aggregates of divalent impurities and charge- compensating vacancies, contain the extra vacancies needed for transport through- out their entire volume. Further, the puzzle as to how large colloidal silver blobs (several pm in diameter) could rapidly develop within the grain boundary region during electrodecoration experiments [IO] can now be accounted for by the existence of loosely structured segregates into which silver can be injected.

The usefulness of the gold decoration technique in examining segregation phenomena at grain boundaries is now being further explored by an investigation of how

12 L.B. Harris, .V.G. Cutmore / Grain houmiar_y segregation phenorner~

these phenomena vary with doping and annealing of specimens and with grain boundary parameters. In this connection it will be important to obtain quantitative data on how segregation of particular impurities varies as a function of distance

from the interface. Data of this kind, using modern surface analysis techniques, has already been obtained for oxides [21,22].

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Grain boundary segregation phenomena observed by gold decoration of NaCl bicrystals