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The Growth Kinetics
3.1 Introduction
The trivalent rare earth ions easily combine with oxalate ligand to form the
metdic complexes having the general formula R2(C204)3.fi0 (R = Ce, La, Nd, . .) [I ] . Halide or nitrate salts of rare earths when admixtured with oxalic acid react to
form the corresponding oxalates. Mixed double rare earth oxalates can be
prepared using mixed reagents having the rare earth ions in the required ratio. The
proposed reaction is
A and B stands for the rare earths to be incorporated in the crystal.
If aqueous solutions of reactants are directly mixed together, there will be
precipitation or microcrystallisation due to the fast chemical reaction. But when
one employs gel technique, the three dimensional network o f gel medium
provides a controlled diffusion environment to the ions which is necessary for the
crystallisation 123.
This chapter reports the growth of single crystals of cerium oxalate
(CeOx), cerium lanthanum oxalate (CeLaOx) and cerium neodymium oxalate
(CeNdOx) in hydro silica gel by single diffusion method employing chemical
reaction. Detailed investigation has been made on the effects of various
parameters that governs the size and perfection of the crystal.
3,2 Preparation of Gel
Finely powdered sodium silicate (Loba chemicals) was dissolved in
double distilled water and kept undisturbed for two days for sedimentation of the
insoluble particles. Decanted clear solution was filtered and stored as stock
solution. The density d, of the stock solution was determined very accurately
using a density bottle as it is highly critical in determining nucleation density,
size, transparency, etc. of the crystals to be grown [3]. For any volume V, of the
sodium meta silicate (SMS) solution of density d,, volume of the stock solution V,
whose density is d, was calculated using the formula 141
V, = V, (d, l )/(d,-1 ); d, > d,
V, was made up to V, by adding distilled water. The pH of SMS solution must be
lowered for enabling gelation. Adding a proper acid to this solution can do this. In
the present single diffusion method employing chemical reaction, one of the
reactant is oxalic acid. It was incorporated into the gel before setting. The strength
of analar grade oxalic acid is decided on considering the density of SMS solution.
For higher densities, acid strength should be reduced for proper gelation.
Here SMS solution was added drop wise to acid solution with rigorous
stirring and not the other way. If acid is added drop by drop to SMS solution,
colloidal solution is formed. pH of the resulting solution was measured using a
digital pH meter immediately after the addition of the SMS solution. Gelation period
varied fiom a few minutes to several days depending on pH, density of SMS solution,
room temperature etc. When the gelling time is very short, solution must be poured
into the test tubes very quickly. The crystallisation vessels were corning glass tubes of
length 15 cm and diameter 1.5 cm. Mouth of the tubes were kept closed so that no
atmospheric impurity let in. The test tubes were kept undisturbed till gel set .It is seen
that gel is set earlier on warm days than on rainy days.
The Growth Kinetics 37
SMS solution can also be neutralised by acids like acetic acid which do
not take part in chemical reaction along with the rare earth salt and then adding
oxalic acid as the supernatant solution. But the addition of rare earth salt directly
to SMS solution resulted in flocculent gel [5] .
3.3 Supernatant Solution
Cerium nitrate, lanthanum nitrate and neodymium nitrate (99.99% pure)
supplied by the Indian Rare Earth Co. were used as the source of rare earth ions.
The concentration of the rare earth ions can be varied. If rare earth salt alone was
taken as the outer reactant, inspite of controlled diffusion by gel, immediate
reaction with oxalate ion was observed as a white thin film of precipitate at the gel
interface. But when the supernatant solution was acidified with FINO3, precipitate
formation was avoided. Formation of the first visible nuclei was only after a few
days depending on the strength and quantity of the acid used. Detailed studies are
in section 3.4.3.
3.4 Growth of Cerium Oxalate Crystals
The growth of cerium oxalate crystals were accomplished by the controlled
diffusion of cerium ions through silica gel impregnated with oxalic acid. The hydro
silica gel was prepared from SMS solution as described earlier. Cerium nitrate of
required strength was poured gently over the set gel as the supernatant solution.
Care was taken for the non incorporation of any foreign nuclei. The slowly
diffusing rare earth ions combined with oxalate ions to form crystals of cerium
oxalate. In the absence of any seed crystals and foreign nuclei, homogcneous
nucleation is expected and it can be seen that homogene~us nucleation
predominates in gel method [3,6]. The possible chemical reaction is
But even after a few days, only a precipitate column extending to the bottom of
the gel could be seen in the growth system (Fig. 3.la). This difticulty was
overcome by acidifying the supernatant solution with concentrated HNO.3. Feed
solution and nitric acid were taken in the ratio 3:2. No precipitate occurred at the
3 8 Chanter 3
gel interface and tiny crystals of cerium oxalate were observed after two days.
Here nitric acid helps to dissolve zhe micro crystals hence the formation of bigger
crystals. Thus the number of nuclei is reduced. Only those nuclei with sufficient
size grew ro become big crystals (Fig, 3.1b). T I is found that the quantity and
strength of ni.tric acid in the supernatant solution highly influenced the
rnorpho1,ogy of the grown crystals [7] .
Number of nuclei, size and quality of the crystals depend on several factors
such as SMS density, strength of oxalic acid, PI-I of gel, concer~tration of rare
earth salt solution and acidity of feed solution. 'These are discussed in detail in the
following sections.
Fig 3.la Precipitate column without acidifying the feed
solution
Fig 3,lb Crystallisation when feed solution i s
acidified
3.4.1 Effect of SMS density, strength of acid, pH valcie arzd time of gelntiun
SMS solution of densities ranging fiom 1.02 gmtcc to 1.06 g d c c can be used
to make hydro silica get [8]. Here SMS of relative densities 1.03, 1.04 and 1.05 were
prepared. Each was mixed with 1 N oxalic acid so as to give gels of dif'ferent pH values
The Grow~h K~nei ics 39 - -. ..
ranging from 4 to 7. Time for gel to set is much greater when lower pH is selected for
lower densities. Hmcc lower pH values are selected only for higher densities. Time of
gelation, transparency and stability of gel in each case are studied. Variation of gelling
time with relative density and pH are given in figures 3.2 and 3.3 respectively. lt can
seen that at pH 6 the gelling time is constant irrespective of the density of the gel. Gel -- .
of relative densities 1.03 and 1.04 are transparent.
o ! 1 1 I I I
1.030 t .W5 1 .OU1 1 ,a45 1.060
Relative density
Fig.3.2 Variation of gelling time with relative density
PH
Fig. 3.3 Variation of gelling time with pH
ARer keeping the set gel for one more day in each case, 0.5 M cerium
nitrate acidified with concentrated HN03 was poured gently down the sides of the
tubes. With in two days well faceted, shining crystals appeared in the gel medium.
The growing crystals were regularly observed for 3 weeks. After that the fully
grown crystals were harvested and examined by the microscope. Results are
tabulated in table 3.1. In most cases growth region extended upto 3 crn in the gel
column. In the top half of the growth region, nucleation density and size are much
greater than in the bottom half. Good quality crystals are obtained in both regions.
Table 3.1 Effect of SMS density, strength of acid, pH value and time o f gelation on the growth of CeOx crystal
A pprox . number
100
90
1 10
80
120
140
large
large
large
Relative density
1.03
1.04
I 1.05 !
I
Av, size (mm3)
1 ~ 2 . 5 ~ I
1 . 5 ~ 3x 2
1 .5x2x 1
1 x2x 1
1 x 13x1
1x2x0+5
1 x l x 1
1 ~ 1 ~ 0 . 5
1 ~ 1 ~ 0 . 5
Time of gelation
(hr) 52
24
instant
168
36
24
144
32
24
pH
5
6
7
4
5
6
4
5
6
Nature of crystallisation
Crystallisation region is 1.5 cm M o w gel solution interface. Larger crystals on top layer and small, transparent, shining crystals are towards bottom. Mostly rhombic prism. A few twinned as well as single crystals appear on the gel interface. Same as above. More transparent crystals. Rhombic, thick prism crystals in the upper layer, thin hexagonal pnsm like crystals in the lower layer. A single large interfacial crystal is observed on the gel interface. Thick growth in the upper layer. Thin hexagonal crystals in the bottom region. Mostly twinned and layer shctured crystals with rough s k s . Crystallisation region is 0.5 cm below gel solution interface. Smaller crystals towards bottom are smooth and transparent with less imperfections A few crystals in between the gei interface and the main growth region. trans par en^, small needle shaped crystals are towards bottom. Crystals in between the gel interface and the main growth region are most transparent. Truncated, rhombic prism crystals are on the top layer and hexagonal towards bottom.
Crystallisation region is just below the gel interface. Very large number of nnall crystals Thick top layer and less dense bottom region. Very dense on top and very small crystals on bottom layer Hexagonal. Cubic and hexagonal
The GI-OIVZ~ Kinetics 4 1
From the above table, it can be seen that the change in the relative density
beyond 1.04 give only very small crystals. Further experiments has been done by
varying the strength of the oxalic acid to 0.5 N in the case of gel having densities
of 1.03 and 1.04. Good quality crystals cannot be formed beyond 1 N and below
0.5 N. From these experiments it is observed that optimum sized crystals are
grown when 1 N oxalic acid has been used as the inner reactant.
3.4.2 Effect of concentrntion of feed solittion
By using I N oxalic acid, concentration of feed solution was varied by
keeping gel density 1.03 in one case (Fig. 3.4) and 1.04 in another case. III both
experiments pH was kept at 6. The growth period was 28 days. Results are tabulated
in table 3.2.
Table 3.2 Variation i m the growth of CeOx crystals with eotlcentratiorr of f e d solution
Density (gm/cc)
3.4.3 Variation qm wct acidity of feed solution
Acidity of k d solution is found to influence the nucleation. growth
and morphology of the crystals [6]. In one case, strength of nitric acid (Fig 3.5)
Strength of feed solution (M)
2
Fig 3.5 G r o w t h of CeOx crystals with variation in .strength of HN03 (30%, 50% and 70%)
No. of crystals Large
1.03'
1.04
'0.25 40 Upper layer of -1y'tm crystals. . . Larger towards lows layers
120
90
20
200
140
1
. 0.5
0.25
2
1
0.5
Max. length of crystals (mm)
1.5
~ e n e r a l characteristics
Uniform distribution of smalt crystallites
2
,‘
1.5
1
Crystats from top to bottom are all of the same size. Thicker, multifaceted cubodial crystals One interfacial crystal of length 5mm. Larger crystals are in the middle layer. More tansparent, rhombic and hexagonal in morphology. A cluster of ten crystals above gel solution interface Zig Zag pattern of growth, nu~nbcr very large, size very small All are most of the same size fiom top to bottom A cluster of 3 interfacial crystals, 3mm each, middle layer of large
, mber9. low layer of smaller ones
. . The Growth Kinetics 43 -
and in another case volume of 100% nitric acid in the feed solution were varied. 'l'his is
tbr a gcl of density 1 -03 @cc, strength of oxalic acid IN, pH 6, concentration of feed
solution 0.5M and growth period 28 days. Results are tabulated in tables 3.3 and 3.4.
YO
30
40
50
I Ratio of feed solution to HNOl 1 Nature o f growth I
Max. length OF crystals
(mm)
60
70
100
6:4 Normal size
A pprox. number
Thin. needle like, clusters, branched, twinned
Mostly hexagonal, clear, transparent
Hexagonal, cubic and rod like. An opaque impurity which is well inside the crystal seen in many
2 [ Very large
Table 3.3 Variation in the growth of CeOx crystals with strength of HNO, in the feed solution
2.5
3 4
8:2
5:s I Formation of crystals very slow
Nature of crystallisation
2.5
4
Number of crystals very large, size small
Very large
200
1 50
1 10
90
It is seen that concentrated-HN03 40% by volume of feed solution give
the best result.
Thicker, short, cubic and hexagonal
Thicker, small and big
Bigger, clear, well faceted
2:8
3.5 Growth of Cerium Lanthanum Oxalate Crystals
No formation - .-
The growth of cerium l a n w u m oxalate crystals were accornplishcd by
the controlled diffusion of cerium and lanthanum ions through silica gel
impregnated with oxalic acid. The hydro silica gel was prepared from sodium
meta silicate solution as described in the previous section. A mixture of 0.5 M solution
of cerium nitrate and lathanwn nitrate in 1 : 1 ratio, acidified with concentrated HN03,
was poured gently over the set gel. The slowly difiqing cerium and lanthanum ions
react with oxalate ions to form crystals of cerium lanthanum oxalate. 'The possible
chemical reaction is
Table 3.4 Variation in the growth of CeOx crystals with ~ & m e of H N 0 3
Due to the close similarity in crystal structure, ionic size, ionicity and
electronic conf guration of cerium and lanthanum cations, they can be supposed
to be substitutionally incorporated in the oxalate lattice 191. Ratio of incorporation
of the two rare earths in the growing crystal is according to their proportion in the
feed solution as will be shown later in the EDXRF studies. Experiments have been
done changing density, pH, concentration, etc. as in the case of CeOx crystals. No
appreciable change has been observed in this case as compared to CeOx. However
concentration of feed solution makes some subtle changes it1 the growth of these
crystals. The optimised growth parameters for cerium oxalate crystals are
applicable here and general growth characteristics are identical as expected. But
inspite of similar physical and chemical properties, each rare etrth element has its own
identity as can be seen fiom the growth kinetics. General characteristics of growth of
cerium lanthanum oxalate crystals with variation of concentration of feed solution is
given in table 3.5 and in figure 3.6. Keeping gel density 1.03 gm/cc, gel pH 6 and
using 1N oxalic acid, concentration of feed solution was varied.
Fig 3.6 Growth system of CeLaOr crystals under different concentrations of 2M, IM and 0.5M feed solution
Table 3.5 Variation in growth of CeLaOx crystals with concentration of feed solution
CeLaOx crystals are found to originate at a region below the gel interface.
This region is at a distance greater than that of the region where CeOx crystals are
originated. Moreover, in this case lesser number of crystals are observed than in
the case of CeOx crystals.
---
Gwml characteristics
Smaller crystals on top layer and larger towards bottom.
Twinned, centre lined and scratched crystals. Larger crystals on top layer and smaller towards bottom.
Three interfacial crystals of size 3 to 4rnm.Smaller towards bottom layers
Very small crystals above gel interface
3.6 Growth of Cerium Neodymium Oxalate Crystals
Max. length d crystals (mm>
2.5
3
5
2
---
Strength solution (M)
2
1
0.5
0.25
The growth of ceriurn neodymium oxalate crystals were accomplished by the
con~olled diffusion of cerium and neodymium ions through silica gel impregnated
with oxalic acid. The hydro silica gel was prepared from sodium meta silicate
solution as described in the previous section. A ~tlixture of 0.5M soiution of cerium
nitrate and neodymium nitrate in the proper ratio, acidified with concentrated
HNO3, was poured gently over the set gel. The slowly diffusing cerium and
neodymium ions react with oxalate ions to form crystals of cerium neodymium
oxalate, The possible chemical reaction is
No' of crystals
120
70
45
I
The optirnised growth parameters f i x cerium oxalate and cerium lanthanum
oxalate are applicable here and general growth chwacteristics are identical. Variation
of growth characteristics of cerium neodymium oxalate crystals with concentration of
feed solution is given in table 3.6. Figure 3.7 shows crystals grown under
concentrations of 1M, 0.5M and 0.25M. Keeping gel density 1.03 gmlcc, pH 4 and
using 1 N nxalic acid, concentration of feed solution was varied.
Fig. 3.7 Growth system o f CeNdOx crystals under different concentrations of l M , 0.5M and 0.25M feed solution
Strength of No. of Led solution
(MI
Very large . Max. length of crystals
( 1 ~ )
General characteristics
Uniform distribution of small crystals. Multjfaceted, not so transparent.
Crystals with rough surface, irregular morphology.
Ten interfacial crystals of size 3 to 4mm. Smaller, well faceted towards botto~n layers.
In the cerium neodymium combination, crystallisation region is more near
0.25
in the gel interface. First nucleation can be observed with in 24 hours. Number of
crystals is very large and average size is small compared with the other two.
Table: 3.6 Variation in the growth of CeNdOx crystals with concentration of feed solution
A Comparative Study of the Growth Kinetics of CeOx, CeLaOx and CeNdOx Crystals
Sarne as above. 70
A careful, comparative observation of the growth of the rare earth
2
coinpounds CeOx, CeLaOx and CeNdOx crystals in gel medium is presented in
table 3. 7. This is for a gel of density 1.03 gidcc, strength of oxalic acid IN, pH 6 ,
The GI-owrh Kine~ics 47
concentration of feed solution 0.5M, concentrated HN03 40% by volume of feed
solution and growth period 28 days. Growth system is shbwn i.n figure 3.8.
Fig. 3.8 CeOx, CeLaOx and CeNdOx crystals grown under identical situations
Tahle 3. 7 A comparative study or growth kinetics of CeOx ,CcLaOx and CeNdOx crystals. 13istance of formation of crystals from gel interface 'a', Length of crystallisation region 'b',
approximate number of crystals ' c'
Time of growth
(days)
2
3
10
Formation of interfacial crystals began after 10 days. After the growth of 4
weeks, maximum size of the crystal was up to 4 mm in length for CeOx, 5mn for
CeLaOx and 3mm for CeNdOx crystals. CeOx and CeLaOx are colourless, while
CeNdOx are rose in colour. Morphology is almost same for al l the crystals.
CeOx
Number of crystals shows a wide variation in all the cases. General characteristics
a (cm)
1.25
1.25
1.5
for CeOx crystals are in between the other two.
CeLaOx CeNdOx
b (cm)
1.5
2
2.5
a
(cm)
0.5
0.4
0.25
c (spprox.)
10
15
4 5
c (approx.)
25
5 0
90
a (cm)
1.5
1.5
2
b (cm)
1
1.5
2
b (cm)
2
2.5
3
C C
(approx.)
120
150
>200
48 Chapter 3 . ., - .- - ...
Attainment of super saturation is rapid in the case of Ce-Nd combination.
Super saturation region is at a greater depth for Ce-I,a combination. This may be
because, as soon as the Ce-Nd combination enters the gel medium, they collect as
much as possible anions. But Ce-La combination has to travel more to find their
counterparts. This may be because of the change in rate of diffusion caused by the
following reasons.
1. Ionic size of lanthanum is slightly greater than that of neodymium.
Assuming the gel density to be the same, lanthanum ions take more
time for diffusion.
. . 11 . The chemical potential of the ions involved are different. This change
can arise fiorn the difference in electronegativity and electron af'f'ity of
these elements. This can also affect the diffusivity.
3.8 Conclusion
Morphology, size and number of crystals are mainly depending on (i)
density of the gel, (ii) strength of oxalic acid , (iii) pH, (iv) concentration and
acidity of feed solution.
Gels with higher densities set faster than low density gels. Very low
density gels have poor mechanical strength. Transparency of the gel medium
decreases and growth media become more hard with increase of gel density. The
advancement of the crystallisation zone is found to be retarded by the increase in
gel density and the size of the grown crystal is found to be very small. This may
be due to the reduction in the pore size at higher densities of the gel [81. Quality of
the grown crystal is affected as the growth media become harder. 'The hard gel
exerts more residual stress on the growing faces of the crystal and deformation
takes place.
In the present case, since oxalic acid is the acidifying agent lor gelling
process, low pH means higher concentration of oxdale ions. So nucleation dcrisity
is higher. As the pH of the gel medium increases, nucleation density decreases,
helping the formation of large crystals. At higher pH, gel become hard and again
The Growth Kinetics 49
nucleation density is higher. Thus a medium pH is suitable. At higher pH, the
three dimensional fibrous net work of the gel changes to a loosely bound plate like
structure which lacks cross linkage [lo].
Nucleation density increases with the concentration of the rare earth ions.
Increase in the availability of the rare earth ions above a certain limit affect the
quality of the crystal. Regular morphology is lost though the size improves.
Increase in supersaturation change the habit of the crystal [ll]. Decrease in the
concentration below the certain limit reduces the chance of nucleation and size of
the crystals. Increasing the acidity by the addition of HN03 help to dissolve the
micro crystals hence reducing the nucleation centres and increase the availability
of rare earth ions at the growth centres. This will result in bigger and good quality
crystals.
By changing the growth parameters like density of the gel, pH of the
medium, concentration and acidity of feed solution, etc., optimum growth conditions
for the better quality CeOx, CeLaOx and CeNdOx crystals has been established.
References
1. Moeller T., Chem. Rev., 65 (1965).
2. Arora S. K., in: Progress in Crystal Growth and Characterisation, ed,
Pamplin B. R., Pergamon Press, Oxford, vol. 4, ( 1 982).
3. Henisch H. K., Crystal Growth in Gels, Pennsylvania State University
Press, (1 970).
4. Sivanesan G., Growth of Ferro Electric Crystals in Silica Gel and Their
Characterisation, Ph.D Thesis, Department of Physics, R. S. G. College,
Thanjavur, India, (1 992).
6. Vasudevan S., Nagalingam S., Dhanashekaran R., and Kamasamy P.,
Cryst. Res. Technol., 16, (1981) 293.
7. Pate1 M. B. and Pandya J. R., Current Trends in Crystal Growth and
Characterisation, ed, Byrappa K., Media International, India, ( I 99 1 ) 227.
8. Henisch 11. K., Crystals in Gels and Liesegang Rings, Cambridge
University Press, (1 988).
9. Joseph C., and lttyachen M. A., Cryst. Res. Technol., 30 (1 995) 1 59.
10. Halberstadt E. S., Henisch H. K., Nick1 .I. and White E.W., J . Colloid and
Interface Sci., 24 (1 969) 461.
1 1. Desai C. C., Ramana M. S. V., J . Cryst. Growth, 102 (1 990) 191,
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