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,