chapter 4 gel growth and kinetics of cadmium...

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CHAPTER 4

GEL GROWTH AND KINETICS OF CADMIUM OXALATE

4.1 INTRODUCTION

Many workers have made contributions to the understanding of growth of crystals. First theoretical prediction of crystal growth came from Gibbs1) who gave an idea about growth of natural mineral crystals. His surface energy theory developed by Curie2), and Wulff3). Volmer4) suggested that growth of a crystal takes place by adsorption of atoms or molecules, layer-by-layer, on the crystal surface. Later, several notable modifications of this theory providing more detailed description regarding the growth of layers in crystal growth were proposed. A review of the historical development at different stages of crystal growth is given by Wells5) and Buckly6). We have gone through various growth techniques including the famous gel technique, in chapter 2 and we are committed to make use of this technique for the chosen material of cadmium oxalate. This is the simplest and least expensive method, involving no use of sophisticated high temperature equipments for growing crystals. Importantly, this is the only suitable and applicable technique for cadmium oxalate, because this material is sparingly soluble in water (0.005 gm/100 ml H2O at 18 oC), it melts highly incongruently, and it decomposes at higher temperature, before melting

Moreover, the different techniques for crystal growth from melt, vapor or solution that require critically controlled variation in temperature have the following inherent difficulties. 1. Crystalline imperfections are more apt to occur due to the lattice deformation by

pronounced thermal vibrations. 2. The chances of lattice contamination by impurities from the container and

environment are profusely increased due to latter’s higher solubility at higher temperature.

3. Point defects and lattice strains are frequently introduced into the growing matrix during the range of cooling cycle.

Crystal growth from gels has attracted much attention because of its unique

characteristic of suppression of nucleation centers. The method offers the following attractive advantages. 1. A gel acts as a three dimensional porous bulk which supports crystal growth. 2. The crystals can be observed practically during all stages of their growth. 3. The gel medium considerably prevents convection currents and turbulence.

Ch.4.Gel Growth and Kinetics of cadmium oxalate

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4. The gel does not hinder in time and space the necessary and sufficient supply of input reactants to the growth sites.

5. Crystal quality is not greatly influenced by changes in environmental temperature or pressure.

6. By remaining chemically inert and harmless, the gel framework acts like a soft three-dimensional crucible in which the crystal nuclei are delicately held in the position of their formation, thereby minimizing interaction and preventing damage, if any, due to impact of propagating ions with the bottom or walls of the container.

7. The gel being soft yields mechanically to the growing crystals. 8. Thermodynamic considerations ensure that since the growth proceeds at near

ambient temperatures, the grown crystals will contain relatively less concentration of equilibrium defects.

9. Since the gel reduces, in effect, the speed of chemical reaction, crystals could be made to grow much larger than those grown by similar reaction in water or in molten state by double decomposition processes.

10. All the nuclei are spatially separated whereby the detrimental effects due to precipitate-to-precipitate interaction are drastically diminished.

11. One can control diffusion rates and nucleation probability experimentally and thus design one’s own crystallization equipment for obtaining different sizes and morphology of crystals.

12. The growth procedure is highly economical, and it can be usefully employed in even smaller laboratories which do not possess sophisticated equipment to grow perfect crystals.

4. 2 PREPARATION OF AGAR GEL

The gel solution was prepared by dissolving the as-available commercial grade agar-agar powder-carbohydrate polymer (derived from seaweeds) in double distilled water at the boiling temperature. The gel formation is known to take place due to (catalytic) polymerization when the base solution is mixed with any mineral or organic acid. The resulting gel is neither liquid nor solid, it is also not a simple three-dimensional network, but actually it consists of sheet-like structures of varying degree of surface roughness and porosity, forming interconnected cells7). The gelation behavior depends on many factors such as the density, pH, nature, aging of the gel and the temperature. The details of the process of gelation are already discussed in chapter 2. At the moment of gelation, viscosity of the solution increases to such an extent that the structure is ‘frozen in’. Among the factors which could contribute to orders of magnitude increase in viscosity during densification are loss of OH,


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polymerisation and structural relaxation. The gel is considered ready for use when it has a high degree of homogeneity, whence it is felt vibrating during the finger stroke on the wall of test tube, since it resists pouring. However, the gelling process is not completed at that point of time because the gel medium always evolves with time. As the pH of gel decreases, it requires relatively higher gelation time and it becomes transparent and soft too.

4. 3 GROWTH EXPERIMENT

Cadmium oxalate has been found to decompose (see chapter 6 for details) at higher temperatures, much below its melting point, the oxalate group being weakly ionically bonded to the cation. Secondly, it is insoluble in water, and third it does not vaporize or sublime. Therefore, none of the high temperature melt, or vapour, or aqueous solution, or hydrothermal methods of crystal growth is applicable to this material.

There is a variety of ways to grow good crystals using hydrogels as described in chapter 2, but they all are variants of two basic and simple methods shown in Figs. (2.7) and (2.8), which are used hereunder. The third method i.e. double diffusion method (Fig. 2.10), employs U-tube for pure-neutral gelation, it is exploited here for the growth of cadmium oxalate single crystals. The apparatus used for crystallization consists of a borosilicate glass U-tube with both ends open, arm length 25 cm and diameter 2.5 cm, placed vertically erect on a wooden stand. The chemicals used are: 1. Commercial grade Agar agar 2. Cheti-Chem ‘ExcelaR’ Cadmium Chloride (99 %) 3. Qualigens ‘‘ExcelaR’ oxalic Acid Cadmium Chloride (99.5 %)

The gel solutions of different mass density ranging from 1.3 to 1.7% were prepared by dissolving 1.3 gm to 1.7 gm of agar-agar powder in 100 ml double distilled water at the boiling temperature. The process of gelling is found to be exothermic. The pH of the resulting gel solutions were constantly measured using a pH meter (CONTROL DYNAMICS, Digital pH meter, model – APX 175). The gel solutions in fixed amount were transferred to the appropriate growth apparatus, before setting, without giving any chance to the formation of air bubbles, by pouring the gel solution to fall steadily along the walls of the U-tubes. After that, mouths of the U-tubes were carefully closed with cotton, essentially to prevent the entry of dust particles, fast evaporation, and contamination of the exposed gel surface. The gel setting period varied from 3 to 6 days depending on the setting condition employed and the environmental temperature. The graphical variation of pH obtained on different mass densities of gel solutions is shown in Fig. 4.1. Evidently, the denser gels have lower pH value.


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Feed solutions of different molarities were prepared by dissolving cadmium chloride and oxalic acid separately in double distilled water and then filtering by micrograin filter paper. After ensuring firm gel setting, the feed solutions were poured above the set gel, with the help of a pipette, being allowed to fall along the wall of the U-tubes so as to prevent the gelled surface from breaking or microcracking. Solution of cadmium chloride of a particular concentration (1.0 to 10.0 M) was poured over the set gel in one arm of the U-tube and oxalic acid of a particular concentration (0.5 to 2.0 M) to the other arm of the U-tube over the set gel. The poured supernant liquids diffuse slowly into the gel medium and react with each other, thus giving rise to the slow and tiny precipitated nuclei of CdC2O4. At some stage in the gel, when the concentration of the diffusant is optimum or appropriate, a few nuclei begin to form. It is believed that these initially formed nuclei act as sinks which result in the establishment of radial diffusion patterns that, in turn, reduce the reagent concentration in the neighboring sites. Subsequent increase of the diffusant concentration at the precipitate site serves to enhance the growth rate with no new nucleations7). The following reaction responsible for crystal growth takes place in the gel medium.

CdCl2 + H2C2O4 → CdC2O4 + 2HCl (1)

Since the crystals growing in gel system compete with one another for solute transfer through the established radial diffusion channels, leading to a kind of competition which limits their size and perfection, it is desirable to severely suppress nucleation until ideally just one crystal grows at a predetermined site. With this concept in mind, the growth has been studied by changing the possible growth parameters, viz. gel density, gel aging, amount and concentration of supernant liquid, and temperature of the neutral gel.

The double diffusion technique has proved successful in the case of cadmium

oxalate8). The process of crystallization is in congruence of the direct union of cadmium and oxalate ions almost in and near the horizontal portion of the U-tube, with an average of one nucleation site per cubic centimeter. The large size crystals have been obtained using this set up. This may be because both the reactants have almost equal degree of vertical as well as lateral freedom to diffuse to the reacting sites, as against single tube system in which only one of the reactants enjoys only vertical freedom for diffusion to occur. It is pertinent to note that the process of crystal growth in agar-agar gel U-tube is extremely slow, the growth takes about 2-3 months to saturate. This results in perfection of the product, in contrast to the relative less perfect yield in silica gel using single as well as double-diffusion method whence the growth is normally completed within a month. Crystals obtained in agar-agar gel


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were flawless and transparent. Selective chemical etching showed a high degree of perfection in these crystals, the average dislocation density being 102 cm-3. Some crystals of cadmium oxalate growing in a typical U-tube apparatus are shown in Fig. 4.2(a, b). The best transparent single crystals of cadmium oxalate trihydrate have been obtained with the following optimized values: Growth temperature : 30 oC Gel density : 1.5 % Gel set time : 4 days Ageing time : 1 day Concentration of reactants : 2.0 M CdCl2 and 2.0 M oxalic acid Amount of supernant liquid : 20 ml

Some of the crystals obtained are displayed in Fig. 4.3, indicating their size, visual perfection and transparency. Our crystals (maximum 17 mm x 9 mm x 4 mm) are on an average larger in size compared to those (maximum 5 mm x 3 mm x 2 mm) grown by Shedam and Rao9) and (maximum 7 mm with pure gel and 10 mm with impure gel grown) by Arora and Abraham10). It is implied, therefore, that the use of agar-agar gel with U-tube set up is more advantageous than sodium metasilicate (SMS), Na2SiO3 gel with single tube system. Perfect crystallinity of the grown crystal was confirmed from the powder XRD pattern (see chapter 5)

4. 4 GROWTH KINETICS

It has been found that the crystals at substantial depths in the gel grew more slowly than those near the top interface, this is because of the thinner concentration channels7) in the gel bulk. Nucleation density and growth rates have both been found to be affected by

1. factors associated with the gel, e.g. age and density and pH of gels.

2. factors associated with the supernant ions e.g. concentration and amount of cadmium chloride or oxalic acid, or both.

4.4.1 EFFECT OF GEL DENSITY

Solutions of agar-agar gel of different densities (1.3 to 1.7%) at room temperature (30 oC) were prepared in appropriate amount. Then, after gelation, feed solution i.e. CdCl2 of a particular concentration (1.0 M, 1.5 M, 2.0 M) was poured above the set gel in one arm of the U-tube and oxalic acid of a particular concentration (1.0 M, 1.5 M, 2.0 M) to the other arm of the U-tube over the set gel, with the help of a graduated pipette. The crystal count as the total number of crystals


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grown and their total weight were recorded after the growth was observed to have been completed. Recorded data are graphically plotted as shown in Figs. 4.4(a) and (b) respectively. The actual crystals, as an illustrative case, growing in such varying conditions are displayed in Fig. 4.4(c). The following observations are note worthy:

1. Transparency of the gel is, found to decrease with increase in gel density. Also, high density gels take lesser time to set and they are mechanically stronger than the lower density gels.

2. The nucleation count as well as their weight increase steadily and uniformly with increase in gel density upto 1.5% and then decrease with gel density beyond 1.5%.

3. For the value of gel density much lower than 1.5%, the gel is found to be mechanically weak since it takes long time to set and sometimes it is ruptured before setting, hence not suitable for growth. In such low density gel medium (around 1.3 to 1.4%), the fewer crystals grown are relatively larger in size but the soft gel gets incorporated into the matrix, resulting into not-so-perfect product.

4. At higher gel density, much beyond 1.5%, the product is accompanied by continuous deterioration of quality; the crystals grown were contaminated, translucent-to-opaque and ill defined. At gel density much above 1.5 %, the gel is almost turbid and hence not appropriate for study.

5. At a given gel density, the number of crystals increase with the reactant molarity. 6. Around gel density of 1.5%, with concentration 2.0 M CdCl2 and 2.0 M oxalic

acid, the crystals have better transparency, perfection and well-developed shape.

Probably close linked pores in higher dense gels are not able to communicate and this results translucent to opaque gels. Our observation contradicts with Shedam and Rao9) who reported that the nucleation density decreases with increase in gel density. This contradiction relies on the fact that in the experiments of Shedam and Rao9), the gel pH was varied by use of perchloric acid, keeping the amount of oxalic acid fixed. Obviously, with that situation9), the decrease in nucleation density with increasing gel density is attributed to smaller pore concentration and poor communication among the pores in the case of denser gels.

4.4.2 EFFECT OF CONCENTRATION OF SUPERNANT SOLUTION

Gels of fixed density at three different values (1.3 to 1.7%), were allowed to set in a number of U-tubes. Then 20 ml of 2.0 M oxalic acid and 20 ml of CdCl2, of different concentrations ranging from 2.0 M–10.0 M in a step of 2.0 M was poured above the set gel in separate limbs and crystal count as total number of crystals grown and their total weight were recorded. The observed variation is shown in Fig. 4.5(a,b), while Fig. 4.5(c) illustrates the actual crystals growing in U-tubes, showing the effect.


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Similarly, to examine the effect of the concentration of the other supernant liquid (oxalic acid) on the nucleation density, gels of fixed density at three different values (1.3 to 1.7 %), were allowed to set in a number of U-tubes. After setting, 20 ml of CdCl2 having the fixed concentration of 2.0 M, and 20 ml of oxalic acid of concentrations varying from 0.5 M to 2.0 M in a step of 0.5 M was poured above the set gel in separate limbs and the crystal count as the total number of crystals grown and their total weight were recorded. The observed variation is shown graphically in Fig. 4.6(a,b), while Fig. 4.6(c) illustrates the actual crystals growing in U-tubes showing the effect. The following observations are noteworthy:

1. On increasing the concentration of CdCl2, the nucleation density, so also the weight, decreases but this is accompanied by continuous deterioration of crystal quality.

2. Although at much higher concentration (above 6.0 M) the count decreases and correspondingly the size slightly increases, but the yield is not visibly perfect.

3. Good quality yield with more weight occurs at concentration around 2.0 M CdCl2 with gel density 1.5%. This is in consonance with the earlier observation in sec. 4.5.1, supported by increased weight due to prolific nucleation, with CdCl2 going from 2.0 to 6.0 M at gel density of 1.7% (Fig. 4.5 b).

4. The trend of the effect of oxalic acid is found to be different from that of cadmium chloride, which is accounted for by their much different speedes and sizes.

It is worthwhile to expect increase in nucleation because of the enhanced

availability and probability of Cd2+ ions to react with the COO2- ions in the medium. Also, the increasing ionic concentration at greater depth is responsible for the increased volume of crystallization zone as concentration of CdCl2 increases. This is thoughtful line of action as evinced by John and Ittyachen11). But in our case, in contradiction to above, availability of more and more of concentration of CdCl2 amounts to reducing gel density indirectly, because of higher solubility of CdCl2 (140 gm in 100 ml H2O at 20 °C) compared to oxalic acid. This is in support to the previous observation (sec.4.5.1) that lower density gel yields greater counts.

With regard to the effect of the concentration of oxalic acid, the following

observations are noteworthy: 1. On increasing the concentration of oxalic acid, the nucleation density and weight

of crystals obtained increase noticeably. This is due to the enhanced availability and probability of oxalate ions to react with the Cd2+ ions in the medium.

2. For the concentration below 2.0 M, crystals are comparatively smaller size, and visibly imperfect yield are observed. This is justified, as the insufficient oxalate ions below 2.0 M result in restricted growth.


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3. For good crystal harvest, the count is more with more product weight, with gel density 1.5% at a given molarity 2.0 M of oxalic acid. This is in consonance with the earlier observation in sec. 4.5.1.

4.4.3 EFFECT OF AMOUNT OF SUPERNANT SOLUTION

Gels of fixed density at three different values (1.3 to1.7 %), were allowed to set. Then, CdCl2 and oxalic acid having a fixed concentration of 2.0 M and varying in amount (20 to 80 ml) in step of 20 ml was poured above the set gel in separate limbs. The observation recorded as the total number of crystals grown and their weight are shown graphically in Fig 4.7 (a, b) and a representative of the actual tubes showing the crystal growth under the varying condition is shown in Fig. 4.7 (c). The following observations have been pointed out.

1. There is an overall increase of crystal count as well as weight with increasing quantity of CdCl2 and oxalic acid (2.0 M).

2. Below 40 ml CdCl2 and OA (2.0 M) the increase is quite marked and a rapid rate.

3. Beyond 40ml CdCl2 and oxalic acid (2.0 M) the increase is relatively little.

4. Beyond 40ml CdCl2 and oxalic acid (2.0 M) the crystals obtained are transparent, hence a good deal perfect. These crystals are larger in size as well.

Evidently, with the increase in CdCl2 and oxalic acid (2.0 M) amount, the concentration of reaction nuclei increase and at greater depth below the gel interface, the effective concentration of nuclei decreases. The increase in diffusion depth and hence greater crystal count with concentration may be attributed to an increase in the hydrostatic pressure correspondingly.

4.4.4 EFFECT OF GEL AGING

Solutions of agar-agar gel of different densities (1.3 to 1.7%) at room temperature (30 oC) were prepared. Then these (in fixed amount) were transferred to the growth apparatus before setting, and allowed to set and then allowed to age for different periods (1-6 days). Then, CdCl2 and oxalic acid having the fixed concentration of 2.0 M and fixed amount (20 ml each) were poured above the set gel in separate arms of the U-tube. The crystal count and their weight were made after the growth was observed to be completed. The observed variation is graphically plotted as shown in Fig. 4.8(a, b). The actual crystals growing in such varying conditions are displayed representative in Fig. 4.8(c). The observed effect is summarized below: 1. The number of crystals and their weight decrease with aging time, on the whole.


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2. With aging time, set-gel transparency decreases. It has been already pointed out that the transparency of the gel is, in general, found to decrease with increase in density of gel. Consequently, gel aging and density are directly related. At gel density above 1.5 %, with higher aging period the gel is almost turbid and hence not appropriate for study.

3. For the value of gel density lower than 1.5% with lower aging period (1-3 days), the gel is found to be mechanically weak and also it takes long time to set and sometimes it is ruptured before setting, hence not suitable for growth.

4. Reasonably good quality and moderate size crystals are obtained in the range of gel density 1.5% with aging period of 1 day.

Gel aging has the effect of reducing the cell size and, consequently, the rate of diffusion of cadmium ions (Cd+2) into the gel3,12). As a matter of fact, the longer a gel sets at room temperature, greater is the amount of water evaporating out of the gel. The effect of water evaporation may be considered in two ways, viz., before and after the formation of gel framework. Before the gel is set, the evaporation of water causes an increase in gel density, which seemingly decreases the diffusivity of (Cd+2) ions in the gel, thereby decreasing the number of nucleation sites, as observed. On the other hand, after the gel is set, evaporation of water causes not only the lack of ionic carriers in the channel of the gel framework, but also discontinuities in the channel due to shrinkage of pores in the gel13). Both these effects do adversely affect the diffusion of cadmium ions and hence the reaction velocity is lessened, resulting into decreased count as well as the weight, as observed.

4.4.5 GROWTH RATES

Three methods are generally employed to study the growth rate of crystals. 1. Linear growth rate measurements, using optical microscope. 2. Photographic method 3. Overall growth rate measurements, by weighing method In the present work, under the optimum condition as obtained out already in this chapter to grow perfect, transparent and well-faceted crystal, we have microscopically measured the linear growth of the crystal along b axis (in the direction of horizontal longer axis) and a axis (in the direction of horizontal shorter axis) when the crystal is being grown inside the gel. We have used the optical microscope having magnification 10x and resolution 1 part in 100 i.e. 0.01. We focused on a single crystal growing inside the gel and carried out the growth kinetic measurement. For the purpose, cross-wire of the microscope is focused on one corner or tip of the crystal and the displacement was determined by increase in the time rate


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of dimension. We have, thus measured growth of the crystal along two axes at the same interval of time. The graph of crystal growth, in mm versus time interval in days, is shown in Fig. 4.9. It is noteworthy that the crystal dimension increases linearly upto 7 weeks, then, later it becomes steady and saturated i.e. no further increase in the growth is observed. The growing face is vicinal and nonsingular due to attachment of steps and kinks, so there may be linear or parabolic increase of dimensions. The surface energy of a growing face is usually anisotropic because it depends on the density of steps and kinks, and so it cannot increase indefinitely, the growth being limited by surface kinetic processes such as desolvation, integration of on-coming molecules on nonsingular faces, the growth gets saturated. The crystal growth along b axis is found higher than that of a axis. It is not possible to measure the crystal growth along the c axis as the geometry of the set up of U tube prevents accessibility in that direction.

4. 5 NUCLEATION CONTOL

The standard procedures for nucleation control, in case of the crystals grown in sodium metasilicate gel using single tube system, are available in the literature10,13,18). Many times, Liesegang ring formation occurred which was detrimental to a quality crystal product. Shedam and Rao9) showed nucleation control by the use of perchloric acid set neutral gel. However, the crystal size in their situation20) is still smaller than what we observed in U-tube system in our case without the formation of Liesegang rings, even without the specific nucleation control procedure. The concentration programming was tried for a period of 15 days by varying the concentration of the supernant solution from 0.1 M to 1.5 M. This also has not evinced favourable results. It might be that the molarity 0.1 M is too large to study the effect. In the case of using agar-agar gel medium, the best yield of good crystal quality is obtained with the moderate concentration of the supernant solutions, CdCl2

and oxalic acid. To prevent unwanted behavior of cadmium oxalate nucleation, without the growth of single crystals in sodium metasilicate gel using single tube technique, cadmium oxalate should be slightly soluble in water so that excess nuclei will get dissolved, allowing only a few remaining potential nuclei to grow. This is a self controlling phenomenon. In certain other cases, this is achieved indirectly by the reaction wastes which could dissolve the excess nuclei. In the case of cadmium oxalate trihydrate, the solubility is too low (0.005 in 100ml of water at 18 °C) which accounts for the failure of conventional methods of nucleation control. The agar-agar gel with U-tube system is found to be the most favorable, especially for the growth of larger size and good quality crystals of cadmium oxalate.


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4.6 CRYSTAL MORPHOLOGY

Cadmium oxalate trihydrate single crystals grown from agar-agar gel belong to triclinic class with the three crystallographic axes of unequal lengths making oblique angles (≠90°) with each other as shown in Fig 4.10. The system has two

symmetry classes: pinacoidal class - 1 and pedial class –1. The former class has the symmetry consisting of a 1–fold axis of rotary inversion which is equivalent to a symmetry centre, while the latter class has a 1 – fold rotation axis which is equivalent

to no symmetry. Each pinacoidal form of class 1 becomes two pedions. Figure 4.11 shows a triclinic pinacoid (or parallelohedron). The pinacoids intersect on one axis and are parallel to the other two axes. This is a one-fold axis of rotoinversion, which may be viewed as the same as having a center of symmetry. Out of the four different orders of pinacoids, the cadmium oxalate trihydrate crystals predominantly show morphology belonging to the third order pinacoid and basal pinacoid (Fig.4.12a). The most frequently occurring habits of the grown crystals are as shown in Fig. 4.12(b, c, d). Figure 4.12(a) shows the basic system while 4.12(b) shows the later developed stage. Evidently, pronounced growth along c-axis is favoured. In general, the morphology (Fig. 4.12a, b) of the grown crystals does not critically depend on any of the parameters viz, gel density, concentration of supernant liquid, etc. In addition to the morphology obtained as mentioned above, the new faces especially p and b as seen in Fig. 4.12(c, d) are developed in the case of two directional diffusion i.e. in U – tube assembly. One is, therefore, inclined to believe that the U-tube setup shows preferential growth along b-axis. The basic x-ray data of the grown crystals have been determined and reported in the subsequent chapter.

The crystals are found frequently twinned according to certain laws. The

crystals belonging to a triclinic system are known to be twinned according to one or both of two laws: the albite and the periciline law. Twinned crystals are usually designated as either contact twins or penetration twins. Contact twins have a definite composition surface separating the two individuals, penetration twins are made up of interpenetrating individuals having irregular composition surface, and the twin law defined by a twin plane. Albite twin is an example of contact twin. The cadmium oxalate trihydrate crystal illustrates the best the twinning example of triclinic system. They are almost universally twinned according to the albite law, with the side pinacoid {010} as the twin plane, shown in Fig. 4.13(a). The angle between the basal plane and the twinning plane is about 94°. Another twinning less commonly found in cadmium oxalate crystals, shown Fig. 4.13(b), is the penetrating twinning type.


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4.7 CONCLUSIONS

1. Gel method using agar-agar gel medium is quite suitable for the growth of well-developed, well-faceted and transparent crystals of cadmium oxalate.

2. Condition for the growth of good quality crystals is optimized and U-tube system is found favourable for the growth of larger size and better quality crystals.

3. The waste product of the reaction viz. HCl in the present case adversely affects the size and transparency of the growing crystals.

4. Gel density, pH and ageing, and reactant concentration and amount need to be played with, for controlling nucleation sites at a given temperature.

5. Cadmium oxalate trihydrate crystals belong to triclinic pinacoid variety and are twinned according to albite and periciline laws.

REFERENCES:

1) J. W. Gibbs, Collected works (Longman’s Green and Co. London,1878) 325 2) P. Curie, Bull. Soc. France Mineral 8 (1885) 145 3) G. wulff, Z. Krist. 34 (1901) 449 4) M. Volmer, Z. Phys. Chem. 102 (1922) 268 5) A. F. Wells, Crystal Growth, Am. Rep. Chem. Soc. London 43 (1946) 62 6) H. E. Buckley, Crystal Growth ( John Wiley, New York, 1951) 7) V. S. Joshi and M. J. Joshi, Indian J. Phys. 75A (2001) 159 8) B. P. Agrawal, K. M. Chauhan and M. M. Bhadbhade, Ind. J. Pure & Appl.

Phys. 37 (1999) 395 9) M. R. Shedam and A. Vankateshwara Rao, Bull. Mater. Sci. 16 (1993) 309 10) S. K. Arora and Tomy Abraham, J. Cryst. Growth 52 (1981) 851 11) M. V. John and M. A. Ittyachen, Bull. Mater. Sci. 21 (1998) 387 12) S. Sengupta, T. Kax and S. P. S. Gupta, J. Mater. Sci. Lett. 9 (1990) 334 13) H. W. Liaw and J. W. Faust, Jr., J. Cryst. Growth 13/14 (1972) 471


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Fig. 4.1 Graphical variation of pH for different mass density of agar agar gel solution

(a) (b)

Fig. 4.2 (a,b) A typical U-tube set up showing single crystal growth of cadmium oxalate trihydrate.

7.2

7.3

7.4

7.5

7.6

7.7

7.8

7.9

1.2 1.3 1.4 1.5 1.6 1.7Gel density

pH


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Fig. 4.3 Grown crystals of cadmium oxalate trihydrate

10

15

20

25

30

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1.2 1.3 1.4 1.5 1.6 1.7 1.8Gel density (ρ)

Tota

l num

ber o

f cry

stal

s, N

1.0 M

1.5 M

2.0 M

Fig. 4.4 (a) Graphical representation of the effect of gel density on the number of crystals grown

20 mm


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0

0.5

1

1.5

2

1.2 1.3 1.4 1.5 1.6 1.7 1.8Gel density (ρ)

Tota

l wre

ight

of c

ryst

als,

w (g

m)

1.0 M

1.5 M

2.0 M

Fig. 4.4 (b) Graphical representation of the effect of gel density on the weight of crystals grown

Fig. 4.4 (c) Pictorial representation (as an illustration) of the effect of gel density on the growth of crystals. Concentration of CdCl2 and O.A.:2.0 M, Gel density:1.3%, 1.4%, 1.5%, 1.7% from left to right side


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10

20

30

40

50

1 2 3 4 5 6 7 8 9 10Concentration of CdCl2 (M)

Tota

l num

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f cry

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1.3

1.5

1.7

Fig. 4.5 (a) Graphical representation of the effect of concentration of supernant solution, CdCl2 on the number of crystals grown

1

1.5

2

2.5

3

3.5

4

1 2 3 4 5 6 7 8 9 10Concentration of CdCl2 (M)

Tota

l wei

ght o

f cry

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(gm

)

1.3

1.5

1.7

Fig. 4.5 (b) Graphical representation of the effect of concentration of supernant solution, CdCl2 on the weight of crystals grown


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Fig. 4.5 (c) Pictorial representation (a representative case} of the effect of concentration of supernant solution CdCl2 on the growth of crystals. Gel density: 1.5%, Concentration of OA: 2 M fixed, CdCl2: 2 M, 4 M, 6 M, 8 M, 10 M from left to right side

10

20

30

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0 0.5 1 1.5 2 2.5Concentration of Oxalic acid (M)

Tota

l num

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stal

s, N

1.3

1.5

1.7

Fig. 4.6 (a) Graphical representation of the effect of concentration of supernant solution, oxalic acid on the number of crystals grown


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1

2

3

4

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6

7

0 0.5 1 1.5 2 2.5

Concentration of Oxalic Acid (M)

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)

1.3

1.5

1.7

Fig. 4.6 (b) Graphical representation of the effect of concentration of supernant solution, oxalic acid on the weight of crystals grown

Fig. 4.6 (c) Pictorial representation (an illustrative case) of the effect of concentration of supernant solution oxalic acid on the growth of crystals.Gel density: 1.5 %, Concentration of CdCl2: 2 M fixed, Oxalic acid: 0.5 M, 1 M, 1.5 M, 2 M from left to right side


77

5

15

25

35

45

55

10 20 30 40 50 60 70 80 90Amount of supernant solution, ml

Tota

l num

ber o

f cry

stal

s, N

1.3

1.5

1.7

Fig. 4.7 (a) Graphical representation of the effect of amount variation of supernant solution on the number of crystals grown

1

2

3

4

5

6

7

10 20 30 40 50 60 70 80 90Amount of supernant solution, ml

Tota

l wei

ght o

f cry

stal

s, w

(gm

)

1.3

1.5

1.7

Fig. 4.7 (b) Graphical representation of the effect of amount variation of supernant solution on the weight of crystals grown


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Fig. 4.7 (c) Pictorial representation (a representative case}of the effect of amount variation of supernant solution CdCl2 and oxalic acid (each 20 ml, 40ml, 60ml, 80 ml and 100ml from left to right side) on the growth of crystals. Gel density: 1.5 %

20

30

40

50

60

0 1 2 3 4 5 6 7days

Tota

l num

ber o

f cry

stal

s, N

1.3

1.5

1.7

Fig. 4.8 (a) Graphical representation of the effect of aging of gel on the number of crystals grown


79

2

3

4

5

6

0 1 2 3 4 5 6 7days

Tota

l wei

ght o

f cry

stal

s, w

(gm

)

1.3

1.5

1.7

Fig. 4.8 (b) Graphical representation of the effect of aging of gel on the weight of crystals grown

Fig. 4.8 (c) Pictorial representation of the effect of gel aging period (1-6 days from left to right side) on the growth of crystals. Gel density: 1.5 %


80

0

2

4

6

8

10

12

14

0 7 14 21 28 35 42 49 56 63No. of days

Cry

stal

gro

wth

in m

m

along b axis

along a axis

Fig. 4.9 Graphical representation of crystal growth in mm along b and a axis

Fig. 4.10 Triclinic crystallographic Fig. 4.11 Triclinic pinacoid (or axis α, β, γ ≠ 90° pallalelohedron)


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(a) Third order pinacoids and basal pinacoid (b) Rhodonite – like morphology

(c) Chalcanthite – like morphology (d) Anorthite – like morphology

Fig.4.12(a,b,c,d) Schematic diagram showing different morphology obtained: ( ) ( ) ( ) ( ) ( )112,112,110,011,001 xnmMp

Fig.4.13 (a) Albite Twinning (b) Staurolite Twins

chapter 4 gel growth and kinetics of cadmium...

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