Gel Growth and Preliminary Characterization of ... - ACS Publications

Gel Growth and Preliminary Characterization of Strontium Tartrate Trihydrate S. K. Arora,* Vipul Patel, Anjana Kothari, and Brijesh Amin Department of Physics, Sardar Patel University, Vallabh Vidyanagar - 388 120, Gujarat, India

CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 2 343-349

Received June 4, 2003

ABSTRACT: Results of a study on nucleation kinetics, growth, and characterization of strontium tartrate trihydrate single crystals in silica hydrogel are described. The basic structure, composition, and perfection of the grown crystals were determined by XRD, EDAX, and laser scattering tomography. It was found that experimental conditions, such as pH and density of gel, concentration and volume of supernatant liquid, aging and quality of gel, have a strong influence on the nucleation kinetics and growth of the crystals. The effect of experimental conditions on growth kinetics has been qualitatively explained from the standpoint of the classical nucleation theory. 1. Introduction gels1,2

is an inexpensive and Crystal growth from powerful method for growing relatively large single crystals of substances that show poor solubility in water. This method can be used, in the first instance, to ascertain the location of the first precipitation in a system where two counter-diffusing reagents meet to generate the sparingly soluble reaction product. The resulting crystal is normally quite perfect, because the growth medium itself permits the reaction to occur at a reasonably slow and controlled rate. Also, the gel strictly prevents turbulence and, remaining chemically inert, it serves to provide a three-dimensional soft crucible. The two significant factors that affect the growth rate of crystals are the driving force for crystallization and the diffusivity in the gel. These in turn depend, to a great extent, on the concentration of reacting ions in gels3,4 and are primarily governed by gel characteristics such as age, density, and pH. The aim of the present paper is to investigate essentially the effect of these factors on nucleation kinetics of strontium tartrate trihydrate (STT) single crystals in silica hydrogel. Several divalent tartrates5-8 are interesting ferroelectric and/or piezoelectric compounds and exhibit nonlinear optical and spectral characteristics. Consequently, they are used in transducers and several linear and nonlinear mechanical devices.9,10 However, there is scanty information in the literature11,12 on the growth and physicochemical characteristics of strontium tartrate crystals. It was thought, therefore, worthwhile to carry out a detailed study on the gel growth of this compound. Strontium tartrate is sparingly soluble in water; it decomposes before melting11 because the tartrate group is weakly bonded ionically to the cation, and it does not vaporize or sublime. Consequently, growth of strontium tartrate crystals in gels is the only relevant alternative. 2. Experimental The apparatus used for crystallization consists of borosilicate glass tubes of length 15 cm and diameter 2.4 cm placed vertical on a wooden stand. An aqueous solution of tartaric * To whom correspondence should be addressed. Tel: 02692-26844/ 26846. Fax: 091-2692-36475. E-mail: [email protected].

acid of a particular molarity was taken in a beaker and sodium metasilicate (SMS) of a particular specific gravity was added dropwise, using a pipet, constantly stirring the solution in the beaker with a view to avoiding excessive local ion concentration which may cause premature local gelling and make the final medium inhomogeneous and turbid. The gel solution with the desired value of pH, as measured using a digital pH meter (model APX 175, Control Dynamics, India) was transferred to several test tubes, in a fixed amount, without giving a chance for the formation of air bubbles, by allowing the mixture to fall steadily alongside of the test tubes. Mouths of the test tubes were then closed with cotton to prevent fast evaporation from and contamination of the exposed surface of the gel. The gel was usually found to set in 15 min to 15 days, depending upon the environmental temperature and its pH. After ensuring firm gel setting, an aqueous solution of strontium chloride of a particular concentration (0.25-2.5 M) was poured over the set gel, with the help of a pipet, being allowed to fall along the wall of the test tubes so as to prevent the gelled surface from cracking. The supernatant liquid diffuses slowly into the gel column and reacts with the inner reactant, giving rise to the nucleation of SrC4H4O6‚3H2O. At some stage in the gel, when the concentration of the reactants is optimum, a few nuclei begin to form. These initially formed nuclei act as sinks and result in the establishment of radial diffusion patterns which in turn reduce the reagent concentration in the neighboring sites. Subsequent increase of the diffusant concentration increases the growth rate with hardly a few new nucleations,13,14 and this forms the basis of seeded solution growth. The following reaction responsible for crystal growth is expected to take place.

SrCl2 + C4H6O6 f SrC4H4O6 V + 2HCl

(1)

The energetics have been found to favor the reaction to proceed in the forward direction only.15 The problem of nucleation has been of paramount importance,16 since one is always interested to obtain larger and more perfect crystals, required for academic interest. The crystals growing in the system compete, as usual, with one another for solute transfer through radial diffusion channels, but this competition results in limiting size and perfection. It is, therefore, desirable to severely suppress nucleation until ideally just one crystal grows15 at a predetermined site. With this concept in mind, the growth of STT crystals (see the following paragraphs) has been studied by changing the possible growth parameters. The conventional methods of nucleation control such as neutral gel technique, seeding, concentration programming as suggested by Henisch,1 and the impurity impregnation as suggested by Arora15,16 were tried in the present study but with no fruitful outcome.

10.1021/cg030024s CCC: $27.50 © 2004 American Chemical Society Published on Web 01/30/2004

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transparent STT crystals of a size between 3 × 2 × 0.5 and 20 × 12 × 5 mm3. Some of the STT crystals obtained are displayed in Figure 1. They predominantly exhibit the three basic morphological forms of pinacoids and prisms with the dominant (100), (010), and (001) faces. The crystallization process normally took about 15 days to complete, and the crystals at substantial depths below the interface grew more slowly than those near the interface, possibly because of the thinner concentration channels17 in the gel bulk. 4. Nucleation Kinetics

Figure 1. Some typical single crystals of strontium tartrate trihydrate (STT) grown by double decomposition reaction occurring in silica hydrogel (scale in mm).

3. Crystals and Growth Parameters The unhydrated strontium tartrate resulting from the double decomposition reaction 1 above is found to have crystallized as trihydrated, with the chemical formula SrC4H4O6‚3H2O, and this has been confirmed by TGA/ DTA thermograms. A good crop of crystals was obtained with the following optimized parameters. Growth temperature: 30 °C Gel pH: 3.5 Gel density: 1.04 g cm-3 Concentration of reactants: 1 M SrCl2 and 1.5 M dextro tartaric acid Amount of supernatant liquid: 20 mL Aging time: 1 day With the onset of supernatant solution diffusion into the set gel, crystallization was observed to have occurred within a few hours. We obtained slightly yellowish crystals touching the bottom of tubes, along with transparent and faceted crystals within the gel column. The yellowish crystals were strontium tartrate tetrahydrate, but our study here refers to the well-faceted,

The nucleation count was accomplished by taking an average of the number of STT crystals grown in the gel column in sets of three test tubes, and thus the effect of several growth variables, as described below, was studied. (1) Supernatant Solution. Gels of a particular density (1.03-1.05 g cm-3) and a particular pH (2.56.0) were prepared and allowed to set in a number of test tubes. After a fixed period (1 day), 20 mL of feed solution, i.e., SrCl2, of different concentrations from 0.25 to 1.5 M was poured over the set gel and the crystal count was recorded. It is observed (Figure 2) that the count increases noticeably with the concentration of feed solution. Increasing beyond 2.0 M the concentration of SrCl2 favored the formation of turbidity and dendritic morphology (the stem being along the c-axis), which may possibly be attributed to an increased local supersaturation at the reaction sites, while good quality faceted crystals developed at and below 1.5 M concentration. Further, volume of the crystallization zone with respect to the gel interface is also found to increase with an increase in molarity of the feed solution. This is, obviously, due to increasing ionic concentration at greater depths, irrespective of gel densities.17 (2) Gel Density. Gels of different densities at room temperature (30 °C) were prepared by mixing SMS of

Figure 2. Nucleation count as influenced by the concentration of the supernatant liquid (SrCl2) added above the set gel.

Gel Growth of Strontium Tartrate Trihydrate

Crystal Growth & Design, Vol. 4, No. 2, 2004 345

Figure 3. Nucleation count as influenced by the density of gel prepared using SMS solution.

Figure 4. Nucleation count as influenced by gel aging, determined after gelation.

specific gravity varying from 1.03 to 1.08 g cm-3 impregnated with 1.5 M tartaric acid. We found that the lesser the SMS density, the better is the set-gel transparency. The gel pH was maintained at 4.0 by varying the amount of tartaric acid. After gelation, the feed solution (0.5, 1.0, and 1.5 M) was placed above the set gel. It was observed that the high-density gels take lesser time for setting and are mechanically stronger than the low-density gels. However, they contaminate the growing matrix with silica, in particular, and the resulting crystal quality and morphology are affected severely. The variation of nucleation count with gel density is graphically plotted as shown in Figure 3. It may be noteworthy that the gel density of 1.04 g cm-3 gave well-developed, transparent crystals of medium

size. The chances of growth of dendritic crystals increased and the length of dendrites also increased with increasing SMS density. (3) Gel Aging. Gels of fixed pH 4.0 and density 1.04 g cm-3 were allowed to age for different periods (between 0.25 and 122 h) before adding a fixed amount (20 mL) of the feed solution (0.5, 1.0, 1.5 M SrCl2). The nucleation count was recorded for the three molarity values of SrCl2 and is reproduced graphically in Figure 4. Evidently, aged gels can be preferred for controlling nucleation sites. (4) Gel pH. Gels of a fixed density (1.04 g cm-3), but having different pH values, were set in different test tubes by adding to SMS a mixture of 36.3 mL of 1.5 M tartaric acid with different amounts (between 0 and 112

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Figure 5. Nucleation count as influenced by gel pH, controlled by addition of optimum amounts of acetic and tartaric acids.

mL) of 1.5 M acetic acid. All other parameters, e.g., aging time (1 day), temperature (30 °C), concentration of supernatant liquid (0.5, 1.0, and 1.5 M), amount of gel and SrCl2 (20 mL) each were kept fixed. In terms of its transparency, the gel quality was found to deteriorate with an increase in pH. The crystals growing at pH > 6 were translucent-to-opaque and ill-defined, possibly due to an uptake of silica particles by the growing matrix. A decrease in pH from 6.0 to 4.5 tends to yield dendritic morphology. The observed change in the nucleation count with gel pH is graphically illustrated in Figure 5. Once again, a higher crystal count with increasing concentration of supernatant liquids at a given gel pH is observed (Figure 2). However, the effect of gel pH at values below 2.0 could not be carried out. (5) Amount of Supernatant Liquid. Gels of a fixed density (1.04 g cm-3) and pH (4.0) were allowed to set and then age for a fixed period of 24 h. After proper setting, supernatant liquid (SrCl2) having the same concentration (0.5, 1.0, and 1.5 M) but varying in amount (8-23 mL) was incorporated above the set gel. It was found that as the amount of the supernatant liquid decreases the nucleation count decreases and the depth of the crystallization zone from the gel surface also decreases. This observation, as recorded, is shown in Figure 6. Perfection and purity of the product were also found to increase with decreasing amount of SrCl2. (6) Thickness of Neutral Gel. The solution of reaction or “growth” gel was poured to set into different test tubes, all aged for 1 day, and then above it a “neutral” gel having the same parameters as the reaction gel, with varying thickness (1.3-3.9 cm), was placed. After setting of the neutral gel, an aqueous solution of SrCl2 having 0.5, 1.0, or 1.5 M concentration was poured, and the resulting crystal count was made. Its variation with the neutral gel thickness is shown in Figure 7.

The experimental results, described above, of the dependence of the number of the nuclei on the gel parameter reveal three trends: (1) the number of nuclei increases exponentially or linearly in some cases (Figures 2 and 6), decreases exponentially in some other cases (Figure 4) while decreases practically linearly in the remaining cases (Figure 3, 5, and 7) with the gel parameter. These trends are consequences of three factors: (1) three-dimensional (3D) nucleation rate, (2) diffusion rate in the gel due to changes in the hydrostatic pressure caused by the height of supernatant solution, and (3) diffusion rate in the gel due to changes in the gel structure, e.g., pore size, cross linkages of cell boundaries, etc. The governing expression in all these cases is the 3D nucleation rate J (the number of critical sized nuclei formed per unit time per unit volume), expressed in the theory of nucleation of a nonelectrolyte18,19 by

(

J ) J0 exp -

∆G* kT

)

(2)

where the change in the activation energy ∆G* for the formation of a metastable 3D spherical nucleus of critical radius r* is given by

∆G* )

16πγ3Ω2 3(kT ln c/c0)2

(3)

In eq 3 the factor 16/3 is the so-called shape factor for a spherical nucleus, γ is the interfacial tension between the developing crystalline surface and the surrounding solution in which it is located, Ω is the volume of molecules/atom participating in nucleation, k is the Boltzmann constant, T is the temperature in Kelvin, and c/co ) S is the supersaturation ratio (co is the equilibrium solution concentration which is related to solubility product, while c is the actual solution con-



Figure 6. Nucleation count as influenced by the amount of the supernatant liquid (SrCl2) incorporated above the set gel.

Figure 7. Nucleation count as influenced by the thickness of neutral gel which is set above the growth gel.

centration which is related to the ionic strength and the height of the supernatant solution). From eq 3, it follows that the supersaturation ratio S and the interfacial surface energy γ are the possible factors that affect the isothermal rate of nucleation. However, since the gel growth occurs in the same medium, one may suppose that there is no influence of γ on the nucleation rate. Since nucleation of our compound in gels occurs as a result of reaction between two substances, it may be argued that the supersaturation for nucleation and

growth is determined by diffusion of the reactants in the gel column. This implies that, apart from the concentration of reactants that determine the value of supersaturation, diffusion processes determine the supersaturation required for nucleation. The factor controlling the diffusion processes is the structure of the gel medium itself. With increasing molarity of SrCl2 solution, the other parameters remaining constant, the probability of Sr2+ ions to react with the already available tartrate ions in

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Figure 8. Laser scattering tomogram of the grown single crystal in antiparallel (a) and parallel (b) mode taken using a He-Ne laser beam.

the medium is increased. Consequently, the relative supersaturation, and hence the crystal count, increases rapidly in the medium, as observed in Figure 2 (cf. eq 2). However, when the quantity of supernatant solution is increased, hydrostatic pressure above the gel column increases. This increased hydrostatic pressure leads to an increased diffusion of ions in the gel medium, resulting in an increased nucleation rate (see Figure 6). The increase in the nucleation rate due to increased diffusion may be described in terms of the diffusion length LD,20,21 which is directly proportional to aggregation period τ, i.e.,

LD ) (Dτ)1/2

(4)

where D is the diffusion/transport coefficient. Further, increasing the gel age has the effect of reducing the average cell size in view of evaporation of water out of the gel. This decrease in the average cell size of the gel, in its turn, results in a decrease in the diffusion length for ions in the gel column, thereby reducing the supersaturation available for nucleation. Consequently, the nucleation rate is lowered, as observed in Figure 4. An increase in the gel density is also expected to have a similar effect (Figure 3). However, it seems that the change in the supersaturation caused by an increase in the gel density is relatively small, and this results in a linear dependence rather than an exponential one. A noticeable, almost linear, decrease in the nucleation count with increasing gel pH, as seen in Figure 5, may equally be explained in an analogous manner, and may be attributed to improper formation of cells at higher pH values of gels.22 It is interesting to note that both gel pH and the neutral gel thickness show quite similar influences on the crystal count (Figures 5 and 7). The similarity suggests that the effect of an increase in the neutral gel thickness is essentially to suppress the diffusion of ions along the gel column. 5. Characterization 5.1 Crystallographic Data. Crystallinity of the grown STT crystals was confirmed by X-ray diffraction.

The crystal has a monoclinic perovskite structure with space group P21. The computed unit cell dimensions are a ) 0.755 nm, b ) 1.006 nm, c ) 0.647 nm; R ) 90°, β ) 102°, γ ) 90°. These values are in very good agreement with the data reported by Ambady.23 Further, since no elemental peak other than that of strontium was seen in the EDAX trace, it is believed that our crystals do not contain new impurities. The pycnometric density of the single crystal is found to be 2.054 g cm-3. 5.2. Laser Scattering Tomography. In this method,24 a red He-Ne laser beam is focused to a diameter of about 20 µm with microscope objective and applied onto the lateral face of crystal to receive the beam impact. Owing to the transparency of the grown crystals in the visible domain, the laser beam enters the material as a quasiparallel beam illuminating the particles located along the path. Scattered light from defects in the sample is imaged on a film surface by an object lens. Scattered light from outside the beam path is removed by a slit, just in front of the film. The linear image is detected by a CCD camera through the upper face of the sample, and it is digitized in a computer. The thickness of the tomographic surface is directly related to the size of the laser spot. The setup installed at Center for Materials Research (CMR), Stanford University, USA, was employed for precise recording of images of the inside of a crystal, using parallel and antiparallel laser beams. The example of a crystal imaged in this way is shown in Figure 8a,b. The figure reveals that the concentration of distortion/defects decreases with increasing depth from the top surface down to 486 µm. In addition, one can see more clearly in Figure 8b the extended/stretched image, representing lateral dislocations, because they stay in both parallel and antiparallel directions of the laser beam. The fact that at greater depths this density is almost negligible implies overall low dislocation concentration in the grown crystals. However, a good concentration of tiny inclusions and such small defects25 does exist close to the as-grown (100) surface itself. Further, the irregularly distributed 3D elevated fine spots, whose number density reduces again with depth



(from the top to 486 µm) and which do stay and appear as streaked with antiparallel laser beam, are dislocations parallel to the (100) face. Again, one can visualize only a few countable dislocations (102-103 cm-2), thereby implying good crystal quality. 5.3. Magnetic Susceptibility Measurements. Magnetic susceptibility provides a wealth of information26-28 on magnetic moments associated with localized inherent electrons. Therefore, measurements were carried out on the grown STT crystals at room temperature (30 °C) using Faraday’s method.29 These experiments were repeated a number of times, and the average weight loss/gain was calculated for a given electrical current. In this particular method, random orientation of particles in the sample can be assumed, since good powdering and packing procedures were followed. The gram susceptibility χg of the sample was determined using the relation,

χg ) (R + β dw)/W

(5)

where the correction factor due to displacement of air R ) kV (the volume susceptibility of air k ) 0.029 × 10-6, and V is the volume of the Gouy tube), β is the tube constant at a given electric current, W is the weight of the sample, and dw is the change in sample weight on applying the magnetic field. The calibrant used in the experiment was Hg[Co(CNS)4] whose susceptibility is 16.44 × 10-6 at 20 °C. The values obtained for STT are

gram susceptibility χg ) -9.73 × 10-8 molar susceptibility χm ) -2.82 × 10-5 It was observed for the powdered STT samples that an increase in the magnetic field intensity results in a noticeable decrease in the weight of the sample. Also, the susceptibility is found negative, implying a diamagnetic behavior of the material and that the induced magnetic moments tend to oppose the applied field. 6. Conclusions Silica gel provides indeed a suitable medium for the growth of large size, well-faceted, and transparent crystals of STT. The process of crystallization follows classical laws of the 3D nucleation and diffusion theory. The nucleation count decreases with an increase in the specific gravity of the gel, aging period, and pH as well as the neutral gel thickness. An increase in the concentration and the amount of SrCl2, however, results in a faster increase of the nucleation count. Acknowledgment. S.K.A. is highly grateful to Prof. Robert S. Feiglson for hosting his visit to CMR, Stanford University, USA, and to Dr. Robert De Mattei for the kind help in carrying out laser scattering tomography experiments on our crystals. V.P. is thankful to Depart-

ment of Science & Technology, New Delhi, for the award of JRF. Thanks are also due to the anonymous referee for his critical comments and useful suggestions on the manuscript.

References (1) Henisch, H. K. In Crystal Growth in Gels; Pennsylvania State University Press: University Park, PA, 1973; p 20. (2) Henisch, H. K. In Crystals in Gels and Liesegang Rings; Cambridge University Press: Cambridge, 1988. (3) Liaw, H. W.; Faust, J. W., Jr. In American Conference on Crystal Growth; Gaithersburg, Maryland, 1969. (4) Liaw, H. W.; Faust, J. W., Jr. J. Cryst. Growth 1972, 13/ 14, 471. (5) Brehat, F.; Wyncke B. J. Phys. B: At. Mol. Opt. Phys. 1989, 22, 1981. (6) Nakatani, N. Jpn. J. Appl. Phys. 1991, 30, 1961. (7) Rethinam, F. J.; Arivuoli, D.; Ramasamy, S.; Ramasamy, P. Mater. Res. Bull. 1994, 29, 309. (8) Selvarajan, P.; Das, B. N.; Gon, H. B.; Rao, K. V. J. Mater. Sci. Lett. 1993, 12, 1210. (9) Torres, M. E.; Lo´pez, T.; Peraza, J.; Stockel, J.; Yanes, A. C.; Gonza´lez-Silgo, C.; Ruiz-Pe´rez, C.; Lorenzo-Luis, P. A. J. Appl. Phys. 1998, 84, 5729. (10) Torres, M. E.; Lo´pez, T.; Stockel, J.; Solans, X.; GarciaValle´s, M.; Rodriguez-Castello´n, E.; Gonza´lez-Silgo, C. J. Solid State Chem. 2002, 163, 491. (11) Rahimkutty, M. H.; Rajendra Babu, K.; Shreedharan Pillai, K.; Sudarsana Kumar, M. R.; Nair, C. M. K. Bull. Mater. Sci. 2001, 24, 2, 249. (12) Suryanarayana, K.; Dharmaprakash, S. M. J. Phys. Chem. Solids 1997, 58, 10, 1599. (13) Joshi, V. S.; Joshi, M. J. Indian J. Phys. 2001, 75A, 159 (14) Suryanarayana, K.; Dharmaprakash, S. M.; Sooryanarayana, K. Bull. Mater. Sci. 1998, 21, 87. (15) Arora, S. K.; Abraham, T. J. Cryst. Growth 1981, 52, 851. (16) Arora, S. K. J. Prog. Cryst. Growth Charact. 1981, 4, 345. (17) John, M. V.; Ittyachen, M. A. Bull. Mater. Sci. 1998, 21, 387. (18) Sangwal, K., Ed. Elementary Crystal Growth; Saan Publishers: Lublin, Poland, 1994; p 46. (19) Mullin, J. W. Crystallization, 4th ed.; Butterworth-Heinemann: Oxford, U.K., 2001; p 182. (20) Pimpinelli, A.; Villain, J. Physics of Crystal Growth; Cambridge University Press: Cambridge, U.K. 1998; p 181. (21) Omar, M. A. Elementary Solid State Physics, 1st Indian reprint; Addison-Wesley Publishers: Reading, MA, 2000; p 310. (22) Desai, C. C.; Ramana, M. S. V. J. Cryst. Growth 1990, 102, 191. (23) Ambady, G. K. Acta Crystallogr., Sect B 1968, 24, 1548. (24) Moriya, K. Observation of Defects in Crystals by Laser Scattering Tomography; Mitsui Mining & Smelting Co., Ltd.: Japan, 1989. (25) Nango, N.; Furuya, H.; Furukawa, J.; Ogawa, T. Jpn. J. Appl. Phys. 1995, 78, 2892. (26) Mott, N. F. Rev. Mod. Phys. 1968, 40, 677. (27) Austin, I. G.; Elwell, D. Contemp. Phys. 1970, 11, 455. (28) De Vries, A. B.; Hass, C. J. Phys. Chem. Solids 1973, 34, 1651. (29) Bates, L. F. In Modern Magnetism; Cambridge University Press: London, 1951.

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