Growth of a-niso 4 6H 2 O Crystals at High Rates

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1 Crystallography Reports, Vol. 50, No. 5, 2005, pp Translated from Kristallografiya, Vol. 50, No. 5, 2005, pp Original Russian Text Copyright 2005 by Manomenova, Rudneva, Voloshin, oboleva, Vasil ev, Mchedlishvili. CRYTAL GROWTH Growth of a-nio 4 6H 2 O Crystals at High Rates V. L. Manomenova, E. B. Rudneva, A. É. Voloshin, L. V. oboleva, A. B. Vasil ev, and B. V. Mchedlishvili hubnikov Institute of Crystallography, Russian Academy of ciences, Leninskiœ pr. 59, Moscow, Russia labsol@ns.crys.ras.ru Received February 10, 2005 Abstract The high-rate growth of nickel sulfate hexahydrate NiO 4 6H 2 O (α-nh) crystals up to mm 3 in size is described for the first time. The data on the distribution of related impurities in the {011} and {001} growth sectors of α-nh crystals grown at different rates are reported. The transmission spectra of both growth sectors of these crystals are obtained. The structural quality and the optical properties of rapidly and slowly grown α-nh crystals are compared. It is established that the {011} growth sector of crystals grown at rates exceeding 5 mm/day shows the best characteristics for application in UV filters Pleiades Publishing, Inc. INTROUCTION Crystals of nickel sulfate hexahydrate NiO 4 6H 2 O (α-nh) are widely used in optics. These crystals exhibit optical rotatory dispersion in the wavelength range nm [1] and circular dichroism in the visible spectral range [1, 2]. In addition, inversion of the sign of the rotatory power is observed in α-nh at a wavelength of 503 µm [4]. Crystals of the α-nh phase are one of the most effective band filters in the solar blind spectral range. Their effectiveness is related to their unusual bandlike absorption spectrum in the range nm, which contains only three transmission bands (peaked at 250, 490, and 880 nm). At the same time, the transmission in the UV region exceeds 80% [4]. The NiO 4 6H 2 O compound has two polymorphic modifications: α-nh and β-nh. Crystals of α-nickel sulfate hexahydrate NiO 4 6H 2 O have a saturated blue-green color. They belong to the tetragonal system with the sp. gr. P and unit-cell parameters a = b = Å and c = Å [4]. Figure 1 shows the habit of α-nh crystals. Crystals of the β-nh modification are bright green; they belong to the monoclinic system with the sp. gr. C2/c and unit-cell parameters a = Å, b = Å, c = Å, β = 9.88, and Z = 8 [4]. Crystals of the α-nh phase decompose in air, losing water, even at room temperature [5]. Figure 2 shows three portions of the solubility curves corresponding to the solid phases NiO 4 7H 2 O, α-nio 4 6H 2 O, and β-nio 4 6H 2 O [5, 6]. The point X at a temperature of 31.5 C is the point of the NiO 4 7H 2 O α-nio 4 6H 2 O + H 2 O liq phase transition [7], and the point Y at 53.3 C is the point of the α-nio 4 6H 2 O NiO 4 6H 2 O phase transition. The phase β-nio 4 6H 2 O is stable at temperatures up to 118 C [5]. A number of studies are devoted to the growth of α-nh crystals from aqueous solutions [4, 8, 9]. Both six-water nickel sulfate NiO 4 6H 2 O [4, 9] and sevenwater nickel sulfate NiO 4 7H 2 O were used as initial raw material for crystal growth [8]. In [4, 9], crystals were grown from solutions of NiO 4 6H 2 O of stoichiometric composition. Crystals mm 3 in size were grown by the method of decreasing temperature. (113) X (001) (102) (101) Z (012) (011) Fig. 1. Habit of an α-nh crystal [4]. (110) Y /05/ $ Pleiades Publishing, Inc.

2 878 t, C 120 MANOMENOVA et al. H 2 O 4, wt % NiO 4, wt % X Y NiO 4 7H 2 O α-nh β-nh H 2 O A NiO 4 6H 2 O 80 NiO 4, wt % Fig. 2. olubility of NiO 4 hydrates in water [5]. Fig. 3. Phase solubility diagram of the NiO 4 H 2 O 4 H 2 O system at 50 C [11]. In [8], on the basis of the analysis of the NiO 4 H 2 O 4 H 2 O system, the solution composition corresponding to the point A in Fig. 3 (in wt %: NiO 4, 28.00; H 2 O 4, 13.00; H 2 O, 59.00) was chosen for the crystal growth. It was shown previously [10] that, to grow crystals of complex compounds, it is reasonable to use mother liquors whose compositions (in wt %) lie on the solubility curve at the maximum distance from the points of invariant equilibria. The composition used in [8] was chosen according to this principle. Crystals up to mm 3 in size were grown by the method of decreasing temperature. In the above-mentioned studies, the crystal growth was performed in the dynamic mode at temperatures from 53 to 32 C; the rate of decrease in temperature of the solutions did not exceed 1 K/day, and the growth rate of α-nh crystals in the [001] direction was no more than 1 mm/day. The purpose of this study is to develop a method of rapid growth of large α-nh single crystals of good optical quality and investigate the transmission spectra, the content of related impurities, and the actual defect structure of the crystals grown. EXPERIMENTAL To grow α-nh crystals, we used hermetically sealed crystallizers with volumes from 1 to 5 l equipped with an automatic cooling system. The temperature in the growth crystallizers was controlled by microcontrollers, which provided an accuracy of maintaining temperature of ±0.02 K. olutions prepared from seven-water nickel sulfate NiO 4 7H 2 O of reagent grade, sulfuric acid H 2 O 4 of high purity grade, and triply distilled H 2 O (taken in different ratios) were used as mother liquors. The saturation temperature of the solutions was C. Immediately before filtration, the solutions were superheated by K above the saturation temperature. Crystals were grown from aqueous solutions by cooling. The seed crystals (plates in the (001) cut, mm 3 in size, cleaved in the (001) cleavage plane) were installed either on immobile platforms (in this case, solutions were stirred by agitators) or on rotating platforms. tirring of solutions was reversible; the stirring rate ranged from 30 to 60 rpm. The concentrations of impurities in α-nh crystals were analyzed by spark mass spectrometry on a JM- 01-BM2 double-focusing mass spectrometer (JEOL, Japan). The mass spectra were recorded on UV-4 photographic plates. Fig. 4. Rapidly grown α-nh crystals.

3 GROWTH OF α-nio 4 6H 2 O CRYTAL 879 Table 1. Growth parameters for α-nh crystals grown in different temperature regimes Crystal aturation temperature of solution, C Rate of solution stirring, rpm Average cooling rate, K/d R [001], mm/d Growth duration, d Crystal mass, g Crystal sizes x y z, mm To measure the transmission spectra of the samples studied, we used a PECOR UV VI automatic double-beam spectrophotometer, which makes it possible to measure transmission spectra in the range from 185 to 800 nm. The structure of the crystals grown was analyzed by Lang X-ray topography in MoK α1 radiation using P-50 photographic plates intended for nuclear study. REULT AN ICUION Figure 3 shows the 50 C isotherm for the ternary system NiO 4 H 2 O 4 H 2 O [12]. Initially, α-nh crystals were grown from solutions of the same composition as in [8] (in wt %): NiO 4, 28.00; H 2 O 4, 13.00; and H 2 O, (Fig. 3, point A on the solubility isotherm). However, when attempts were made to increase the rate of growth of α-nh crystals from the noted solutions by increasing the solution supercooling from 0.3 to 1.6 K (which corresponds to the change in the supersaturation from 0.4 to 1.9%), the crystals grown contained microcracks and inclusions of mother liquor. The supersaturation was calculated by the formula σ c r ( T) c 0 ( T) = c 0 ( T), where c 0 (T) and c r (T) are, respectively, the equilibrium and real concentrations of a solution in molar fractions at temperature T. With a further increase in supersaturation, spontaneous crystallization from solution occurs. In this context, we carried out a series of experiments aimed at determining the solution composition applicable for rapid growth of α-nh crystals. As a result, a solution composition was determined that made it possible to reach the supersaturation σ = 4 5.5% without intense nucleation. A series of α-nh crystals up to mm 3 in size (Fig. 4) were grown from solutions of this composition in different temperature regimes. The average cooling rates during rapid growth and slow growth were 5 7 and K/day, respectively. The normal growth rate of faces R in the direction [001], R [001], was 5 7 and mm/day during rapid and slow growth, respectively. Table 1 contains the growth parameters for three crystals grown in 1-l crystallization vessels. We compared the structural quality and optical properties of rapidly and slowly grown crystals. From the point of view of application of the crystals under study as filters in the mid-uv range, they should have maximum transmission in this wavelength range and minimum transmission in the visible spectral range. To compare the efficiency of UV transmission of different growth sectors of α-nh crystals, we measured the optical transmission spectra of samples cut from the {011} and {001} growth sectors of the crystals grown at different rates (Fig. 5). (These sectors occupy the major part of a crystal.) All samples were 5-mm-thick plates cleaved in the (001) cleavage plane. It was found that the {011} growth sector of rapidly grown crystals has the best optical characteristics: its transmittance T is 85 86% at λ = nm (shortrange UV) and relatively low in the visible range. The UV transmittance of the {001} sector of rapidly grown crystals is comparable in magnitude with that of the {011} and {001} growth sectors of slowly grown crystals (T ~ 75 77%). In the visible spectral range, the T, % λ, nm Fig. 5. pectral characteristics of the {011} and {001} growth sectors of α-nh crystals grown at different rates: the {001} sector of rapidly (dashed line) and slowly (dash dotted line) grown crystals and the {011} sector of rapidly (solid line) and slowly (dotted line) grown crystals.

4 880 MANOMENOVA et al. g[004] (a) g[020] (b) ZI 1 cm (c) (d) g[020] g[004] V 1 cm Fig. 6. The X-ray projection topographs of (100) cuts of (a, b) slowly and (c, d) rapidly grown α-nh crystals. esignations: () seed, () sectorial boundaries, () dislocations, (ZI) zonal inhomogeneities, (V) vicinal sectorial boundary; g is the diffraction vector. transmittances of both growth sectors of slowly grown crystals exceed that of the rapidly grown crystals. One of possible factors affecting the optical characteristics of crystals is the presence of unintentional impurities. Therefore, we analyzed their content in the crystals grown. The concentrations of most studied impurities, such as Be, Na, P, c, Ti, V, Cr, Mn, Ga, Ge, As, e, Br, Rb, r, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, n, b, Te, Cs, Ba, Hf, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, Th, U, and all lanthanides, were below the detection limit (< wt %). Table 2 contains the data on the distribution of impurities whose concentration exceeds wt %. Mass-spectrometric analysis shows that the major impurities in the crystals under study are Zn, Co, Cu, I, F, and Cl (> wt %). Impurities of Mg, Ca, Fe, Al, i, and B are present in smaller amounts. The high content of Zn, Co, Cu, Fe, and Mg may be related to the large values of the distribution coefficients of these impurities, which are very likely to exceed unity. The impurity distributions in α-nh crystals grown at different rates are different. The data of Table 2 indicate that the maximum content of Zn, Co, Cu, Al, Mg, I, and B is found in rapidly grown crystals; note that in the {001} growth sector the concentrations of Zn, Cu, Al, and B are several times higher than in the {011} sector. For Mg and Co impurities, the situation is directly opposite: their concentrations in the {001} growth sector of the rapidly grown crystals are lower by a factor of In slowly grown crystals, the content of related impurities is the same in both growth sectors. Higher concentrations of the halogens Cl and F are observed in

5 GROWTH OF α-nio 4 6H 2 O CRYTAL 881 Table 2. istribution of impurities in α-nh crystals grown by different methods Element Concentration, 10 4 wt % rapidly grown α-nh slowly grown α-nh {011} sector {001} sector {001} and {011} sectors Li <0.01 < B Be <0.005 <0.005 <0.005 F Mg Al <0.07 i Cl K <0.1 <0.1 3 Ca Fe Co Cu <0.1 Zn I these crystals in comparison with the rapidly grown ones. uch a complex redistribution of unintentional impurities in the growth sectors of α-nh crystals grown at different rates makes it impossible to draw unambiguous conclusions about the effect of specific impurities on the transmission spectra of these crystals. It is well known that the optical properties of crystals are also significantly affected by their internal defect structure. To compare the quality of rapidly and slowly grown α-nh crystals, we obtained a series of X-ray projection topographs. The topographs in Fig. 6 demonstrate the typical structure of crystals grown at different rates. The {011} growth sector of a slowly grown crystal contains distinct stripes of zonal inhomogeneity (Fig. 6b), which may be responsible for the decrease in the transmission of this sector in the visible spectral range in comparison with the {001} sector. A large number of growth dislocations directed from the seed region perpendicular to the (001) face are observed in the {001} growth sector of a slowly grown crystal (Fig. 6b). The presence of one vicinal sectorial boundary in this sector suggests the presence of only one growth hillock on the surface of the (001) face and, on the whole, is indicative of rather high homogeneity of this sector. ectorial boundaries, dividing different growth sectors, can be seen in both topographs. Figures 6c and 6d show the defect structure of the rapidly grown crystals. The image of the sectorial boundary between the {011} and {001} growth sectors has a brighter contrast. On the basis of the change in the contrast at the sectorial boundaries, we may conclude that the lattice parameters in the {011} growth sector are smaller than in the {001} sector. In the {011} sector, there are no growth defects for both reflection vectors. In the {001} sector of a rapidly grown crystal, the concentration of growth dislocations is much higher than in the same sector of a crystal grown at a lower rate. The broken line, corresponding to the vicinal sectorial boundaries, indicates the presence of competing growth hillocks. Under these conditions, the formation of inclusions of the solution [12], which lead to higher scattering of radiation, is very likely. We believe this mechanism to be the most likely reason for the decrease in the transmission of the samples cut from the {001} growth sector. Analysis of the topographs showed that the {011} growth sector of the rapidly grown crystal is more uniform (also compared to the sectors of the slowly grown crystal). This circumstance is, in turn, in good agreement with the obtained maximum values of the transmittance of the {011} sector of the rapidly grown crystals. CONCLUION Transparent α-nh single crystals up to mm 3 in size were grown at high rates. (The normal growth rate is up to 7 mm/day.) The transmission spectra of the {011} and {001} growth sectors of crystals grown at different rates were measured. It is shown that the {011} sector in the crystals grown at rates above 5 mm/day has the best characteristics to be used in UV filters. This conclusion is in agreement with the results of the analysis of X-ray projection topographs, which showed a high degree of structural quality of this sector of rapidly grown crystals. A difference in trapping of impurities by {001} and {011} faces during rapid growth is revealed. ACKNOWLEGMENT This study was supported by the president of the Russian Federation (the program Leading cientific chools, project no. Nh ). REFERENCE 1. Ya. O. ovgiœ and I. G. Man kovskaya, Fiz. Tverd. Tela (t. Petersburg) 40 (9), 1608 (1998) [Phys. olid tate 40, 1460 (1998)]. 2. J. R. L. Moxon, A. R. Renshaw, and I. J. Tebbutt, J. Phys. : Appl. Phys. 24, 1187 (1991). 3. K. tadnicka, A. M. Glazer, and M. Koralewski, Acta Crystallogr., ect. B: truct. ci. 43, 319 (1987). 4. H. Youping,. Genbo, Y. Xianchum, et al., J. Cryst. Growth 169, 193 (1996).

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