Growth of K(D x H 1 x ) 2 PO 4 Single Crystals from Solutions in the K 2 O P 2 O 5 (D,H) 2 O System

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1 Crystallography Reports, Vol. 47, No., 00, pp Translated from Kristallografiya, Vol. 47, No., 00, pp. 44. Original Russian Text Copyright 00 by Soboleva, Voloshin, Kirikov, Biglova. CRYSTAL GROWTH Growth of K(D x H x ) PO 4 Single Crystals from Solutions in the K O P O 5 (D,H) O System L. V. Soboleva, A. E. Voloshin, V. A. Kirikov, and M. V. Biglova Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskiœ pr. 59, Moscow, 7333 Russia voloshin@ns.crys.ras.ru Received December 7, 000; in final form, April 5, 00 Abstract Based on the analysis of the K O P O 5 D O solubility phase diagram, the optimum conditions of KD PO 4 crystallization the compositions of mother solutions and the temperature range of crystallization in the KH PO 4 D O system have been determined. The technique of K(D x H x ) PO 4 growth is developed. The DKDP single crystals with deuterium concentration up to 88 wt % are grown on DKDP seeds from KH PO 4 solutions in D O by the method of temperature decrease. 00 MAIK Nauka/Interperiodica. INTRODUCTION Crystals of the KH PO 4 (KDP) family, in particular KDP and K(D x H x ) PO 4 (DKDP), are widely used in various electrooptical and nonlinear laser devices. The DKDP crystals may exist in two polymorphous modifications the tetragonal modification (the symmetry class 4m ) isomorphous to KDP and the monoclinic one (the symmetry class ). It is known that the electrooptical properties of tetragonal DKDP crystals are much better than those of KDP []. A partial replacement of hydrogen in KH PO 4 by deuterium leads to the formation of mixed K(D x H x ) PO 4 crystals with physical properties dependent on the deuterium concentration possessing ferroelectric phase transitions []. This study is aimed at () the determination of the optimum growth conditions (the optimum composition of the mother solutions, the temperature range of crystallization) from the analysis of the known physical and chemical data of the solubility phase diagrams K O P O 5 H O, K O P O 5 D O, and KH PO 4 D O (5 C); () the development of the method of growth of K(D x H x )PO 4 crystals in dynamic mode; and (3) growth of DKDP crystals from solutions in the KH PO 4 D O system. CHOICE OF THE SOLUTION COMPOSITIONS AND GROWTH OF K(D x H x ) PO 4 SINGLE CRYSTALS Development of the method of K(D x H x ) PO 4 growth from the solutions of the K O-P O 5 (D x H x ) O system is based on the analysis of the physical and chemical characteristics of the solubility phase diagram of the ternary K O P O 5 H O [, 3] and K O P O 5 D O systems [4]. As is shown in [5], the phase diagrams of the ternary systems can be used to determine the optimum growth conditions for large stoichiometric solid single crystals of optical quality in the dynamic mode. The solubility phase diagram of the ternary K O P O 5 H O system [, 3] has nine branches of solubility curves corresponding to different compositions of solid phases (Fig. ). The KH PO 4 solid phase is dissolved congruently and is stable in the temperature range from 0 to C. Its solubility curve has a singular point A corresponding to the molar ratio K O : P O 5 = :. The solution composition corresponding to the A point on the solubility curve remains unchanged with the variation of equilibrium conditions, C K O, wt % A C P O 5, wt % K : P = : Fig.. Isotherms of KH PO 4 solubility in the ternary K O P O 5 H ) system [, ] at 0 (curve ), 5 (curve ), and C (curve 3); A is a singular point /0/4-030$ MAIK Nauka/Interperiodica

2 GROWTH OF K(D x H x ) PO 4 SINGLE CRYSTALS 3 K O, mol % K : P = : A P O 5, mol % Fig.. Solubility isotherms in the ternary () K O P O 5 H O, () K O P O 5 D O systems at 5 C; A is a singular point. which provides the crystallization of KH PO 4 from the KH PO 4 H O solutions in the temperature range from 0 to C [6]. In the K O P O 5 D O system, which is studied at 5 C [4] (Fig. ), eight different solid phases are formed, and the phase diagram shows a complete analogy with the branches of the solubility curves of solid phases in the K O P O 5 H O system [] with the KD PO 4 phase being congruently soluble. It is found that the KD PO 4 phase dissolves in heavy water better than the KH PO 4 phase in H O, which can be explained by the formation of the stronger deuterium bonds (more stable with respect to hydrogen ones) 3 between the PO 4 ions and the D O molecules [4]. The determination of the optimum growth conditions for DKDP crystals is based on the closeness of the physical and the chemical characteristics of the solubility isotherms of KH PO 4 and KD PO 4 crystals in the ternary K O P O 5 H O and K O P O 5 D O systems at 5 C [, 4] (Fig. ). Thus, it is possible to construct the solubility branch for KD PO 4 at C, with due regard for the fact that the KH PO 4 solubility at C is about 5% higher than at 5 C (Fig. ). The solubility isotherms of the K O P O 5 H O system at C and the K O P O 5 D O system at 5 and C are shown on the triangular Gibbs diagram in Fig. 3 to facilitate the use of the concentration data of their components. On this diagram, point corresponds to the saturation point of the KH PO 4 solution at C (K O =.00, P O 5 = 5.00, and H O = wt %); H O, D O KOH H O K O K 3 PO 4 3H O 3K HPO 4 KH PO 4 H O K H PO wt % CRYSTALLOGRAPHY REPORTS Vol. 47 No. 00 KD PO 4 KH 5 (PO 4 ) KH PO 4 K HPO 4 KH PO 4 H O Fig. 3. Solubility isotherms of KH PO 4 in the K O P O 5 H system at C (crosses); KD PO 4 in the K O P O 5 D O system at 5 C (circles) and C (dashed curve); () the composition of the saturated KH PO 4 solution at C; () the composition of the saturated KD PO 4 solution at 5 C; (3) the probable saturation point in the KD PO 4 solution at C; and (4) the point corresponding to the KH PO 4 supersaturation in the KH PO 4 D O system at C. P O 5

3 3 SOBOLEVA et al. Compositions of the solution corresponding to points, 3, and 4 in the K O P O 5 D O and KH PO 4 D O systems Component, wt % point is the saturation point of the KD PO 4 solution at C; and 4 is the point corresponding to a certain supersaturation of the KH PO 4 solution in the KH PO 4 D O system at C. The solubility of KD PO 4 in D O at C calculated from the known data [, 7] is equal to wt % of KDP and wt % of D O, which is close to the composition of point 4. The ternary K O P O 5 D O system used for growth of K(D x H x ) PO 4 crystals with different deuterium concentration allows one to use K O, P O 5, and D O components (in the form of KOH, H 3 PO 4, and D O solutions) in certain weight ratios to prepare necessary mother solutions. The binary KH PO 4 D O system in the ternary K O P O 5 D O system allows one to use KH PO 4 and D O as the initial components for preparing mother solutions. Using this approach, airtight setup, and a number of necessary kinetic parameters, we managed to grow K(D x H x ) PO 4 crystals with the maximum possible deuterium concentration. x, % 0 0 Points 3 4 K O P O 5 D O P O K O D O KH PO 4 D O KH PO D O t, C Fig. 4. Regions of existence of () the monoclinic and () the tetragonal phases in the K(H,D) PO 4 solutions; data [5] are shown by open circles; data [], by filled circles; data [] by crosses; and x is the degree of solution deuteration. The solution compositions at points, 3, and 4 are listed in the table and show close values of KH PO 4 and KD PO 4 solubility in D O. The compositions of the components in the KH PO 4 D O system are determined by the lever rule [8] and are also listed in the table. The data obtained lead to the conclusion that the compositions of mother solutions used for growing K(D x H x ) PO 4 crystals with the maximum deuterium concentration from the solutions in the KH PO 4 D O system correspond to the compositions of the saturated solutions at point 3 and certain supersaturation at point 4 on the K O P O 5 (H,D) O solubility phase diagram at C (see table). Polymorphism of DKDP crystals hinders the choice of growth conditions for the tetragonal DKDP crystals, since the existence of the tetragonal or the monoclinic phase in the mother solution depends on the temperature and the deuterium concentration in this solution. We analyzed the data [9 7] on the dependence of crystallization of the tetragonal and the monoclinic phases of KD PO 4 crystals and their mixture (mainly, under the conditions of spontaneous crystallization) on the degree of deuteration ( 0%), ph-value (.6 4.) and the temperature (5 0 C) of the K(H,D) PO 4 solutions in (H,D) O. It is shown that under the above conditions, there are three crystallization regions those of the tetragonal and the monoclinic K(H,D) PO 4 phases and the mixture of these two phases (Figs. 4, 5). The tetragonal phase is stable below the solubility curve of K(H,D) PO 4 in the temperature range from 0 to 0 C, ph 4, and the deuteration degree from to 0% (Fig. 4). Similar features are also seen in Fig. 5; in this case, the decrease in from 4. to.8 results in the decrease of the existence region of the tetragonal phase in terms of temperature (7 C) and deuterium concentration (85 0%). Based on the analysis of the known data and preliminary experiments on DKDP crystallization in the dynamic mode, we used the mother solutions with the following characteristics: the temperature ranging within C, ph, ranging within 5 3, and the deuteration degree, about 98%. The K(D x H x ) PO 4 single crystals were grown from solutions on seeds by the temperature decrease method. To prepare the mother solution for growing DKDP single crystals, we used a high-purity grade KH PO 4 and D O with the deuterium concentration of about 98%. The initial composition of the mother solution corresponded to the composition of point 4: KDP concentration wt % and D O concentration wt %. The DKDP single crystals were grown from solutions containing 67 g KDP in 300 ml D O (i.e., the composition corresponding to point 4) with the decrease of the temperature from to 45 C at the rate CRYSTALLOGRAPHY REPORTS Vol. 47 No. 00

4 GROWTH OF K(D x H x ) PO 4 SINGLE CRYSTALS 33 x l, % ph 4. ph.8 0 t, C Fig. 5. Regions of spontaneous crystallization of solid phases in the K(H,D) PO 4 solutions; the tetragonal phase is shown by crosses; the monoclinic phase, by triangles; and the mixture of two phases, by open circles; x is the degree of solution deuteration. x s of temperature decrease equal to C per day (4 h), and ph ranging within 3 5 (the variation in ph was attained by addition of (D,H) 3 PO 4 ). The reaction in the solution ( C) proceeded by the scheme D KH PO 4 + D O O K(D x H x ) PO 4 + H O. () H O x l Fig. 6. Distribution of deuterium between the K(H,D) PO 4 crystals and the saturated solution. Curves are plotted by the data reported by different authors in accordance with the equation given in [7]. Open circles indicate data [4] at 30 C; triangles, data at 45 C; half-filled circles, data at C; rhombuses indicate data [5]; crosses, data [6]; filled circles, data [7]; and x s is the degree of crystal deuteration, and x l is the degree of solution deuteration. In comparison with the stoichiometric concentration of the components in the reaction (), we had to use an amount of D O about times higher in the mother solutions. The deuterium concentration in the crystals grown was determined from the temperature of the tetragonal orthorhombic phase transition and was equal to wt %. Proceeding from the exchange reaction [3], H O + D O HDO () and the deuterium concentration in crystals grown, we can rewrite reaction () in the schematic form as D KH PO 4 + D O O K( D 0.87 H 0.3 ) PO H 4 O (3) + ( H.7 D 0.8 ) O+ D O. It is known that the deuteration degree in K(D x H x ) PO 4 crystals is lower than in mother solutions. This leads to the assumption [9] that the change in deuterium concentration occurs during crystal growth. Proceeding from data [7] (Fig. 6) and the maximum deuterium concentration (~88 wt %) in the crystals grown, we established that in the growth process the concentration of deuterium in the mother solution was about 9 wt %. CONCLUSIONS The analysis of the physical and chemical characteristics of the formation of the KD PO 4 solid phase in the K O P O 5 D O and KH PO 4 D O systems in the temperature range 5 C has been performed. The method for growing K(D x H x ) PO 4 crystals in dynamic mode has been developed. The optimum conditions for growing DKDP crystals have been determined. The K(D x H x ) PO 4 single crystals with the maximum deuterium concentration up to 88 wt % have been grown from the KH PO 4 solutions in (D,H) O on a DKDP seed by the temperature decrease method. REFERENCES. L. N. Rashkovich, KDP-Family Single Crystals (Adam Hilger, Bristol, 99).. L. G. Berg, Izv. Akad. Nauk SSSR, Ser. Khim., No., 47 (938). 3. L. G. Berg, Izv. Akad. Nauk SSSR, Ser. Khim., No., (938). 4. L. V. Barkova and I. N. Lepeshkov, Zh. Neorg. Khim. 3 (5), 43 (968). 5. L. V. Soboleva, Neorg. Mater. 3 (5), 64 (995). 6. L. V. Soboleva, Neorg. Mater. 3 (8), 07 (996). 7. L. V. Barkova and I. N. Lepeshkov, Zh. Neorg. Khim. (6), 4 (966). CRYSTALLOGRAPHY REPORTS Vol. 47 No. 00

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