Study of phase equilibria in the Rb 3 H(SO 4 ) 2 -RbH 2 PO 4 -H 2 O system

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Study of Phase Equilibria in the Rb3H(SO4)2–RbH2PO4–H2O System V. A. Komornikova, V. V. Grebeneva, P. V. Andreevb, and E. V. Dmitrichevaa a

Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskii pr. 59, Moscow, 119333 Russia b Lobachevsky State University, Nizhni Novgorod, 603950 Russia e-mail: [email protected] Received November 12, 2014

Abstract—Phase equilibria in the Rb3H(SO4)2–RbH2PO4–H2O system have been studied by the method of additives under isothermal conditions (at 25.0°С). The limits of the crystallization ranges of the compounds are preliminarily determined. Concentration conditions for the crystallization of Rb2(HSO4)(H2PO4) and Rb4(HSO4)3(H2PO4) compounds are found. DOI: 10.1134/S1063774515030086

INTRODUCTION The proton subsystem and its disorder in crystals are of fundamental scientific interest. Superprotonic crystals—hydrogen-containing salts with the general formula MmHn(AO4)(m + n)/2 ⋅ xH2O (where M = Cs, Rb, K, NH4, A = S, Se, P, As)—can be considered model objects for studying the proton subsystem. In contrast to other superprotonic materials, hydrogen bonds in crystals of this group can be delocalized as a result of phase transitions; this delocalization results in high intrinsic proton conductivity. In this case, the proton disorder is a property of the system rather than a consequence of doping [1]. The superprotonic conductivity was found for the first time in CsHSO4 [2] and CsH2PO4 [3] crystals in the 1980s. Afterwards, crystals of the MmHn(AO4)(m + n)/2 ⋅ xH2O family have attracted much attention. A large number of studies were devoted to the structural features of phases with high proton conductivity, mechanisms of high proton conductivity, and the properties of these crystals. The most complete review can be found in [1]. It should be noted that there are only a few studies on the search for and preparation of new compounds with proton conductivity in the MmHn(AO4)(m + n)/2 ⋅ xH2O family, in contrast to the known proton conductors. For example, the CsHSO4–CsH2PO4––H2O water–salt system was systematically investigated only recently [4]. As a result of that investigation, a new compound, Cs6(H2SO4)3(H2PO4)4, with high proton conductivity was found [5, 6]. In this family of crystals, the RbHSO4–RbH2PO4–H2O system is also promising for both fundamental and applied research. No systematic investigations of phase equilibria at specific temperatures aimed at searching for new com-

pounds have been performed for this system. What is more, the only known compounds of this system, Rb2(HSO4)(H2PO4) and Rb4(HSO4)3(H2PO4) [7], were grown by analogy with the compounds βCs3(HSO4)2(H2 – x(P1 – xSx)O4) [8] and Cs2(HSO4)(H2PO4) [9] of the cesium system. The structures of Rb2(HSO4)(H2PO4) and Rb4(HSO4)3(H2PO4) were solved at room temperature; however, their properties were not investigated in detail. Thus, the purpose of our study is to determine the limiting conditions for the crystallization from the aqueous solutions of compounds in the Rb3H(SO4)2–RbH2PO4–H2O system