Phase relations in the Me2MoO4?In2(MoO4)3?Hf(MoO4)2 systems, where Me = Li, K, Tl, Rb, Cs 126 D O I: 1 0. 15 82 6/ ch im te ch .2 01 8. 5. 3. 01 J. G. Bazarova*, Yu. L. Tushinova, V. G. Grossman, Ts. T. Bazarova, B. G. Bazarov, R. V. Kurbatov Baikal Institute of Nature Management Siberian Branch of the Russian Academy of Sciences 8 Sakh’yanovoi St., Ulan-Ude, Buryat Republic, Russian Federation *E-mail: jbaz@binm.ru Phase relations in the Me2MoO4–In2(MoO4)3–Hf(MoO4)2 systems, where Me = Li, K, Tl, Rb, Cs The Me2MoO4–In2(MoO4)3–Hf(MoO4)2 systems where Me = Li, K, Tl, Rb, Cs were studied in the subsolidus region using an X-ray powder diffraction. Quasi-binary joins were revealed, and triangulation carried out. The formation of ternary mo- lybdates Me5InHf (MoO4)6 for Me = K, Tl, Rb, Cs and Мe2InHf2(MoO4)6.5 for Me = Rb, Cs was established. Keywords: phase relations, triangulation, solid-phase reactions, X-ray phase diffraction, molybdates. Received: 26.09.2018. Accepted: 12.10.2018. Published: 31.10.2018. © Bazarova J. G., Tushinova Yu. L., Grossman V. G., Bazarova Ts. T., Bazarov B. G., Kurbatov R. V., 2018 Introduction Ternary molybdates attract atten- tion due to their catalytic and ionexchange properties and the diversity of their crystal structures. The MoO6 octahedra are usually highly distorted because of the relatively small effective radius of the Mo6+ ion in the oxygen environment, which is favorable for the formation of low-symmetry crystal structures. Systematic studies of multicomponent systems allow obtaining the large amount of  data which make it possible to  iden- tify regularities of  the phase formation in related systems. In our previous works, the phase equilibria in the Me2MoO4–R2 (MoO4)3–Hf(MoO4)2 (Me = Rb, Cs; R — trivalent metals) systems were studied [1, 2]. The purpose of this work was to estab- lish the phase formation in the Me2MoO4– In2(MoO4)3–Hf (MoO4)2 systems where Me = Li, K, Tl, Rb, Cs. Experimental Subsolidus phase relations in  the Me2MoO4–In2(MoO4)3–Hf(MoO4)2 (Me = Li, K, Tl, Rb, Cs) systems were studied within the temperature range 450–550 °C using the intersecting joins method. The correspondent molybdates of lithi- um, potassium, thallium, rubidium, ces- ium, indium and hafnium were used as  starting components for  studying the phase equilibria in  the Me2MoO4– In2(MoO4)3–Hf(MoO4)2 (Me  = Li, K, Tl, Rb, Cs) systems. In order to avoid MoO3 losses due to the sublimation, annealing was started at  400 °C.  Synthesis of  thal- Bazarova J. G., Tushinova Yu. L., Grossman V. G., Bazarova Ts. T., Bazarov B. G., Kurbatov R. V. Chimica Techno Acta. 2018. Vol. 5, No. 3. P. 126–131. ISSN 2409–5613 127 lium molybdate Tl2MoO4 was performed according to the following reaction: Tl2O3+MoO3 → Tl2MoO4+O2↑ while the temperature was gradually in- creased in the range of 400–550 °С for 50 h. Binary alkali molybdates Me2MoO4 (Me  = Li, K, Rb, Cs) were synthesized by  the solid-state reaction using stoi- chiometric mixtures of alkali carbonates or  nitrates with molybdenum trioxide for 80–100 h. Hafnium molybdate was prepared by step annealing of stoichiometric mix- tures of HfO2 and MoO3 within the tem- perature range 400–700 °C for 100–150 h. Indium molybdate was synthesized from indium oxide (III) In2O3 and molybdenum oxide (VI) MoO3 by  solid-state reaction at 500–700 °C. X-ray powder diffraction (XRD) mea- surements were performed using a Bruk- er D8 Advance diffractometer (Bragg — Brentano geometry, Cu Kα radiation, secondary monochromator, maximum angle 2θ = 100°, scan step 0.02°). Results and discussion Information about the known phases in  the side quasi-binary systems, which formed studied quasi-ternary Me2MoO4– In2(MoO4)3–Hf(MoO4)2 (Me = Li, K, Tl, Rb, Cs) systems, required for triangulation, was taken from the literature. According to Solodovnikov et al. [3], the Li2MoO4– Hf(MoO4)2 system contains a lithium haf- nium molybdate Li10–4xHf2+x(MoO4)9 (0.21 ≤ x ≤ 0.68). Two types of  double molyb- dates, Me8Hf(MoO4)6 and Me2Hf(MoO4)3 (Me  = K, Tl, Rb, Cs), are formed inside the Me2MoO4–Hf(MoO4)2 systems [4–6]. An existence of  the double molybdates, namely: Li3In(MoO4)3, MeIn(MoO4)2 (Me = Li, K, Tl, Rb, Cs), and Me5In(MoO4)4 (Me = Tl, Rb) was confirmed in the Me2MoO4– In2(MoO4)3 systems [7–10]. No interme- diate compounds were found inside the In2(MoO4)3–Hf(MoO4)2 system [11]. Taking into account the aforementioned data, the phase formation in the Me2MoO4– In2(MoO4)3–Hf(MoO4)2 (Me = Li, K, Tl, Rb, Cs) systems were studied by means of so- called “intersection joins method”. Within this approach, we analyzed the XRD results for the samples representing the intersec- tion points of  the joins that connect the starting components and phases inside the quasi-binary systems. This makes it pos- sible to  establish the quasi-binary joins and, as  a  result, to  implement the trian- gulation of  the system. Since the phase relations in the K2MoO4–In2(MoO4)3 and Cs2MoO4–In2(MoO4)3 systems enriched by either potassium molybdate or by ce- sium molybdate were found to  be non- quasibinary, the studies of the Me2MoO4– In2(MoO4)3–Hf(MoO4)2 (Me  = K, Cs) systems were limited to the Hf(MoO4)2– Me8Hf(MoO4)6–MeIn(MoO4)2–In2(MoO4)3 (Me = K, Cs) regions. The results obtained are presented in Fig. 1 and Fig. 2. All systems under investigation can be categorized into three groups depend- ing on the phase compositions of the bi- nary subsystems and triple molybdates. The first group comprises the Li2MoO4– In2(MoO4)3–Hf(MoO4)2 simple eutec- tic system without intermediate phases inside. The second group consists of  the Me2MoO4–In2(MoO4)3–Hf(MoO4)2 sys- tems where Me = K and Tl, with one in- termediate phase, denoted in  Fig.  1 as S — Me5InHf(MoO4)6 (5:1:2 mole ratio). The third group includes the Me2MoO4– 128 In2(MoO4)3–Hf(MoO4)2 systems where (Me  = Rb, Cs), with two intermediate phases: S1 − Me5InHf(MoO4)6 (5:1:2 mole ratio) and S2  — Me2InHf(MoO4)6 (2:1:4 mole ratio). S i n g l e - p h a s e s a m p l e s o f Me5InHf(MoO4)6 (Me  = K, Tl, Rb, Cs) Fig. 1. Subsolidus phase relations in the Me2MoO4–In2(MoO4)3–Hf (MoO4)2 (Me = Li, K, Tl) systems: S − Me5InHf(MoO4)6 (5:1:2 mole ratio) Fig. 2. Subsolidus phase relations in the Me2MoO4–In2(MoO4)3–Hf(MoO4)2 (Me = Rb, Cs) systems: S1 − Me5InHf (MoO4)6 (5:1:2 mole ratio); S2 — Me2InHf(MoO4)6 (2:1:4 mole ratio) 129 and Me2InHf(MoO4)6 (Me = Rb, Cs) were prepared by  annealing the stoichiomet- ric mixtures of quasi-binary molybdates at 450–600 °C for 80–100 h. Ternary mo- lybdates Me5InHf(MoO4)6 (Me  = K, Tl, Rb, Cs) and Me2InHf(MoO4)6 (Me = Rb, Cs) are insoluble in  water and usual or- ganic solvents, but were found to be soluble in HCl aqueous solution. T h e t e r n a r y m o l y b d a t e s Me5InHf(MoO4)6 (Me = K, Tl, Rb, Cs) are located inside the triangle that is formed by  the double molybdates MeR(MoO4)2, Me8Hf (MoO4)6 and Me2Hf(MoO4)3 in its vertices. The number of  phases formed in  the systems under consideration increases as  the size of  the singly charged alkali cation increases. The only exception is thallium-containing system. A distinctive feature of thallium is that it combines pro- perties of alkali metals, such as potassium, rubidium, and cesium, together with those related to heavy metals, such as copper (I), silver, and lead [12]. The single crystals of  new ternary potassium indium hafnium molybdate K5InHf(MoO4)6 were grown by  fluxed- melt crystallization with spontaneous nu- cleation [13]. The composition and crys- tal structure of  as-grown single crystals were refined using X-ray diffraction data (a CAD-4 automated diffractometer, Mo Kα radiation, 1498 reflections, R = 0.0252). The crystal structure was solved as trigonal with the following unit cell parameters: a = 10.564 (1) Å, c = 37.632 (4) Å, V = 3637.0 (6) Å3, Z = 6, space group R3c. A three- dimensional mixed framework of  the structure is formed by Mo tetrahedra and two independent (In, Hf ) octahedra, which are connected through the shared vertices. Two types of potassium atoms occupy the large voids within the framework. The dis- tribution of In3+ and Hf 4+ cations over two different sites was refined as  presented in the caption for Fig. 3. Fig.  4 illustrates the IR and Raman spectra for  the triple rubidium indium Fig. 3. Mixed framework of MoO4 tetrahedra (blue color) and two types of octahedra (In, Hf )O6 in the K5InHf(MoO4)6 crystal structure. M(1) = 0.413(1)Hf + 0.587(1)In (olive color); M(2) = 0.587(1)Hf + 0.413(1)In (burgundy color) 130 hafnium molybdate Rb2InHf2(MoO4)6.5. Since the oscillation frequencies in the IR and Raman spectra differ from each other, one can assume that ternary molybdate Rb2InHf2(MoO4)6.5 and its analogues are centrosymmetric. Conclusions The phase equilibria in quasi-ternary salt systems were studied; six new com- pounds were identified inside the stu- died systems. The phase relations in  the Me2MoO4–In2(MoO4)3–Hf(MoO4)2 (Me = Li, K, Tl, Rb, Cs) systems are influenced by  the size factor and the nature of  the singly charged alkali cation. Acknowledgements The work was carried out according to the state assignment BINM SB RAS (project no. 0339-2016-0007) and RFBR, grants Nos. 18-03-00799. References 1. Bazarov BG, Klevtsova RF, Bazarova TsT, Glinskaya LA, Fedorov KN, Tsyren- dorzhieva AD, Chimitova OD, Bazarova JG. Phase equilibria in  the systems Rb2MoO4–R2(MoO4)3–Hf(MoO4)2 (R = Al, In, Sc, Fe (III)) and the crystal structure of double molybdate RbFe(MoO4)2. Russ J Inorg Chem. 2006;51(7):1111–5. 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