Phase equilibria in the Tl2MoO4–R2(MoO4)3–Zr(MoO4)2 (R = Al, Cr) systems: synthesis, structure and properties of new triple molybdates Tl5RZr(MoO4)6 and TlRZr0.5(MoO4)3 218 Grossman V.G., Bazarov B.G., Bazarova Zh.G. Chimica Techno Acta. 2017. Vol. 4, No. 4. P. 218–223. ISSN 2409–5613 D O I: 1 0. 15 82 6/ ch im te ch /2 01 7. 4. 4. 02 V.G. Grossman1, B.G. Bazarov1,2, Zh.G. Bazarova2 1Baikal Institute of Nature Management, Siberian Branch of Russian Academy of Sciences, 8 Sakh’yanovoi St., Ulan-Ude, 670047, Russian Federation e-mail: grossmanv@mail.ru 2Buryat State University, 24a SmolinaSt., Ulan-Ude, 670000, Russian Federation Phase equilibria in the Tl 2 MoO 4 –R 2 (MoO 4 ) 3 –Zr(MoO 4 ) 2 (R = Al, Cr) systems: synthesis, structure and properties of new triple molybdates Tl 5 RZr(MoO 4 ) 6 and TlRZr 0.5 (MoO 4 ) 3 The Tl 2 MoO 4 –R 2 (MoO 4 ) 3 –Zr(MoO 4 ) 2 (R = Al, Cr) systems were studied in the subsolidus region using X-ray powder diffraction and differential scanning calo- rimetric (DSC) analysis. Quasi-binary joins were revealed, and triangulation was carried out. New ternary molybdates Tl 5 RZr(MoO 4 ) 6 (5:1:2) and TlRZr 0.5 (MoO 4 ) 3 (1:1:1) (R = Al, Cr) were prepared. The unit cell parameters for the new com- pounds were calculated. Keywords: phase equilibria, synthesis, systems, thallium, zirconium, iron, aluminum, crystal structure, space group. Received: 23.10.2017; accepted: 01.12.2017; published: 25.12.2017. © Grossman V.G., Bazarov B.G., Bazarova Zh.G., 2017 Introduction This paper is a continuation of our systematic studies of phase relations in the Tl2MoO4–R2(MoO4)3–A(MoO4)2 (R – trivalent metals, А = Zr, Hf ) ternary salt systems [1]. Earlier we studied phase equilibria in the Tl2MoO4–Fe2(MoO4)3– Hf(MoO4)2 system [2]. Subsolidus phase diagrams for this system and constituent double systems were constructed, and triple molybdates Tl5FeHf(MoO4)6 and TlFeHf0.5(MoO4)3 had been detected. The aims of the present study include (1) in- vestigation of phase equilibria in the ter- nary salt systems Tl2MoO4–R2(MoO4)3– Zr(MoO4)2 (R = Al, Cr), (2) determination of optimal condition for the solid state synthesis of ternary molybdates found in these systems, and (3) determination of crystallographic and thermal characteris- tics of the obtained compounds. Experimental Subsolidus phase relations in the Tl2MoO4–R2(MoO4)3–Zr(MoO4)2 (R = Al, Cr) systems were studied in the subsolidus region (500–550  °C) using the intersect- ing joins method. The corresponding molybdates of thallium, aluminum, chromium and zir- conium were used as initial components for studying the phase equilibria in the Tl2MoO4–R2(MoO4)3–Zr(MoO4)2 (R = Al, 219 Cr) systems. Synthesis of Tl2MoO4 was carried out according to the reaction Tl2O3+MoO3→Tl2MoO4+O2↑ at gradu- ally increasing temperature in the range 400–550  °С for 50  h. The high tempera- ture modification of Zr(MoO4)2 was pre- pared by annealing of stoichiometric mixture of binary oxides ZrO2 and MoO3 at 400–700  °С for 100 h. Aluminum mo- lybdate and chromium molybdate were obtained by calcination of stoichiometric mixtures of precursors Al(NO3)3·9H2O, Сr2O3, and MoO3 in the temperature range 400–650 °С for 100 h. The initial stage of each synthesis was chosen as 400 °С since MoO3 possesses high volatility at tempera- ture about 600  °С. To ensure better ho- mogenization, the reaction mixtures were ground in ethanol every 20–30 h during firing. After annealing, the samples were slowly cooled in the furnace. The non- equilibrium samples were additionally an- nealed. It was assumed that equilibrium is reached if the phase composition of the samples remains unchanged during two consecutive anneals. The crystallographic parameters of the synthesized compounds were close to those reported in literature [3–5]. X-ray powder diffraction (XRD) meas- urements were performed using Bruker D8 Advance diffractometer (Bragg–Bren- tano geometry, Cu Kα radiation, second- ary monochromator, maximum angle 2θ=100°, scan step 0.02°). The differen- tial scanning calorimetric (DSC) analy- sis of the samples was carried out using NETZCH STA 449C (Jupiter, Germany) thermoanalyzer. Compounds’ pellets were placed in a Pt-crucible, heated up and then cooled down in argon atmosphere with the heating and cooling rate of 10 K/min. Results and discussion The information about the phase formation in the Tl2MoO4–R2(MoO4)3– Zr(MoO4)2 (R = Al, Cr) systems, which represent the bounding sides of the stu- died system, were taken from previous papers [3–5]. The formation of double molybdates with general composition TlR(MoO4)2 was detected in the bound- ary Tl2MoO4–R2(MoO4)3 systems [3]. Two double molybdates Tl8Zr(MoO4)6 and Tl2Zr(MoO4)3 were formed in the Tl2MoO4–Zr(MoO4)2 system [4]. No in- termediate compounds were found in the R2(MoO4)3–Zr(MoO4)2 systems [5]. In order to find new triple molybdates, the subsolidus phase equilibria in the Tl2MoO4–R2(MoO4)3–Zr(MoO4)2 (R = Al, Cr) systems were studied at 500–550  °C and its triangulated phase diagrams were constructed. Solid-state interac- tions between Tl2MoO4, R2(MoO4)3, and Zr(MoO4)2, which occurred over wide ranges of temperature and concentration, led to the formation of new triple molyb- dates Tl5RZr(MoO4)6 (5:1:2 mole ratio) S1 and TlRZr0.5(MoO4)3 (1:1:1 mole ratio) S2. Compound S2 was found at the intersection point of the R2(MoO4)3–Tl2Zr(MoO4)3 and TlR(MoO4)2–Zr(MoO4)2 joins. The triple molybdate (S1) locates inside the triangle with the double molybdates TlR(MoO4)2, Tl8Zr(MoO4)6 and Tl2Zr(MoO4)3 in its vertices. The phase relations in the Tl2MoO4– R2(MoO4)3–Zr(MoO4)2 (R = Al, Cr) sys- tems are shown in Fig. 1. Individual Tl5RZr(MoO4)6 (R = Al, Cr) oxides were prepared by firing at 450– 550 °С for 150–200 h, and the molybdates TlRZr0.5(MoO4)3 (R = Al, Cr) were ob- tained by firing at temperatures from 500 to 600 °С for 100–150 h. 220 The single phase Tl5AlZr(MoO4)6 was not synthesized under the conditions of our experiment. The analysis of X-ray diffraction pat- terns of the compounds obtained shows that the reflection positions and their intensity ratio for TlRZr0.5(MoO4)3 and Tl5RZr(MoO4)6 (R = Al, Cr) are similar to TlFeHf0.5(MoO4)3 [2] and Rb5ErHf(MoO4)6 [6], respectively. It could be concluded that TlRZr0.5(MoO4)3 is isostructural to TlFeHf0.5(MoO4)3 and Tl5RZr(MoO4)6 is isostructural to Rb5ErHf(MoO4)6. Three dimensional framework of the TlFeHf0.5(MoO4)3 crystal structure (a = b = 13.0324(2) Å, c = 11.8083(3) Å, V = 1736.87(6) Å3, ρcalc=4.757 g/cm 3, space group R3, Z = 6) is composed of the Mo- tetrahedra sharing O vertices with the (Fe, Hf )O6 octahedra, with thallium atoms oc- cupying wide channels in the framework [2] (Figs. 2 and 3). The arrangement of Tl atoms (pink spheres) in the structural channel in TlFeHf0.5(MoO4)3 is shown in Fig. 3. Rb5ErHf(MoO4)6 possesses the trigo- nal crystal structure: a = 10.7511(1) Å, c = 38.6543(7) Å, V = 3869.31(9) Å3, ρcalc=4.462 g/cm 3, Z = 6, space group R3c [6]. The three-dimensional framework of the structure is formed of the MoO4 tet- rahedra, which are sharing corners with two ErO6 and HfO6 octahedra (Fig. 4). Two types of Rb atoms occupy large cavi- ties in the framework. The particular ar- Fig. 2. The framework of TlFeHf0.5(MoO4)3 crystal structure that consists of MoO4 tetrahedra and (Fe, Hf )O6 octahedra in the projection of layer onto the (001) plane (Tl atoms are represented by pink spheres) Fig. 3. The fragment of the TlFeHf0.5(MoO4)3 crystal structure that is projected onto the (010) plane Fig. 1. Subsolidus phase relations in the Tl2MoO4–R2(MoO4)3–Zr(MoO4)2 (R = Al, Cr) systems: S1 −Tl5RZr(MoO4)6 (5:1:2 mole ratio) and S2 – TlRZr0.5(MoO4)3 (1:1:1 mole ratio) Tl2MoO4 Zr(MoO4)2 R2(MoO4)3 S1 S2 1:1 4:1 1:1 221 rangement of Rb atoms in the structural channel is shown in Fig. 5. The distribu- tion of the Er3+ and Hf4+ cations over two positions is obtained during the structure refinement. The unit cell parameters for the synthe- sized triple molybdates TlRZr0.5(MoO4)3 (R = Al, Cr) were refined using uniquely-in- dexed lines for the TlFeHf0.5(MoO4)3 single crystal [2]. The lines for Tl5CrZr(MoO4)6 were indexed using Rb5ErHf(MoO4)6 [6] as analogous isostructural compound. The unit cell parameters that were refined using the TOPAS-4 software are listed in Table 1 along with the melting points of the corresponding compounds. Fig. 6 il- lustrates good coincidence of the experi- mental and calculated profiles. As shown in Table 1, the unit cell pa- rameters and volume of TlRZr0.5(MoO4)3 (R = Al, Cr) and Tl5CrZr(MoO4)6 increase with the substitution of 6-coordinated alu- minum cations (r = 0.535 Å) with a larger chromium cation (r = 0.615  Å) [7]. The linear dependence of the unit cell volume on the trivalent element radii is also in ac- cordance with TlRZr0.5(MoO4)3 (R = Al, Table 1 The unit cell parameters and melting points of TlRZr0.5(MoO4)3 (R = Al, Cr) and Tl5CrZr(MoO4)6 in comparison with literature data Compound Unit cell parameters (Å) T, °C a, Å c, Å V, Å3 space group; Z TlFeHf0.5(MoO4)3 [2] TlAlZr0.5(MoO4)3 TlCrZr0.5(MoO4)3 13.0324(2) 12.5935(6) 12.6961(6) 11.8083(3) 11.5946(8) 11.7022(9) 1736.87(6) 1592.5(2) 1633.6(2) R3; 6 811 751 842 Rb5ErHf(MoO4)6 [6] Tl5CrZr(MoO4)6 10.7511(1) 10.4047(8) 38.6543(7) 37.5322(3) 3869.31(9) 3518.8(6) R3c; 6 730 599 Fig. 4. Complex framework of the Rb5ErHf(MoO4)6 crystal structure built of the MoO4 tetrahedra and (Er, Hf )O6 octahedra. Red parallelogram shows the projection of a layer onto the (001) plane. The Rb atoms are represented by the orange spheres Fig. 5. Crystal structure of Rb5ErHf(MoO4)6 as projected on plane (133) 222 Cr) and Tl5CrZr(MoO4)6 belonging to one structural family. Two endothermic effects are observed on the DSC curves (Fig. 7). The first one corresponds to some structural changes that, however, are not accompanied by the change of a structural type. The last endo- thermic effect corresponds to the melting of the studied compounds. Conclusions New triple molybdates with general compositions Tl5RZr(MoO4)6 (5:1:2) and TlRZr0.5(MoO4)3 (1:1:1) (R = Al, Cr), re- spectively, were obtained in the thallium- containing systems with trivalent metals and zirconium. The formation of these triple molibdates occurs similarly to the formation of ternary molibdate Tl(FeHf0.5) Fig. 7. DSC curves for TlRZr0.5(MoO4)3 (R = Al, Cr) and Tl5CrZr(MoO4)6 Fig. 6. Measured (red), calculated (black) and differential (blue) powder diffraction patterns for TlCrZr0.5(MoO4)3 (a), TlAlZr0.5(MoO4)3 (b) and Tl5CrZr(MoO4)6 (c) a b c 223 (MoO4)3 in previously studied Tl2MoO4– Fe2(MoO4)3−Hf(MoO4)2 system [2]. The phase relations do not change with haf- nium (r = 0.71  Å) being substituted with zirconium (r = 0.72 Å) [7]. Acknowledgements This work was carried out according to the state assignment BINM SB RAS (project no. 0339-2016-0007). References 1. Grossman VG, Bazarov BG, Bazarova ZhG. Subsolidus phase diagrams for the Tl2MoO4–Ln2(MoO4)3–Hf(MoO4)2 systems, where Ln = La–Lu. Russ J Inorg Chem. 2008;53(11):1788–94. DOI:10.1134/S003602360811020X 2. Grossman VG, Bazarov BG, Klevtsova RF, Glinskaya LA, Bazarova ZhG. Phase equilibria in the Tl2MoO4–Fe2(MoO4)3–Hf(MoO4)2 system and the crystal struc- ture of ternary molybdate Tl(FeHf0.5)(MoO4)3. Russ Chem Bull. 2012;61(8):1546–9. DOI:10.1007/s11172-012-0202-7 3. 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Revised effective ionic radii and systematic studies of interatomic dis- tances in halides and chalcogenides. Acta Cryst. 1976;A32:751–67. DOI:10.1107/ S0567739476001551 Cite this article as: Grossman VG, Bazarova BG, Bazarova ZhG. Phase equilibria in the Tl2MoO4– R2(MoO4)3–Zr(MoO4)2 (R = Al, Cr) systems: synthesis, structure and properties of new triple molybdates Tl5RZr(MoO4)6 and TlRZr0.5(MoO4)3. Chimica Techno Acta. 2017;4(4):218–23. DOI:10.15826/chimtech/2017.4.4.02.