Exploration of the Structural and Vibrational Properties of the Ternary Molybdate Tl₅BiHf(MoO₄)₆ with Isolated MoO₄ Units and Tl⁺ Conductivity

The phase relations in the subsolidus region of the Tl2MoO4-Bi2(MoO4)3-Hf(MoO4)2 system were studied with the "intersecting cuts" method. The formation of the novel ternary molybdate Tl5BiHf(MoO4)6 is found in this ternary system. The compound has a phase transition at Tpt = 731 K (ΔH = -3.15 J/g) and melts at Tm = 871 K (ΔH = -41.71 J/g), as determined by a thermal analysis. Tl5BiHf(MoO4)6 single crystals were obtained by the spontaneous nucleation method. The crystal structure of Tl5BiHf(MoO4)6 was revealed by structure analysis methods. This molybdate crystallizes in the trigonal space group R3̅c with the unit cell parameters a = 10.6801(4) Å, c = 38.5518(14) Å, V = 3808.3(2) Å3, and Z = 6. The vibrational characteristics of Tl5BiHf(MoO4)6 were determined by Raman spectroscopy. The Tl5BiHf(MoO4)6 conductivity was measured at frequencies of 0.1, 1.0, and 10 kHz in the temperature range of 293-773 K; in this temperature range, the conductivity level was 10-12-10-7 S/cm.