A new Tl_4.86Fe_0.82Hf_1.18(MoO₄)₆ ternary molybdate: crystal structure and properties

Single crystals of Tl_4.86Fe_0.82Hf_1.18(MoO₄)₆ [a = b = 10.5550 (3), c = 37.7824 (9) Å, γ = 120°] are obtained by the self-flux method in the Tl₂MoO₄-Fe₂(MoO₄)₃-Hf(MoO₄)₂ system. On the differential scanning calorimetry curve in the temperature range 320-350 K and at T ∼ 690 K, endothermic peaks are observed. The second harmonic generation test shows an excess of the signal of the quartz standard by almost three times at room temperature. In the range 320-340 K its intensity decreases by almost three times and at T ∼ 700 K it drops to zero. In the same interval, the temperature dependences of the unit-cell parameters and volume show stepwise changes. According to the X-ray diffraction data, the crystal structure consists of nonpolar and polar domains with different local symmetries. The structure is a three-dimensional framework consisting of alternating (Hf,Fe)O₆ octahedra connected by MoO₄ tetrahedra. Hf and Fe atoms occupy mixed Hf/Fe positions with different probabilities: 0.77:0.23, 0.50:0.50 and 0.32:0.68. Tl cations are located inside the framework in zigzag channels extended along the a and b axes. The thallium arrangement is disordered, i.e. it involves additional positions and vacancies. The complex crystal structure has been solved using the nonstandard space group R1, taking into account the local symmetry R3c for the Mo atoms and mixed Hf/Fe positions mainly occupied by Hf atoms. The possible paths of ion transport are analyzed. The energy required to overcome the potential barrier between sites of Tl cations to migrate, which corresponds to the activation energy of conductivity, is estimated. The ion current is shown to be most probable in the ab plane.