A new Tl4.86Fe0.82Hf1.18(MoO4)6 ternary molybdate: crystal structure and properties

• Published in: Acta crystallographica Section B, Structural science, crystal engineering and materials Structural science, crystal engineering and materials, 2020-10, Vol.76 (5), p.839-849
• Main Authors: Novikova, Nataliya E., Grossman, Victoria G., Bazarov, Bair G., Verin, Igor A., Dudka, Alexander P., Stefanovich, Sergei Yu, Bazarova, Jibzema G.
• Format: Article
• Language: English
• Subjects: Calorimetry; Cations; Crystal structure; Crystals; ion conductivity; Ion currents; Ion transport; Lattice vacancies; Potential barriers; Room temperature; Second harmonic generation; Single crystals; ternary molybdates; Tetrahedra; Thallium
• ISSN: 2052-5206; 2052-5192; 2052-5206
• DOI: 10.1107/S2052520620010768

Single crystals of Tl4.86Fe0.82Hf1.18(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. Single crystals of Tl4.86Fe0.82Hf1.18(MoO4)6 were obtained by the self‐flux method from high‐temperature solutions in the Tl2MoO4–Fe2(MoO4)3–Hf(MoO4)2 system. The crystal structure consists of nonpolar and polar domains with different local symmetries and can be described as a whole by the nonstandard space group R1 with local symmetry Rc for some atoms.