| Autor: |
Novikova NE; Shubnikov Institute of Crystallography of Federal Scientific Research Centre 'Crystallography and Photonics' of Russian Academy of Sciences, Leninskii Prospekt 59, Moscow, 119333, Russian Federation., Grossman VG; Baikal Institute of Nature Management, Siberian Branch, Russian Academy of Sciences, Sakh'yanovoi 8 St., Ulan-Ude, Buryatia 670047, Russian Federation., Bazarov BG; Baikal Institute of Nature Management, Siberian Branch, Russian Academy of Sciences, Sakh'yanovoi 8 St., Ulan-Ude, Buryatia 670047, Russian Federation., Verin IA; Shubnikov Institute of Crystallography of Federal Scientific Research Centre 'Crystallography and Photonics' of Russian Academy of Sciences, Leninskii Prospekt 59, Moscow, 119333, Russian Federation., Dudka AP; Shubnikov Institute of Crystallography of Federal Scientific Research Centre 'Crystallography and Photonics' of Russian Academy of Sciences, Leninskii Prospekt 59, Moscow, 119333, Russian Federation., Stefanovich SY; Department of Chemistry, Lomonosov Moscow State University, Leninskie Gory 1, Moscow, 119991, Russian Federation., Bazarova JG; Baikal Institute of Nature Management, Siberian Branch, Russian Academy of Sciences, Sakh'yanovoi 8 St., Ulan-Ude, Buryatia 670047, Russian Federation. |
| Abstrakt: |
Single crystals of Tl 4.86 Fe 0.82 Hf 1.18 (MoO 4 ) 6 [a = b = 10.5550 (3), c = 37.7824 (9) Å, γ = 120°] are obtained by the self-flux method in the Tl 2 MoO 4 -Fe 2 (MoO 4 ) 3 -Hf(MoO 4 ) 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)O 6 octahedra connected by MoO 4 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. |
