A. G. Kocharova

Mechanism and nature of phase transitions in the (NH4)3MoO3F3 oxyfluoride

AbstractThe temperature dependences of the heat capacity, the unit cell parameter, and the permittivity for the (NH4)3MoO3F3 cryolite (space group Fm

Structure transformations during phase transitions in the K3WO3F3 oxyfluoride

\overline 3

Structural phase transition in elpasolite-like (NH4)2KWO3F3

m) are investigated. It is revealed that the compound undergoes ferroelectric and ferroelastic structural phase transitions at temperatures of 297 and 205 K, respectively. The mechanism of structural distortions is discussed in terms of the entropy parameters, pressure-temperature phase diagrams, and electron density maps for critical atoms. An analysis is made of the influence of the cation size and shape on the phase transitions in oxyfluorides of the general formula A2A′MO3 (A,A′ = NH4, K; M = Mo, W).

Raman spectroscopic study of the phase transitions in the Cs2NH4WO3F3 oxyfluoride

The thermal expansion along the principal crystallographic axes of the (NH4)2WO2F4 and (NH4)2MoO2F4 oxyfluorides has been studied. The anomalous behavior of αi(T) due to the phase transitions has been revealed at T1 = 271.4 K and T2 ≈ 180 K for the molybdate and at T1 = 201.5 K and T2 ≈ 161 K for the tungstate. The quantities dT/dp and dT/dσi, which characterize the dependence of the phase transition temperatures on the hydrostatic and uniaxial pressures, have been determined from analyzing the results of studies of the thermal expansion and heat capacity with the use of the Pippard relations. The p-T and αi-T phase diagrams reflect different characters of the influence of the pressure on the stability of the initial and distorted phases of the oxyfluorides. The magnitudes of the extensive and intensive barocaloric effects determined in the vicinity of the structural phase transitions are as follows: ΔSBCE varies from approximately −10 to −17 J/mol K and ΔTAD ≈ 8−17 K for the molybdate and ΔSBCE varies from approximately −10 to −17 J/mol K and ΔTAD ≈ 8−13 K for the tungstate.

Investigation into phase diagrams of the fluorine-oxygen system: Ferroelastic-antiferroelectric (NH4)2WO2F4-(NH4)2MoO2F4

AbstractThe structures of three phases of the K3WO3F3 crystal have been determined from X-ray diffraction data obtained for a powder sample. The profile and structural parameters have been refined according to the technique implemented in the DDM program. The results obtained have been discussed using the group-theoretical analysis of the complete order parameter condensate, which takes into account the critical and noncritical atomic displacements and allows the interpretation of the experimental data. The sequence of structural transformations is found to be as follows:

The role of structural and nonstructural water in oxyfluoride K2WO2F4 · H2O

Fm\bar 3m\xrightarrow[{(\eta _1 ,0,0)}]{{11 - 10(\Gamma _4^ - )}}I4mm\xrightarrow[{(\eta _1 ,\eta _2 ,0)}]{{11 - 10(\Gamma _4^ - )}}Cm