M. V. Gorev

Phase transitions in elpasolites (ordered perovskites)

Abstract Many compounds with general chemical formula A 2 BB′X 6 and with tolerance factor value t

Role of metal fluoride octahedra in the mechanism of phase transitions in A2BMF6 elpasolites

Sequences of phase transitions (PTs) occurring in (AI)2BIMIIIF6 elpasolites have been studied via X-ray diffraction, differential scanning calorimetry (DSC) and adiabatic calorimetry at low temperature (LT). The substitution of NH4+ ions for alkaline ions in sites with either 6 or 12 coordination number considerably affects both transition temperature and entropy change associated with the transition(s). Two main mechanisms are responsible for the PTs: displacive transitions, as for the Rb2KMIIIF6 series, in which the transitions are associated with small octahedral tilts and are followed by a rather small entropy change; order-disorder transitions of rigid octahedra, as for (NH4)2NH4MIIIF6. In this latter case, the fluoride atoms located at the apices of the octahedra are disordered onto several possible positions in the cubic...

Effect of Cationic Substitution on Ferroelectric and Ferroelastic Phase Transitions in Oxyfluorides A2A′WO3F3 (A, A′: K, NH4, Cs)

Thermal properties and structure of initial and distorted phases of crystals K 2 KWO 3 F 3 , (NH 4 ) 2 KWO 3 F 3 , (NH 4 ) 3 WO 3 F 3 , Cs 2 NH 4 WO 3 F 3 and Cs 2 KWO 3 F 3 were studied to reveal the effect of cationic substitution on disordering and stability of cubic Fm3m phase. It was found that the nature of phase transitions depends on the shape of cation in 8c position.

Thermodynamic properties and p-T phase diagrams of (NH4)3M3+F6 cryolites (M3+: Ga, Sc)

Calorimetric and high pressure experiments are used to establish thermodynamic features of (NH4)3M3+F6 cryolites with M3+: Ga, Sc as they undergo one and three structural phase transitions, respectively. The heat capacity measurements performed between 80 K and 370 K, using an adiabatic calorimeter, have shown that all phase transitions under study are first order with different closeness to the tricritical point. The total entropy change in the Sc compound connected with the successive transformations is almost equal to the entropy change in the Ga cryolite. The effect of hydrostatic pressure has been studied up to 0.6 GPa, using differential thermographic analysis. The p-T phase diagrams of both cryolites were found to be rather complicated, including triple points. The experimental results are considered as connected with order-disorder phase transitions.

Effect of B3+ ion size on the phase transitions in Rb2KB3+F6 elpasolites series

Abstract The physical properties of Rb2KScF6 crystal characterized by the medium size of B3+ atom in Rb2KB3+F6 series are studied. The two successive phase transitions from cubic into tetragonal and then into monoclinic phases are found. The results are discussed in view of the influence of B3+ atom size on structural transformations.

Ferroelastic phase transitions in Rb2KM3+F6elpasolites

Abstract The effect of M3+ ion substitution on the physical properties of Rb2KM3+F6 (M3+ =Ga, Fe, Sc, In, Lu, Er, Ho, Tb) series with elpasolite structure was studied by DTA under hydrostatic pressure, calorimetric, X-ray, elastic and inelastic neutron scattering measurements. There exists a strong dependence of both type and succession of ferroelastic phase transitions on the M3+ ion size. Hydrostatic pressure effect was found partially inconsistent with chemical pressure. Possible models of phase transition mechanism were considered. It was supposed that anharmonic F atom vibrations could play a great role in phase transitions of the series.

Effect of hydrostatic pressure on phase transitions in ABF6.6H2 crystals (A identical to Zn,Co,Mg,Mn,Fe:B identical to Ti,Si)

The effect of hydrostatic pressure on structural phase transitions in some fluorotitanates and fluorosilicates of the ABF6.6H2O family is studied. It is shown that it is possible to divide the crystals of this family into two groups differing from each other in their values of entropy change and shifts of the phase transition temperature T0 under pressure. In some compounds the high-pressure phases and triple points were established. The phase diagram for FeSiF6.6H2O can be considered as a unified diagram describing the possible phase transitions in the crystals studied. The effect of deuteration is investigated in different phases.

Barocaloric Effect in Oxyfluorides Rb2KTiOF5 and (NH4)2NbOF5

Using the data about the temperature–pressure phase diagram and the phase transition entropy at ambient pressure, intensive ΔT AD and extensive ΔS BCE barocaloric effects were estimated near structural order-disorder phase transitions in oxyfluorides Rb2KTiOF5 and (NH4)2NbOF5. It was found that hydrostatic pressure is an effective tool to change the entropy of compounds with ordering ionic groups in structure and to realize the significant caloric effects comparable with the values of electro- and magnetocaloric effects, characteristic for a number of materials, considered as promising solid refrigerants.

Successive and triggered phase transitions in Rb2KM3+F6 elpasolites

Abstract Phase transitions mechanism in Rb2KM3+F6 elpasolites is discussed by using the results of neutron scattering and thermodynamic experiments.

Phase transitions in oxides, fluorides and oxyfluorides with the ordered perovskite structure

Some results of calorimetric and structural investigations of the A 2 A′ MO x F 6 − x crystals series with the elpasolite-cryolite structure are reported. The analysis has exhibited a key role of the MO x F 6 − x species content in the disordering degree and stability of Fm3m phase as well as in the nature of structural distortions.