I. N. Flerov

Phase transitions in elpasolites (ordered perovskites)

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

Perovskite-like fluorides and oxyfluorides: Phase transitions and caloric effects

An analysis of the effect that chemical and hydrostatic pressures have on the thermodynamic properties of perovskite-like fluorine-oxygen compounds A2A′MeOxF6 − x has revealed that materials under-going order-disorder transitions and having significant external-pressure compliance have the highest caloric efficiency. Some of the fluorides and oxyfluorides under study can be considered promising solid coolants.

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...

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

Adiabatic calorimetric study of the intense magnetocaloric effect and the heat capacity of (La0.4Eu0.6)0.7Pb0.3MnO3

\overline 3

Electrocaloric effect and anomalous conductivity of the ferroelectric NH4HSO4

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).

Calorimetric and x-ray diffraction studies of the (NH4)3WO3F3 and (NH4)3TiOF5 perovskite-like oxyfluorides

Heat capacity of the PbMg1/3Nb2/3O3 compound is measured using the methods of adiabatic and differential scanning calorimetry in the temperature range 80–750 K. Two blurred anomalies on the Cp(T) dependence are observed in wide temperature intervals of 200–400 K and 500–700 K. The results of studies are discussed together with data on the structure and phonon spectrum in the framework of spherical random bond-random field model.

Investigation of thermal expansion, phase diagrams, and barocaloric effect in the (NH4)2WO2F4 and (NH4)2MoO2F4 oxyfluorides

The temperature dependences of the intense magnetocaloric effect ΔTAD(T, H) and the heat capacity Cp(T) of the (La0.4Eu0.6)0.7Pb0.3MnO3 manganite are directly measured using adiabatic calorimetry. The experimental dependences ΔTAD(T) are in satisfactory agreement with those calculated from the data on the behavior of the magnetization. The factors responsible for the absence of an anomaly in the experimental temperature dependence of the heat capacity Cp(T) in the range of the magnetic phase transition are discussed.