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A detailed study of the effect of temperature and pressure on heat capacity, entropy and hysteresis phenomena near the ferroelectric phase transition in ammonium sulfate (AS) was performed. An analysis of experimental results within the framework of the phenomenological theory showed that taking into account the temperature-dependent part of the anomalous entropy leads to a significant increase in the barocaloric effect (BCE). The maximum values of extensive and intensive BCE near the tricritical point are outstanding: Δ S B C E m a x ≈ 85 J/kgK, Δ T A D m a x ≈ 12 K and can be achieved at low pressure ∼0.5 GPa. [ABSTRACT FROM AUTHOR]

Calorimetric, dilatometric and differential thermal analysis studies were performed on (NH 4 ) 3 SnF 7 for a wide range of temperatures and pressures. Large entropy ( δS 0 =22 J/mol K) and elastic deformation ( δ (Δ V / V ) 0 =0.89%) jumps have proven that the Pa- 3↔ Pm -3 m phase transition is a strong first order structural transformation. A total entropy change of Δ S 0 =32.5 J/mol K is characteristic for the order–disorder phase transition, and is equal to the sum of entropy changes in the related material, (NH 4 ) 3 TiF 7 , undergoing transformation between the two cubic phases through the intermediate phases. Hydrostatic pressure decreases the stability of the high temperature Pm -3 m phase in (NH 4 ) 3 SnF 7 , contrary to (NH 4 ) 3 TiF 7 , characterised by a negative baric coefficient. The effect of experimental conditions on the chemical stability of (NH 4 ) 3 SnF 7 was observed. [ABSTRACT FROM AUTHOR]

Heat capacity, Mössbauer and Raman spectra as well as magnetic properties of fluoride CsFe 2 F 6 with defect pyrochlore structure were studied. In addition to recently found above room temperature three successive structural transformations Pnma-Imma-I4 1 amd-Fd-3m, phase transition of antiferromagnetic nature with the 13.7 K Neel temperature and a broad heat capacity anomaly with a maximum at about 30 K were observed. The room temperature symmetry Pnma is unchanged at least down to 7 K. Simple model of indirect bond used to estimate the exchange interactions and to propose a magnetic structure model. [ABSTRACT FROM AUTHOR]

Successive phase transitions G1 → G2 → G3 in the double salt (NH 4 ) 3 TiF 7 have been studied by detailed calorimetric, DTA under hydrostatic pressure and dielectric measurements. Rather large entropy jumps at phase transition points were found to be followed by large additional contributions associated with the temperature dependence of the excess heat capacity. The permittivity behaviour and the tangent of the dielectric losses proved the nonferroelectric nature of both transformations. Two triple-points were found on the temperature-pressure phase diagram, suggesting the existence of a hypothetical parent G0 Pm −3 m cubic phase. A direct transformation between high (G0) and low (G3) temperature cubic phases takes place at p > 0.41 GPa with a baric coefficient dT G0 → G3 /dp = −40 K/GPa. [ABSTRACT FROM AUTHOR]