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We report the conventional and inverse barocaloric effect (BCE) in ferroelastics (NH 4 ) 3 NbOF 6 and (NH 4 ) 3 TiOF 5 . We found a rich set of the structural phase transitions and triple points on the T − p phase diagrams. Extensive and intensive BCE near some transformations and around triple points in both crystals are outstanding (|∆ S BCE max | = 30–100 J/kg·K; |∆ T AD max | = 4–22 K) and can be achieved at low pressure 0.01–0.60 GPa. PACS 62.50.-p 65.40.-b 81.30.-t [ABSTRACT FROM AUTHOR]

Abstract: For the first time, structural phase transitions induced by the temperature were found in A x M x IIM(1−x) IIIF3 fluorides with the defect pyrochlore structure (Fd m, Z=8). The room temperature structure of CsFe2F6 was determined using X-ray powder diffraction technique. This phase was found to be ordered with the Pnma space group. The study of the temperature stability of orthorhombic structure by differential scanning calorimeter between 100K and 700K has shown a succession of phase transitions. The Pnma (Z=4)→Imma (Z=4)→I41/amd (Z=4)→Fd m (Z=8) structural sequence was proposed to occur within a rather narrow temperature range 500–560K. The mechanism of structural transition has been mainly associated with the rotation of (MF6) octahedra and small displacements of some Fe atoms. This assumption is in good agreement with the low experimental entropy value, which is characteristic for displacive transformations. [Copyright &y& Elsevier]

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]

Highlights: [•] (NH4)3ZrF7 undergoes the succession of six ferroelastic phase transitions. [•] The transformation between two cubic phases Fm-3m↔F23 is suggested at T 0. [•] Excess entropy does not correspond to strong disorder model of cubic structure. [•] The stability of all phases except G4 increases with pressure increase. [ABSTRACT FROM AUTHOR]

A study of heat capacity, thermal dilatation, permittivity, dielectric loops and susceptibility to hydrostatic pressure was carried out on quasi-ceramic samples of NH 4 HSO 4 obtained from an aqueous solution as well as the melt. The main parameters of the successive P 2 1 / c ( T 1 ) ↔ Pc ( T 2 ) ↔ P 1 phase transitions did not depend on the method of preparation of the samples, and were close to those determined in previous studies of single crystal and powder, except for the sign and magnitude of the baric coefficient for T 2 . Direct measurements of the pressure effect on the permittivity and thermal properties showed d T 2 /d p = −123 K·GPa −1 , which is consistent in terms of magnitude and sign with the baric coefficient evaluated using dilatometric and calorimetric data in the framework of the Clausius-Clapeyron equation. Thus, the temperature region of the ferroelectric Pc phase existence is extended under pressure. A strong decrease in the entropy jump at the Pc ↔ P 1 transformation with an increase in pressure, and the linear dependence of T 2 on pressure, indicate that an increase in pressure shifts this phase transition towards the tricritical point on the T – p phase diagram. A significant barocaloric effect was found in the region of the Pc ↔ P 1 phase transition. [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]

Abstract: Calorimetric, X-ray, dielectric and DTA under pressure measurements have been performed on oxyfluoride (NH4)3Ta(O2)2F4. The succession of nonferroelectric phase transitions was found associated with the order-disorder processes. The comparative analysis tantalate with related niobate has revealed the important role of the central atom in the physical properties behavior, mechanism of structural distortions and barocaloric effect in oxyfluorides with the eight-coordinated anionic polyhedra. [Copyright &y& Elsevier]