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The heat capacity and thermal expansion of potassium heptafluorotantalate were studied. The room temperature phase P 2 1 / c is stable at least to 4 K. The strong first-order phase transition P 2 1 / c - P n m a at T 0 = 486.2 K is accompanied by giant changes in the entropy, Δ S 0 = 22.3 J (mol K) - 1 , and volume strain, δ V 0 / V = - 3.6 % . A rather high sensitivity of K 2 TaF 7 to pressure was found, d T 0 / d p = - 220 K G Pa - 1 . Significant extensive and intensive barocaloric effects are found at low pressure. The possibility of improving the barocaloric properties is discussed. [ABSTRACT FROM AUTHOR]

The heat capacity, thermal expansion, and T-p phase diagrams of (NH4)3TiF7and (NH4)3SnF7were studied in wide temperature and pressure ranges. The total excess entropies at successive Pa-3 ↔ P4/mnc ↔ 4/m (Ti) and single Pa-3 ↔ Pm-3m (Sn) phase transitions are close to each other and characteristic for the order–disorder processes. The Pm-3m cubic phase and direct Pa-3 ↔ Pm-3m transformation were found at high pressure in (NH4)3TiF7. The different sign of baric coefficients for phase transition between two cubic phases in (NH4)3TiF7and (NH4)3SnF7was supposed due to nonlinear phase boundary with pressure in the latter fluoride. [ABSTRACT FROM PUBLISHER]

The (NH)WOF compound undergoes a series of phase transitions: G → 201, K → G → 160, and K → G, with a significant change in entropy (Δ S ∼ Rln10 at the G → G transition), which indicates significant orientational disordering in the G phase and the order-disorder type of the phase transition. X-ray diffraction is used to identify the crystal structure of the G phase as rhombohedral (sp. gr. Cmcm, Z = 4), determine the lattice parameters and the positions of all atoms (except hydrogen), and show that [WOF] ions can form a superposition of dynamic and static orientational disorders in the anionic sublattice. A determination of the orientational position of [NH] ions calls for the combined method of elastic and inelastic neutron scattering. Inelastic neutron scattering is used to determine the state of hindered rotation for ammonium ions in the G phase. Powder neutron diffraction shows that the orientational disorder of NH ions can adequately be described within the free-rotation approximation. [ABSTRACT FROM AUTHOR]

Using the data about the temperature-pressure phase diagram and the phase transition entropy at ambient pressure, intensive ΔTAD and extensive ΔSBCE 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. [ABSTRACT FROM AUTHOR]

Single crystals of (NH4)3SnF7 are obtained from aqueous solutions of (NH4)2SnF6 and excess NH4F during solvent evaporation. Reinvestigation of the room temperature structure reveals that its symmetry is Pa... (Z = 8 single crystal XRD) instead of Pm...m (Z = 1) suggested earlier. The material is further characterized by polarizing optical observations, heat capacity measurements, and powder and single crystal XRD at 100--420 K. A reversible first order structural phase transition between the two cubic space groups Pa... and Pm...m is observed at ≈360 K. The structural model associated with partially disordered and totally ordered high and low temperature phases comply with the observed large phase transition entropy. [ABSTRACT FROM AUTHOR]