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The role of chemical pressure as an effective tool in the processes of formation of initial and distorted (as a result of structural transformations) phases, thermodynamic properties, and direct and inverse barocaloric effects in some complex oxyfluorides and fluorides with octahedral, tetrahedral, and spherical anion and cation groups in the structure has been studied. It is found that, due to the small temperature hysteresis and high baric sensitivity of materials, the maximum values of absolute and integral barocaloric characteristics can be implemented at low pressures. Correspondingly, the temperature range of reversibility of thermodynamic cycles based on fluorides/oxyfluorides as solid-state coolants can be expanded. [ABSTRACT FROM AUTHOR]

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]

Using the data about the temperature - pressure phase diagram and the phase transition entropy at ambient and high pressure, intensive and extensive barocaloric effects were analyzed in the region of single and successive order-disorder phase transitions in fluorides and oxyfluorides with the elpasolites-cryolite structure. It was found that hydrostatic as well as chemical pressure are the effective tools to change the entropy of compounds with disordered ionic groups or atoms in structure and to realize the significant intensive and extensive barocaloric effects comparable with the values of baro- and magnetocaloric effects, characteristic for materials, considered as promising solid refrigerants. [ABSTRACT FROM AUTHOR]

n analysis of the effect that chemical and hydrostatic pressures have on the thermodynamic properties of perovskite-like fluorine-oxygen compounds A A′ MeOF 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. [ABSTRACT FROM AUTHOR]

In order to understand the role of central atom in the nature of phase transitions in two series of ammonium oxyfluorides, A2MeO2F4 and A2A'MeOxF6-x, the permittivity was studied in wide temperature range. In most cases structural distortions are associated with the ferroelasticity. The ferro- and antiferroelectric properties were revealed only in molybdates. [ABSTRACT FROM AUTHOR]

Vibrational and 17O NMR spectroscopy in combination with quantum chemical calculations are used to investigate the hydrolysis of antimony(III) fluoride complexes. A hydrolytic decomposition of SbF3 and [SbF4]− is accompanied by oligomerization with the formation of edge-and corner-connected dimers ([Sb2O2F4]2−, [Sb2OF8]4−) and trimers ([Sb3O3F6]3−, [Sb3OF9]2−) with bridging oxygen atoms. The hydrolysis of [SbF4]− is also characterized by the presence in the solution of a discrete cation of [SbF5]2− which is least hydrolized. Only a partial isomorphic substitution of fluoride ion by hydroxide one is possible, which is reflected in the composition of K2Sb(OH)xF5− x ( x = 0.3) crystals isolated from the fluoride aqueous solution. [ABSTRACT FROM AUTHOR]

For the first time structural transformations were observed for one of the compounds belonging to the family of double salt fluorides A3MeF7 = A2MeF6· AF. Single crystals of (NH4)3TiF7 = (NH4)2TiF6·NH4F were grown by evaporation of the aqueous solution. Polarizing optical observations, heat capacity and birefringence Δn = (noΔne) measurements were performed in a wide temperature range 100-400 K. Two reversible, first order structural phase transitions were found to be followed by an unusual sequence of symmetry changes: cubic (G3) (T2↓ = 287.5 K; T2↑ = 291-310 K) ↔ P4/mnc (G2) (T1? = 358.5 K; T1&#8995 = 360 K) ↔ 4/m (G1). The twin structures observed were described assuming the existence of cubic parent phase G0 with P m3- symmetry. [ABSTRACT FROM AUTHOR]