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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]

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]

Some results of calorimetric and structural investigations of the A2A' MOxF6 - x crystals series with the elpasolite-cryolite structure are reported. The analysis has exhibited a key role of the MOxF6 - x species content in the disordering degree and stability of Fm3m phase as well as in the nature of structural distortions. [ABSTRACT FROM AUTHOR]

The ferroelastic phase transitions are investigated in several series of fluoride crystals belonging to the elpasolite and cryolite families (space group Fm3m) with the general formula A[sub 2]BB′F[sub 6] . The influence of the size and shape of cations and anions on the entropy and the mechanism of structural distortions is discussed. © 2004 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]