Abstract:
In recent years, caloric effects near phase transitions in solids have attracted growing interest from investigators. First, this is due to the possibility of obtaining information about a direct relationship between fundamental values such as entropy, temperature, order parameter, structural disorder and sensitivity to external fields (electric, magnetic, mechanical stress and hydrostatic pressure). [1,2] The second reason is associated with the actual problem of searching for high-performance solid refrigerants and for designing alternative refrigeration cycles which are competitive compared to the traditional vapor-compression cycles. [3,4]
Barocaloric effect (BCE) associated with the reversible change in the entropy/temperature, ΔSBCE / ΔTAD, under pressure variation under the isothermal/adiabatic conditions is a common caloric characteristic for substances of different physical nature.
We performed the analysis of the extensive and intensive BCE in some complex fluorides and oxyfluorides which are very sensitive to a change of the chemical pressure and very often undergo successive order-disorder phase transitions of a ferroelastic nature. Different types of the T - p phase diagrams, including the triple points, are considered in connection with the complicated dependences of T(p) observed experimentally. Analyzed diagrams do not cover all possible variants of the phase transition temperature behavior under pressure. They show, however, which parameters of the phase transitions and phase diagrams should be taken in consideration when analyzing BCE. A very important point is that rather low hydrostatic pressure practically does not affect the entropy of the ferroelastic transformations. Therefore, the behavior of extensive and intensive BCE is not changed with increase in pressure. In the case of close temperatures of the successive phase transitions, there is a possibility to realize extensive BCE as the sum of entropies of two transformations. Due to the large magnitude of the extensive and intensive BCE, complex fluorides and oxyfluorides can be considered as new competitive solid refrigerants.
References:[1] A.M. Tishin and Y.I. Spichkin, The Magnetocaloric Effect and its Applications (Bristol: Institute of Physics Publishing) 2003.
[2] I.N. Flerov, M.V. Gorev, A. Tressaud, N.M. Laptash, Cryst. Rep. 56 (2011) 9-17.
[3] L. Manosa, A. Planes, M. Acet, J. Mater. Chem. A 1 (2013) 4925-4936.
[4] M. Ožbolt, A. Kitanovski, J. Tušek, A. Poredoš, Int. J. Refrig. 40 (2014) 174-88.
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