Investigation of thermal expansion, phase diagrams, and barocaloric effect in the (NH4)2WO2F4 and (NH4)2MoO2F4 oxyfluorides
- 16 January 2010
- journal article
- Published by Pleiades Publishing Ltd in Physics of the Solid State
- Vol. 52 (1), 167-175
- https://doi.org/10.1134/s1063783410010294
Abstract
The thermal expansion along the principal crystallographic axes of the (NH4)2WO2F4 and (NH4)2MoO2F4 oxyfluorides has been studied. The anomalous behavior of α i (T) due to the phase transitions has been revealed at T 1 = 271.4 K and T 2 ≈ 180 K for the molybdate and at T 1 = 201.5 K and T 2 ≈ 161 K for the tungstate. The quantities dT/dp and dT/dσ i , which characterize the dependence of the phase transition temperatures on the hydrostatic and uniaxial pressures, have been determined from analyzing the results of studies of the thermal expansion and heat capacity with the use of the Pippard relations. The p-T and α i -T phase diagrams reflect different characters of the influence of the pressure on the stability of the initial and distorted phases of the oxyfluorides. The magnitudes of the extensive and intensive barocaloric effects determined in the vicinity of the structural phase transitions are as follows: ΔS BCE varies from approximately −10 to −17 J/mol K and ΔT AD ≈ 8−17 K for the molybdate and ΔS BCE varies from approximately −10 to −17 J/mol K and ΔT AD ≈ 8−13 K for the tungstate.Keywords
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