Investigation of thermal expansion, phase diagrams, and barocaloric effect in the (NH4)2WO2F4 and (NH4)2MoO2F4 oxyfluorides

Abstract

The thermal expansion along the principal crystallographic axes of the (NH4)2WO2F4 and (NH4)2MoO2F4 oxyfluorides has been studied. The anomalous behavior of αi(T) due to the phase transitions has been revealed at T1 = 271.4 K and T2 ≈ 180 K for the molybdate and at T1 = 201.5 K and T2 ≈ 161 K for the tungstate. The quantities dT/dp and dT/dσi, 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 αi-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: ΔSBCE varies from approximately −10 to −17 J/mol K and ΔTAD ≈ 8−17 K for the molybdate and ΔSBCE varies from approximately −10 to −17 J/mol K and ΔTAD ≈ 8−13 K for the tungstate.