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

The thermal expansion along the principal crystallographic axes of the (NH4)(2)WO2F4 and (NH4)(2)MoO2F4 oxyfluorides has been studied. The anomalous behavior of alpha (i) (T) due to the phase transitions has been revealed at T (1) = 271.4 K and T (2) a parts per thousand 180 K for the molybdate and at T (1) = 201.5 K and T (2) a parts per thousand 161 K for the tungstate. The quantities dT/dp and dT/d sigma (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 alpha (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: Delta S (BCE) varies from approximately -10 to -17 J/mol K and Delta T (AD) a parts per thousand 8-17 K for the molybdate and Delta S (BCE) varies from approximately -10 to -17 J/mol K and Delta T (AD) a parts per thousand 8-13 K for the tungstate.