Thermal expansion, phase diagrams and barocaloric effects in(NH4)2NbOF5

Abstract

The thermal expansion along thea,b andc axesof a (NH4)2NbOF5 crystal was measured from 120 to 300 K. Anomalies ofα(T) associated with previously reported phase transitions were observed along all directions atT1 = 259.3 Kand T2 = 220.5 K. The analysis of thermal expansion and heat capacity in the frame of the Pippard relationshas permitted us to determine the uniaxial pressure derivatives of the transition temperaturesdT/dσi. TheT–σi phase diagrams have shown a tendency for the intermediate phase to disappear at high stress along theb andc axes. Intensive and extensive barocaloric effects near the structural phase transitions werefound to be comparable with the caloric parameters of some ferroelectrics, ferromagnetsand antiferromagnets.