Effect of deuteration on the barocaloric properties of complex vanadates (NH4)3VOxF6−x (x: 1, 2)

Abstract

We report on the analysis of intensive and extensive barocaloric properties and their sensitivity to deuteration of complex oxyfluorides (NH4)3VOxF6−x (x: 1, 2) undergoing a similar sequence of structural phase transitions. Due to the high sensitivity to hydrostatic pressure and the strong disordering of six-coordinated anionic fluoro-oxygen species, (NH4)3VO2F4 and (ND4)3VO2F4 crystals demonstrate the highest barocaloric efficiency at low pressure, p = 0.1 GPa, during the phase transition from the initial cubic phase: barocaloric coefficients reach large values |ΔSBCE|/p ≈ 400 J/(kg K GPa) and ΔTAD/p ≈ 80 K/GPa. Anionic, [VOF5] → [VO2F4], and cationic, [ND4] → [NH4] substitutions are accompanied by a decrease in the disorder of structural units in the cubic phase, which leads to a decrease in changes in entropy and temperature under pressure. The contribution of thermal expansion of the crystal lattice to the total intensive and extensive barocaloric effects is large and amounts to about 30–40%.