Thermal, dielectric and barocaloric properties of NH4HSO4 crystallized from an aqueous solution and the melt

Abstract A study of heat capacity, thermal dilatation, permittivity, dielectric loops and susceptibility to hydrostatic pressure was carried out on quasi-ceramic samples of NH 4 HSO 4 obtained from an aqueous solution as well as the melt. The main parameters of the successive P 2 1 / c ( T 1 ) ↔  Pc ( T 2 ) ↔  P 1 phase transitions did not depend on the method of preparation of the samples, and were close to those determined in previous studies of single crystal and powder, except for the sign and magnitude of the baric coefficient for T 2 . Direct measurements of the pressure effect on the permittivity and thermal properties showed d T 2 /d p  = −123 K·GPa −1 , which is consistent in terms of magnitude and sign with the baric coefficient evaluated using dilatometric and calorimetric data in the framework of the Clausius-Clapeyron equation. Thus, the temperature region of the ferroelectric Pc phase existence is extended under pressure. A strong decrease in the entropy jump at the Pc  ↔  P 1 transformation with an increase in pressure, and the linear dependence of T 2 on pressure, indicate that an increase in pressure shifts this phase transition towards the tricritical point on the T – p phase diagram. A significant barocaloric effect was found in the region of the Pc  ↔  P 1 phase transition.