Conventional and inverse mechanocaloric effects in single-crystal and ceramic (NH4)3H(SO4)2

Detailed studies of heat capacity, thermal expansion, sensitivity to hydrostatic pressure, as well as mechanocaloric effects were carried out for single-crystal and ceramic (NH₄)₃H(SO₄)₂ undergoing a number of structural transformations at atmospheric pressure: R-3m ↔ (C2/c) ↔ (P2/n)₁ ↔ (P2/n)₂ ↔ P-1. A significant smearing of the anomalous contribution to the deformation of the ceramic sample was observed, especially near first order transformations, while the behavior and values of the anomalous entropy are in satisfactory agreement for both samples. For the first time, the region of the T – p phase diagram, including low temperature phase transitions, was experimentally investigated. A good correspondence was found between the measured and calculated volumetric baric coefficients. A comparative analysis of the baro(BCE)- and piezo(PCE)-caloric effects was carried out using entropy–temperature phase diagrams at various hydrostatic/uniaxial pressures. Inverse BCE is characteristic of all studied phase transitions, which is caused by a decrease in their temperatures under hydrostatic pressure. Due to rather low symmetry of the crystalline phases, (NH₄)₃H(SO₄)₂ demonstrates a strong anisotropy in the thermal expansion which leads in turn to the difference in the values and sign of the linear baric coefficients and, as a result, to conventional and inverse PCE associated with the various crystallographic axes. The caloric parameters of single-crystal and ceramic (NH₄)₃H(SO₄)₂ are analyzed in comparison with some other derivatives of ammonium sulphate.