Phase transitions, baro- and piezocaloric effects in single crystal and ceramics of ferroelectric NH4HSeO4

https://doi.org/10.1016/j.solidstatesciences.2024.107440

A study of heat capacity, thermal dilatation and sensitivity to hydrostatic and uniaxial pressure was carried out on single-crystal and ceramic samples of NH₄HSeO₄. The main parameters of low-temperature successive phase transitions B2 (T₁) ↔ incommensurate IC (T₂) ↔ ferroelectric P1 (T₃) ↔ non-ferroelectric did not depend on the type of samples. The behavior of the volumetric strain and the results of direct measurements of T₃(p) contributed to the resolution of the longstanding problem associated with the ambiguity of the sign of the corresponding volumetric baric coefficient. The role of thermal expansion anisotropy in the formation of the piezocaloric effect (PCE) near the ferroelectric phase transition at T₃ has been studied. Due to the strong difference in the linear baric coefficients, the main contribution to the barocaloric effect (BCE) comes from the inverse intensive and extensive PCE associated with the a-axis. Compared to a single crystal, ceramics demonstrate lower BCE values, which, however, exist in a wider temperature range, which leads to close values of integral caloric parameters. The strong decrease in both BCE and PCE at low-temperature transformations in NH₄HSeO₄ compared to the ferroelectrics NH₄HSO₄ and NH₄NH₄SO₄ is associated with a small change in entropy during three low-temperature phase transitions, ΣΔS_i = 2.52 J/mol∙K, which is a consequence of a high degree of structural ordering in selenate as a result of a high-temperature transformation at T₀ between the superionic and B2 phases, accompanied by a giant change in entropy, ΔS₀≈Rln21.