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

Abstract

The study of heat capacity, thermal dilatation, permittivity, dielectric loops and susceptibility to hydrostatic pressure was carried out on the quasi-ceramic samples of 1 Direct measurements of pressure effect on permittivity and thermal properties showed dT 2 /dp = -123 K•GPa -1 which is consistent well in 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 extends under pressure. Strong decrease in the entropy jump at the Pc ↔ P1 transformation with pressure increase as well as linear dependence of T 2 against pressure lead one to conclude that pressure shifts this phase transition towards the tricritical point on T -p phase diagram. Significant barocaloric effect was found in the region of the Pc ↔ P1 phase transition.

Anisotropy of piezocaloric effect at ferroelectric phase transitions in ammonium hydrogen sulphate

“…Further experimental procedure was organized taking in mind the peculiarities of the phase transitions in NH 4 HSO 4 , Due to a large volume change in the region of the first order phase transition P c ↔ P 1, detected when measuring on quasi-ceramic samples prepared using the solution and melt technology [23],…”

Conventional and inverse barocaloric effects around triple points in ferroelastics (NH 4 ) 3 NbOF 6 and (NH 4 ) 3 TiOF 5

“…The latter situation is less common than the former one. Nevertheless, inverse BCE was recently studied in ferroelectrics BaTiO 3 , (NH 4 ) 2 SO 4 , NH 4 HSO 4 and intermetallic compound La-Fe-Si-Co. [6,7,8,9] A different sign of the change of the unit cell volume is observed quite often at structural transformations in materials of the ferroelastic origin: fluorides, oxides and oxyfluorides with the perovskite-like structure. [5,10] Some of these crystals undergo at ambient pressure only one phase transition G 0 ↔ G 1 which is characterized by large entropy change ∆S and small change in the unit cell volume or in anomalous part of the β value.…”

Features of the Behavior of the Barocaloric Effect near Ferroelectric Phase Transition Close to the Tricritical Point

“…For phase transitions of the first order close to the tricritical point, the pressure behavior of both entropies can be restored analyzing the dependencies S(T, p = 0), ∆S 0 (T, p = 0), δS 0 (p) and T 0 (p) in the framework of the Landau phenomenological theory [16][17][18]. Such an analysis is impossible for AHS due to the almost complete absence at atmospheric pressure of temperature-dependent anomalous heat capacity below the transition Pc ↔ P1 [19]. In the case of AS, the relatively small value of δS 0 /∆S 0 and the wide temperature range of the existence of ∆S(T, p = 0) [11,20] suggest the applicability of the thermodynamic theory to the description of the transformation Pnam → Pna2 1 .…”