Ferroelastic and Ferroelectric Phase Transitions in Fluoro- and Oxyfluorometallates

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

“…That is why for a long time BCE in solids has been studied to a much lesser extent than magneto(MCE)-and electro(ECE)-caloric effects in ferromagnets and ferroelectrics some of which are considered as potential solid refrigerants competitive in comparison with gaseous coolants. [1,2,3,4,5] However, near phase transitions where many properties of solids exhibit strongly anomalous behaviour, a value of the (∂V /∂T ) p derivative can often vary by several orders of magnitude. In such a case both barocaloric characteristics, ∆S BCE and ∆T AD , can reach very large magnitudes comparable with the highest values of extensive and intensive MCE and ECE.…”

“…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. In such a case, in accordance with the Clausius-Clapeyron, dT /dp = δV · δS −1 , and Ehrenfest, dT /dp = T 0 · ∆β · ∆C −1 p , equations, baric coefficients dT G0−G1 /dp is also characterized by small magnitude.…”

Barocaloric effect in ferroelastic fluorides and oxyfluorides

“…Great values of ΔS accompany usually phase transitions associated either with orientational or positional disorder of some structural elements in initial high temperature phase, which disappeared through the process of structural distortion accompanied by the symmetry decrease [25]. As an illustrative example one can refer to the wide series of fluoro-and oxyfluoro-compounds with structure containing cationic and anionic polyhedra, which can be disordered by different ways [21,26].…”

“…Depending on the composition and the Fig. 1a [21,27]. Table 1 demonstrates that changing chemical pressure by variation of the central atom size and the relation between F and O atoms, one can change the temperature, entropy as well as baric coefficient typical for structural transformation.…”

“…Recent reviews on CE of different physical nature [15,16], have underlined that external hydrostatic pressure, up to the present, was mainly used as an instrument for tuning MCE and only episodically for BCE study [17 -20]. The present paper is devoted to BCE in the family of complex fluorides and oxyfluorides with the octahedral anionic groups in structure, which is very rich of compounds undergoing ferroelastic phase transitions accompanied with large values of both the entropy change and baric coefficient dT/dp [21].…”