Chemical pressure as an effective tool for tuning the structural disordering and barocaloric efficiency of complex fluorides (NH4)3MF7 (M: Sn, Ti, Ge, Si)

https://iopscience.iop.org/article/10.1088/1361-6463/ad211b

Double fluoride salts (NH₄)₃M⁴⁺F₇ (M⁴⁺: Sn, Ti, Ge, Si) demonstrate a high efficiency of using chemical pressure as a tool for control and tuning structural ordering/disordering, sensitivity to hydrostatic pressure, successions of the phase transitions, etc and, as a result, for purposeful variation within a wide range of parameters of barocaloric effect (BCE). The conventional and inverse BCEs near the triple points were found on the T − p phase diagrams, combination of which can be used to construct original cooling cycle in narrow temperature and pressure ranges. Reconstructive transformation between two cubic phases, , realized in (NH₄)₃SnF₇ at atmospheric pressure and in (NH₄)₃TiF₇ at  0.4 GPa are characterized by rather low thermal hysteresis,  = 1 K, and a great entropy change,  = 110–152 J (kg · K)⁻¹, depending on the size of the central atom. At above 300–350 K, a contribution to BCE associated with the regular thermal expansion of the crystal lattice becomes comparable to entropy and temperature changes under pressure in the region of the phase transitions. An analysis of the absolute, relative and integral barocaloric characteristics of (NH₄)₃M⁴⁺F₇ compounds showed their high competitiveness with respect to other barocaloric materials considered as promising solid-state refrigerants.