We report giant reversible barocaloric effects in [(CH₃)₄N]Mn[N₃]₃ hybrid organic–inorganic perovskite, near its first-order cubic-monoclinic structural phase transition at T₀ ∼ 305 K.

In this work, “breathing-caloric” effect is introduced as a new term to define very large thermal changes that arise from the combination of structural changes and gas adsorption processes occurring during breathing transitions. In regard to cooling and heating applications, this innovative caloric effect appears under very low working pressures and in a wide operating temperature range. This phenomenon, whose origin is analyzed in depth, is observed and reported here for the first time in the porous hybrid organic–inorganic MIL-53(Al) material. This MOF compound exhibits colossal thermal changes of ΔS ∼ 311 J K^–1 kg^–1 and ΔH ∼ 93 kJ kg^–1 at room temperature (298 K) and under only 16 bar, pressure which is similar to that of common gas refrigerants at the same operating temperature (for instance, p(CO₂) ∼ 64 bar and p(R134a) ∼ 6 bar) and noticeably lower than p > 1000 bar of most solid barocaloric materials. Furthermore, MIL-53(Al) can operate in a very wide temperature range from 333 K down to 254 K, matching the operating requirements of most HVAC systems. Therefore, these findings offer new eco-friendly alternatives to the current refrigeration systems that can be easily adapted to existing technologies and open the door to the innovation of future cooling systems yet to be developed.

Scheme of the different phase transitions of (( R )-(−)-3-hydroxyquinuclidium)[FeCl ₄ ].

In this work, we report a new halometallate [(CH ₃ ) ₃ S][FeCl ₄ ] with plastic crystal behaviour as a new material for multi-energy storage.

Plastic crystals are supramolecular materials that possess a unique high entropy mesophase at elevated temperatures, where a long-range structural symmetry coexists with a local molecular orientational disorder. The transition to mesophase can involve a large entropy change useful for thermal energy storage and influences the temperature range of ferroelectric and piezoelectric properties, important for sensor applications. Synchrotron X-ray diffraction and pair distribution function analysis were used to study the structure, while calorimetry, dielectric, leakage current measurements, and density functional theory were used to investigate the influence of the organic cation on the structure and properties of tetraethylammonium bromotrichloroferrate(III) [(C₂H₅)₄N][FeBrCl₃] and tetramethylammonium bromotrichloroferrate [(CH₃)₄N][FeBrCl₃]. The [(C₂H₅)₄N][FeBrCl₃] mesophase transition had an entropy change of 151.5 J·K^–1·kg^–1, while [(CH₃)₄N][FeBrCl₃] had only 49 J·K^–1·kg^–1. This was explained by the [(C₂H₅)₄N][FeBrCl₃] mesophase having less long-range structural symmetry and more local orientational disorder, of both the cations and anions, compared to [(CH₃)₄N][FeBrCl₃]. Both materials exhibited at least two conductive mechanisms below the transition, vacancy-mediated ionic and electronic conduction. The introduction of anion orientational freedom, as opposed to cation orientational freedom, at the mesophase transition was most influential for the electrical properties.