Barocaloric effects in the vicinity of first-order phase transitions.

Abstract

Barocaloric effects refer to the adiabatic temperature change (ΔT) or isothermal en-tropy change (ΔS) of a solid under the application (or removal) of hydrostatic pressure, which are expected to show a great enhancement near a first-order phase transition. Materials with giant or colossal barocaloric effects hold the potential to offer eco-friendly and more efficient alternatives to solid-state heat-pump technologies for re-placing hazardous gases used in traditional vapor-compression cooling and heating. However, those effects are often limited by the need for large pressures, small temper-ature span and by significant hysteresis. The latter feature increases the input work required to drive the cooling cycle reversibly and reduce the temperature range of op-eration. In this thesis, I studied the barocaloric effect in materials undergoing order-disorder transition and showing mechanical flexibility, especially on molecular crystal and or-ganic-inorganic hybrid perovskites. I found that a molecular crystal of C60 shows giant reversible barocaloric effects at very small pressure changes. I demonstrated that the high sensitivity of the transition to the applied pressure allows for giant reversible baro-caloric effects over a very wide temperature range under moderate pressure changes. II Colossal barocaloric effects so far have only been reported in molecular materials with orientational disorder, for example neopentylglycol. I demonstrated that the lay-ered hybrid organic-inorganic perovskite (C10H21NH3)2MnCl4 also exhibits colossal effects, but with improved reversibility and performance. By tuning the chemical com-position of the organic and inorganic parts, the research offers a path for designing hybrid perovskites with improved barocaloric behaviour. I analysed the anisotropic lattice deformation in two-dimensional hybrid compounds and then investigated the effect of anisotropy on caloric properties. Further studies in-dicated a colossal caloric effect can be achieved in the compound by using uniaxial stress along the direction of the c-axis. The research presents a new perspective in find-ing excellent mechanocaloric effects in strong anisotropic materials.