Manipulating Phase Transitions in Barocaloric Materials

Abstract

Solid-state phase transitions are central to the design of the advanced, responsive materials needed to help address a wide range of pressing global challenges. This is particularly relevant in the context of barocaloric effects—thermal changes induced in a material by applied hydrostatic pressure—that are strongest near first-order phase transitions. Barocaloric effects emerge from an intricate coupling between volume and entropy, offering a versatile mechanism to drive solid state cooling, heat pump, and thermal energy storage cycles. Although critical to realizing the full potential of barocaloric effects, it remains difficult to manipulate the thermodynamics of phase transitions in the solid state, and the microscopic mechanisms responsible for barocaloric effects are not well understood. The work presented in this dissertation describes efforts to manipulate and understand phase transitions in barocaloric materials, aiming to bring molecular insights into this emerging class of thermal materials. Chapter One provides a brief introduction to barocaloric materials as a new class of solid refrigerants for sustainable cooling and heating applications. Chapters Two explores how hydrocarbon order–disorder transitions in two-dimensional metal–halide perovskites drive large barocaloric effects at low pressures. Pressure-induced phase transitions in two representative layered perovskites (C10H21NH3)2MnCl4 and (C9H19NH3)2CuBr4 are evaluated in detail using high-pressure calorimetry and X-ray diffraction. These efforts demonstrate how the confined nature of order–disorder phase transitions and the synthetic tunability of layered perovskites can be leveraged to reduce phase transition hysteresis. Chapter Three describes synthetic efforts to manipulate the thermodynamics of hydrocarbon order–disorder transitions in layered perovskites, with a particular focus on establishing design principles for promoting strong barocaloric effects. A compositionally diverse library of layered perovskites is presented, and relationships between equilibrium structure, composition, and phase-change thermodynamics are investigated. Chapter Four discusses a systematic investigation of phase transitions in di n alkylammonium halides. Specifically, pressure–temperature phase diagrams of three representative compounds (C6H13)2NH2X (X = Cl, Br, I) are explored through a combination of high-pressure calorimetry, X-ray diffraction, and Raman spectroscopy. These results reveal insights into the structural and chemical factors that produce large pressure sensitivity and entropy changes in barocaloric materials, which has exciting implications for tunable thermal storage and solid-state cooling. In Chapter Five, spin-crossover transitions in the molecular iron(II) complex Fe[HB(tz)3]2 (HB(tz)3− = bis[hydrotris(1,2,4 triazol-1-yl)borate]) are investigated as a mechanism to induce reversible barocaloric effects. Notably, in situ powder X-ray diffraction experiments are presented that enable a direct evaluation of dissipated energy during pressure cycling. These efforts establish spin crossover as a key mechanism for the design of barocaloric materials, highlighting opportunities to use coordination chemistry to tailor the thermodynamics of barocaloric effects. Chapter Six discusses a mechanism that renders high-entropy phase transitions extremely sensitive to pressures. Inspired by the fluidizing effects of anesthetic gases on cell membranes, this mechanism leverages the thermodynamic effects of a pressure-transmitting medium on order disorder transitions in layered barocaloric solids. Through this mechanism, hysteresis barriers can be readily overcome, enabling large reversible temperature changes (> 14 °C) to be induced at record low pressures ( 40 bar). This approach substantially reduces the complexity, cost, and power consumption required for operating barocaloric cooling and heating cycles, unlocking the use of solid refrigerants in practical devices.