Exploring the structural and barocaloric properties of hexafluorophosphate based organic ionic plastic crystals

Refrigeration systems account for approximately 17% of the global electricity consumption, a figure that is expected to rise with the ongoing challenges posed by climate change. Current vapour-compression technologies rely on volatile refrigerants with large global warming potentials, creating an urgent demand for solid-state alternatives. One promising approach is the barocaloric effect, whereby the application of hydrostatic pressure can induce changes in isothermal entropy and adiabatic temperature through solid-solid first-order structural phase transitions. While thermoanalytical techniques, such as high-pressure differential scanning calorimetry (HP-DSC), are increasingly used to probe barocaloric materials, detailed insight into the accompanying structural changes is often limited. This thesis aims to characterise and understand the structural phase transitions of hexafluorophosphate-based organic ionic plastic crystals (OIPCs). Structure-property relationships were established by combining HP-DSC experiments with temperature- and pressure-variable X-ray/neutron diffraction measurements. To complement these experimental investigations, ab initio molecular dynamics simulations were performed to provide atomistic insights into the dynamics of this emerging class of thermal materials. Chapter One provides an introduction to barocaloric materials as a new class of solid-state refrigerants for sustainable cooling applications. Chapter Two describes the experimental and computational techniques used to characterise the structural and thermal properties of barocaloric materials. Particular emphasis is placed on high-pressure crystallography and the determination of metrics used to quantify the barocaloric performance. Complementary computational approaches are also outlined to rationalise structure-property relationships. Chapter Three presents an investigation into the temperature-driven phase behaviour of cyanomethyltrimethylammonium hexafluorophosphate ([N111CN][PF6]). Differential scanning calorimetry (DSC) is employed to evaluate the reproducibility, hysteresis, and entropy changes associated with its phase transitions. Notably, different phase behaviour is observed in powdered versus single-crystalline samples. To probe this further, variable-temperature single-crystal and powder X-ray diffraction are used to track the structural evolution across phases. Analysis of the anisotropic contraction within each phase highlights the dominant interionic interactions that influence the behaviour of each phase. Finally, ab initio molecular dynamics (AIMD) simulations are utilised to investigate the origin of the structural disorder observed crystallographically. Chapter Four expands upon the results in Chapter Three, presenting an investigation into the pressure-driven phase behaviour of [N111CN][PF6]. Utilising different rates of compression across variable-pressure powder X-ray diffraction experiments, the role of kinetics in controlling accessible phase transition pathways is revealed. The accessible pressure range is further extended with high-pressure powder neutron diffraction experiments, enabling an assessment of the high-pressure phase stability and allowing bulk moduli and free volume distributions to be extracted. These analyses provide insights into the energetic contributions of volume minimisation across the successive transitions. The barocaloric potential of [N111CN][PF6] is assessed using high-pressure DSC measurements and correlated with the structural phase transitions using a combination of variable-temperature and variable-pressure neutron and X-ray diffraction. Chapter Five presents an investigation of the phase behaviour of hexamethylguanidinium hexafluorophosphate ([HMG][PF6]) and characterises its barocaloric performance. Single-crystal X-ray diffraction models both high- and low-temperature phases, with AIMD simulations providing insight into the origin of disorder. A combination of variable-temperature powder X-ray diffraction under applied hydrostatic pressures and high-pressure DSC measurements allows for an in-depth understanding of the structural origins and performance metrics critical to the barocaloric effect in this OIPC.