The physical properties of a crystalline material depend on the spacial arrangement of its atoms or molecules, as much as on the molecules themselves. Quite often the same molecules can generate two or more different kinds of crystal, by packing together in different ways, leading to materials that are physically distinct but with the same chemical composition (polymorphs). A compound can often prefer to adopt different crystal polymorph structures under different conditions of temperature or pressure. Thus, when the temperature is changed, the crystal lattice can rearrange itself into a new three-dimensional structure - a phase transition. This is important, for example, in the pharmaceutical industry, for example, where different crystal polymorphs of drug compounds can have different solubilities, with the less soluble form being less active. Crystal phase transitions can also have drastic effects on the properties of conducting, magnetic and photonic materials, where small rearrangements of the atoms in a material have large consequences for how their electrons behave. One type of phase change that we have been studying for some time is spin-crossover, which is a rearrangement of the electrons in an atom in response to a change in temperature. This is common in some types of transition metal compound, being particularly prevalent in iron chemistry. While the molecules in a material undergo spin-crossover individually, it leads to large changes in their size and shape which are propagated through the material in the solid state. As one molecule undergoes the transition and changes its size, it causes a change in pressure in the crystal lattice that in turn promotes the transition in its nearest neighbours. These effects are transmitted through a crystal lattice at differing rates, depending on the strength of the interactions between molecules. Hence, whether a particular material undergoes spin-crossover abruptly or gradually, with temperature or with time, is controlled by its crystal packing. Spin-crossover is a rather extreme example of a crystallographic phase change, in terms of the changes involved to the structure of the material. But it can serve as a model for other, more general types of crystal phase behaviour. This project represents a concerted program to improve our understanding of phase changes in crystalline materials, using spin-crossover compounds as a test-bed. We will establish new fundamental principles for engineering phase changes into molecular crystal, that occur under pre-defined conditions (of temperature and/or light irradiation), at different rates, and with the property of hysteresis. As well as synthesising these new materials, this apparently simple objective requires state-of-the-art methods for measuring these structure changes. This will be achieved using new X-ray diffraction techniques, for inducing phase changes in crystals in high yields under controlled conditions, and for interpreting the data that result from these experiments (to deconvolute contributions from the starting and product phases of the material, for example). We will also develop improved methods for simulating the phase change events using computer models, to provide new insight into how the design of the crystal affects the propagation of the phase change through its bulk. The combination of expertise in our consortium will achieve real advances towards solving a problem, that has only been successfully addressed up to now by trial-and-error.