Most materials expand when heated, yet there exist some that contract – a rare phenomenon known as negative thermal expansion (NTE). Uniaxial NTE has been observed over a pronounced temperature range in certain phases of several Ruddlesden-Popper oxides, which are layered perovskites with general formula An+1BnO3n+1 that are characterised by their layer thickness, n. Despite being renowned for possessing many interesting functional properties, ABO3 perovskites, which form the n = • limit of the Ruddlesden–Popper series, rarely exhibit NTE of this kind and over such a wide temperature range. A central theme of this thesis is thus to establish the origin of uniaxial NTE and discover why it is prevalent only in particular phases of low n Ruddlesden–Popper oxides. Since tunability has become a paradigm in the field of functional ceramics, a further aim is to develop ways to engineer oxides with controllable NTE.
This thesis presents mainly the results of first-principles simulations, but also analysis of experimental data and mechanical models to achieve these aims. It is found that uniaxial NTE arises from the combined effects of octahedral tilt vibrations and anisotropic elastic compliance. This compliance is unique to the NTE phase of low n Ruddlesden–Popper oxides and I propose an atomistic model, the corkscrew mechanism, to explain it based on structural arguments. I also find that the magnitude of NTE in Ruddlesden–Popper oxides may be tuned in two ways: by varying the layer thickness – which changes the elasticity; and by altering the chemistry (changing A and B) – which affects the dynamic tilts of BO6 octahedra. Combining these insights gained from atomistic simulations has led to a compound being engineered that displays record uniaxial NTE within a Ruddlesden–Popper oxide due to this mechanism.