| Summary: | Tin molybdate (SnMo₂O₈) is a member of the well-known cubic AM₂O₈ family of materials which are famous for displaying negative thermal expansion. Remarkably, SnMo₂O₈ is the only member that shows the opposite behaviour and "normal" positive thermal expansion. It also shows more complex oxygen-ordering phases than other cubic AM₂O₈ materials. In this thesis, we explore its synthesis, phase transitions and the local structures of each known phase. We hope that studying this NTE counter-example will help us and others to gain a better understanding of how the negative thermal expansion phenomenon occurs in this family, and how it might be exploited in applications. Chapter 1 reviews the basic ideas and concepts related to thermal expansion and describes materials that display the counter-intuitive negative thermal expansion property. Particular emphasis is given on the AM₂O₈ family of materials which is of relevance to the materials studied in this thesis. Chapter 2 describes the characterisation techniques used to investigate the materials studied in this project. Chapter 3 discusses the synthesis of SnMo₂O₈ using a precursor and a gas-solid method. The precursor method was investigated in detail and optimum conditions to obtain large samples of SnMo₂O₈ were found. This allows large quantities of bulk materials to be prepared for the first time. This in turn enables some of the experiments to probe and understand its properties described in later chapters. One significant feature of the work is the difficulty in reproducibility performing chemically-controlled routes to a metastable phase. Chapter 4 investigates the conversion of the amorphous precursor obtained via the precursor method of Chapter 3 into cubic SnMo₂O₈. Reports on the crystallisation of amorphous precursors of SnO₂, MoO₃, Sn₀.₅Zr₀.₅Mo₂O₈ and ZrMo₂O₈ are also given. The amorphous phase is not a simple mixture of its amorphous constituent oxides. We follow key changes in local coordination from precursor to crystalline material using PDF methods. Moreover, the conversion is kinetically controlled, and thus temperature and time are key parameters in order to obtain the cubic phases instead of other polymorphs in the AM₂O₈ family. Finally, a methodology for enriching SnMo₂O₈ samples with ₁₇O is reported for the first time. Chapter 5 investigates the kinetics of phase transformation from α to γ-SnMo₂O₈ for materials prepared via the precursor and gas-solid methods under different temperatures at ambient pressure. Understanding this process helps us understand the origins of controllable negative, zero and positive thermal expansion in the SnMo₂O₈ family. Chapter 6 reports the phase behaviour and thermoelastic properties of SnMo₂O₈, derived from variable temperature and pressure synchrotron powder diffraction data. The bulk modulus and the pressure dependence of the thermal expansion for each known phase of SnMo₂O₈ are reported. We report the counter-intuitive property of warm hardening for the high-temperature β phase. Chapter 7 investigates the differences in the local structures of the different phases of SnMo₂O₈ by means of total scattering analysis using the reverse Monte Carlo method. The difference in the local structures of each phase is used to explain and rationalise the behaviour of all SnMo₂O₈ phases. Overall, SnMo₂O₈ behaves as a quasi-rigid unit structure with highly rigid MoO₄ tetrahedra and less rigid SnO₆ octahedra. The local structure of room-temperature phase deviates heavily from its average structure. Chapter 8 summarises the work discussed in the previous chapters.
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