Synthesis and structural characterisation of SnMo2O8

Tin molybdate (SnMo\textsubscript{2}O\textsubscript{8}) is a member of the well-known cubic AM\textsubscript{2}O\textsubscript{8} family of materials which are famous for displaying negative thermal expansion. Remarkably, SnMo\textsubscript{2}O\textsubscript{8} 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 AM\textsubscript{2}O\textsubscript{8} 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 AM\textsubscript{2}O\textsubscript{8} 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 SnMo\textsubscript{2}O\textsubscript{8} using a precursor and a gas-solid method. The precursor method was investigated in detail and optimum conditions to obtain large samples of SnMo\textsubscript{2}O\textsubscript{8} 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 SnMo\textsubscript{2}O\textsubscript{8}. Reports on the crystallisation of amorphous precursors of SnO\textsubscript{2}, MoO\textsubscript{3}, Sn\textsubscript{0.5}Zr\textsubscript{0.5}Mo\textsubscript{2}O\textsubscript{8} and ZrMo\textsubscript{2}O\textsubscript{8} 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 AM\textsubscript{2}O\textsubscript{8} family. Finally, a methodology for enriching SnMo\textsubscript{2}O\textsubscript{8} samples with \textsuperscript{17}O is reported for the first time. Chapter 5 investigates the kinetics of phase transformation from $\alpha$ to $\gamma$-SnMo\textsubscript{2}O\textsubscript{8} 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 SnMo\textsubscript{2}O\textsubscript{8} family. Chapter 6 reports the phase behaviour and thermoelastic properties of SnMo\textsubscript{2}O\textsubscript{8}, 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 SnMo\textsubscript{2}O\textsubscript{8} are reported. We report the counter-intuitive property of warm hardening for the high-temperature $\beta$ phase. Chapter 7 investigates the differences in the local structures of the different phases of SnMo\textsubscript{2}O\textsubscript{8} 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 SnMo\textsubscript{2}O\textsubscript{8} phases. Overall, SnMo\textsubscript{2}O\textsubscript{8} behaves as a quasi-rigid unit structure with highly rigid MoO\textsubscript{4} tetrahedra and less rigid SnO\textsubscript{6} octahedra. The local structure of room-temperature phase deviates heavily from its average structure. Chapter 8 summarises the work discussed in the previous chapters