Structural studies into AM(_2)O(_7) framework materials and parametric Rietveld refinement

Abstract

This thesis investigates the structural properties of the AM2O7 family of materials, many of which show interesting thermal expansion properties and remarkably complex structures. It also develops Parametric Rietveld refinement, a method of extracting more information from multi-condition powder diffraction data. Chapter 1 reviews the relevant literature. Chapter 2 describes the equipment and analytic techniques used in the work. Chapter 3 describes Parametric Rietveld refinement, which aims to fit a single, evolving structural model to a series of diffraction data collected as a function of time, temperature, pressure or other external variable. This can lead to a number of benefits over conventional, sequential Rietveld refinement: it can increase the precision of refined parameters; allow physically realistic models to be applied during data analysis; and allow 'non-crystallographic’ quantities, such as temperature or rate constants, to be refined directly from diffraction data. Chapter 4 presents a full structure solution of ZrP(_2)O(_7) at room temperature from powder diffraction data. This involved developing a method for combining Rietveld refinement, restraints and simulated annealing. Despite 136 unique atoms and close-to metrically cubic symmetry, the true structure of this orthorhombic material can be refined to a high degree of precision. Chapter 5 is an investigation into the structures and thermal expansion of two AM(_2)O(_7) species, pseudo-cubic SnP(_2)O(_7) and GeP(_2)o(_7). A full structure solution of the room temperature phase of SnP(_2)O(_7) is presented. Two higher temperature phases of SnP(_2)O(_7) and the low temperature phase of GeP(_2)o(_7) are also analysed. Variable temperature data for both materials are presented. Chapter 6 describes the synthesis and characterisation via powder diffraction of solid-state solutions of the general formula Zr(_x)Sn(_1-x)P(_2)o(_7). The variation of properties across the series is investigated both at room temperature and upon heating.