Structure and dynamics in framework materials

Abstract

This thesis details a study of framework materials of the AM2O8 and AM2O7 families, which are of interest due to their unusual thermal expansion properties, phase transitions and often complex structures. The combination of results from NMR and diffraction techniques has enabled several new insights into the structure and dynamics to be made. Chapter 1 reviews the literature on negative thermal expansion materials that are of relevance to this project. Chapter 2 outlines the background to the key experimental techniques employed in this work. Chapter 3 outlines 17O isotopic enrichment techniques and qualitative results for several AM2O8 phases. 17O NMR results for cubic ZrW2O8 including variable temperature spectra and 2DExchange Spectroscopy (EXSY) are presented. The a / ß phase transition and low temperature oxygen mobility in ZrW2O8 are investigated. NMR results enabled the determination of a mechanism of oxygen exchange different to the previously suggested mechanism, which is disproved. The results of in situ diffraction experiments to investigate phase transitions inZrMo2O8 are also presented. 17O enriched samples of cubic-, trigonal- and LT- ZrMo2O8 samples were prepared, and their 17O NMR spectra recorded. Chapter 4 describes quantitative 17O NMR studies used to characterise oxygen dynamics inZrW2O8. The results of a range of NMR experiments, including a combination of 1D EXSY and saturation recovery experiments, are interpreted to give a measure of the rate and the activation energy for oxygen exchange. The experiments and derivation of the required theoretical background are detailed. Chapter 5 outlines a range of structural studies on AM2O7 materials. The room temperature superstructure of HfP2O7 is determined using a combination of X-Ray and neutron diffraction.31P NMR is reported for the high temperature phases of HfP2O7 and ZrP2O7 for the first time. The symmetry of the high temperature phase of HfP2O7 is investigated by Rietveld refinement of neutron diffraction data. Unusual effects in the 51V NMR spectra of HfV2O7 and ZrV2O7 are also reported. Chapter 6 outlines preliminary work on computational methods to investigate the structural dependence of the 31P NMR chemical shift of pyrophosphate materials. DFT calculations of NMR parameters were carried out, and a methodology developed for the derivation of suitable theoretical model structures.