Structural and high pressure studies of some low and negative thermal expansion materials
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The research presented in this thesis focuses on the structural studies and the high pressure behavior of oxide negative thermal expansion (NTE) materials that can be classified as framework materials. First two chapters were devoted to TaO2F which adopts the ReO3-type cubic structure. Our studies under pressure revealed a rather complicated high pressure behavior for this deceivingly simple compound. The diffraction measurements at variable temperature and high pressure indicated that pressure had a significant effect on the linear coefficient of thermal expansion of TaO2F. In the remainder of the thesis, compounds that belong to the Sc2W3O12 family were examined. High-pressure in-situ powder diffraction studies were conducted on Zr2WO4(PO4)2, Zr2MoO4(PO4)2, Hf2WO4(PO4)2, and Sc2W3O12 in order to investigate the effects of pressure on the coefficients of thermal expansion, existence of phase transitions, phase transition pressures and structural changes occurring upon phase transitions.