Exploring the thermal expansion of fluorides and oxyfluorides with ReO₃-type structures: from negative to positive thermal expansion

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Greve, Benjamin K.
Wilkinson, Angus P.
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This thesis explores the thermal expansion and high pressure behavior of some materials with the ReO₃ structure type. This structure is simple and has, in principle, all of the features necessary for negative thermal expansion (NTE) arising from the transverse thermal motion of the bridging anions and the coupled rotation of rigid units; however, ReO₃ itself only exhibits mild NTE across a narrow temperature range at low temperatures. ReO₃ is metallic because of a delocalized d-electron, and this may contribute to the lack of NTE in this material. The materials examined in this thesis are all based on d⁰ metal ions so that the observed thermal expansion behavior should arise from vibrational, rather than electronic, effects. In Chapter 2, the thermal expansion of scandium fluoride, ScF₃, is examined using a combination of in situ synchrotron X-ray and neutron variable temperature diffraction. ScF₃ retains the cubic ReO₃ structure across the entire temperature range examined (10-1600 K) and exhibits pronounced negative thermal expansion at low temperatures. The magnitude of NTE in this material is comparable to that of cubic ZrW₂O₈, which is perhaps the most widely studied NTE material, at room temperature and below. This is the first report of NTE in an ReO₃ type structure across a wide temperature range. Chapter 3 presents a comparison between titanium oxyfluoride, TiOF₂, and a vacancy containing titanium hydroxyoxyfluoride, Tiₓ(O/OH/F)₃. TiOF₂ was originally reported to adopt the cubic ReO₃ structure type under ambient conditions, therefore the initial goal for this study was to examine the thermal expansion of this material and determine if it displayed interesting behavior such as NTE. During the course of the study, it was discovered that the original synthetic method resulted in Tiₓ(O/OH/F)₃, which does adopt the cubic ReO₃ structure type. The chemical composition of the hydroxyoxyfluoride is highly dependent upon synthesis conditions and subsequent heat treatments. This material readily pyrohydrolyizes at low temperatures (~350 K). It was also observed that TiOF₂ does not adopt the cubic ReO₃ structure; at room temperature it adopts a rhombohedrally distorted variant of the ReO₃ structure. Positive thermal expansion was observed for TiOF₂ from 120 K through decomposition into TiO₂. At ~400 K, TiOF₂ undergoes a structural phase transition from rhombohedral to cubic symmetry. High pressure diffraction studies revealed a cubic to rhombohedral phase transition for Tiₓ(O/OH/F)₃ between 0.5-1 GPa. No phase transitions were observed for TiOF₂ on compression. In Chapter 4, an in situ variable pressure{temperature diffraction experiment examining the effects of pressure on the coefficients of thermal expansion (CTE) for ScF₃ and TaO₂F is presented. In the manufacture and use of composites, which is a possible application for low and NTE materials, stresses may be experienced. Pressure was observed to have a negligible effect on cubic ScF₃'s CTE; however, for TaO₂F the application of modest pressures, such as those that might be experienced in the manufacture or use of composites, has a major effect on its CTE. This effect is associated with a pressure-induced phase transition from cubic to rhombohedral symmetry upon compression. TaO₂F was prepared from the direct reaction of Ta₂O₅ with TaF₅ and from the digestion of Ta₂O₅ in hot hydro uoric acid. The effects of pressure on the two samples of TaO₂F were qualitatively similar. The slightly different properties for the samples are likely due to differences in their thermal history leading to differing arrangements of oxide and uoride in these disordered materials. In Chapter 5, the local structures of TiOF₂ and TaO₂F are examined using pair distribution functions (PDFs) obtained from X-ray total scattering experiments. In these materials, the anions (O/F) are disordered over the available anion positions. While traditional X-ray diffraction provides detailed information about the average structures of these materials, it is not suffcient to fully understand their thermal expansion. Fits of simple structural models to the low r portions of PDFs for these materials indicate the presence of geometrically distinct M{X{M (M = Ti, Ta; X = O, F) linkages, and a simple analysis of the TaO₂F variable temperature PDFs indicates that these distinct links respond differently to temperature.
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