Mechanochemically Aided Synthesis of Rhenium Trioxide-Type Fluorides and Related Phases
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Bodine, Daniel LaLonde
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Abstract
A Mechanochemically aided synthetic method was used to synthesis the ReO3-type materials CaZrF6, MgZrF6, CaNbF7, and a new low temperature tetragonal phase of CaZrF6. This method uses high-energy ball milling, which reduces particle size, and allows for lower heating temperatures than conventional methods. CaZrF6 and MgZrF6 were prepared at 500 oC and 550 oC respectively. These temperatures are much lower than those reported in the originally published syntheses. A new tetragonal polymorph of CaZrF6 was prepared by the high energy ball milling of a CaF2/ZrF4 mixture, followed by heat treatment at 325 °C. This phase is thermodynamically stable with respect to the cubic phase at low temperatures and transforms to the cubic phase upon heating above ~400 °C. Tetragonal CaZrF6 is not isostructural with any known alkaline earth AZrF6 phase and displays positive thermal expansion unlike the cubic form. The anion-excess cubic ReO3-type material CaNbF7 was synthesized using a mechanochemical aided route. Cubic CaNbF7 can be prepared between 300 °C and 500 °C, but the room lattice constant varied with synthesis temperature. 19F NMR was performed on a sample made at 325 °C. This showed two isotropic peaks, a main one at 69 ppm and a secondary at 104 ppm. Integration of these peaks showed a ~ 30:1 ratio between the main and secondary peaks. Lattice constant versus temperature showed a strong thermal history dependence. CaNbF7 was found to display PTE in the studied temperature range. This is in contrast with other members of the CaZr1-xNbxF6+x series, which all displayed NTE.
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2024-01-12
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