Solid state structure-reactivity studies of bixbyite-fluorite phases belonging to the scandium-vanadium-oxygen system
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Date
2017
Authors
Samandari, Golnaz
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Abstract
The main focus of this thesis is to study solid-state reaction pathways by controlling structures during the synthesis of scandium vanadium oxides. Besides the synthesis of structures involving different oxidation states, the stable phases were identified by means of ex-situ powder X-ray diffraction as well as in-situ high temperature oxidation. For the synthesis of Sc4/3V2/3O3 (Sc2VO4.5) and Sc2VO5 optimized methods are presented for the formation of a cubic bixbyite-type and tetragonal phase, respectively. In an ex-situ study Sc2VO5 was obtained by oxidation of Sc4/3V2/3O3 (Sc2VO4.5). A deeper understanding of the structural changes can be obtained by focussing on the sub-lattices throughout the reaction pathway.
The phase change from cation-disordered bixbyite Sc2VO4.5 to cation-ordered tetragonal Sc2VO5 was studied through oxidation in trace amounts of oxygen. Moreover, the oxidation of bi-phasic samples of Sc4/3V2/3O3 (Sc2VO4.5) and Sc2VO5 clearly indicates the competition of thermodynamic and kinetic routes during heating. Thermodynamic and kinetic oxidation products were compared based on the high temperature in-situ oxidation as well as ex-situ experiments. The product of topotactic oxidation of bixbyite Sc4/3V2/3O3 (Sc2VO4.5) crystallizes in a defect fluorite structure Sc4/3V2/3O3+ε, (= Sc2VO5+δ). The oxidation of the Sc4/3V2/3O3 (Sc2VO4.5) bixbyite phase results in the formation of the reconstructed tetragonal Sc2VO5 structure at high. This finding is contrasted with the low temperature topotactic oxidation resulting in a disordered defect fluorite phase. Moreover, the conventional solid state method was used to explore the possibility of obtaining tetragonal Sc2VO5. The results showed that solid state methods might not be suitable for the preparation of Sc2VO5.
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Keywords
Solid state, Topotactic, Vanadate