X-RAY STUDIES OF PURE AND DOPED VANADIUM SESQUIOXIDE
Abstract
The crystal structure of V(,2)O(,3) has been studied first with substitution by Cr(,2)O(,3) from 0 to 5% Cr, then as a function of temperature, with 0.325% and 0.350% Cr content, from 23(DEGREES) to 550(DEGREES)C, and finally the distribution of valence electron density in metallic V(,2)O(,3) and insulating 1.0% Cr-V(,2)O(,3) has been mapped, using X-ray diffraction data from single crystals. Doping V(,2)O(,3) with up to 1.25% Cr produces changes in structure consistent with a slight weakening of the bonding metal-metal interactions in the basal plane, leading to an increase in the metal-metal distances coupled with changes which maintain constant metal-oxygen distances. A discontinuity appears at about 1.25% Cr as the first-order transition from metal to insulating behavior occurs with increasing Cr content. No change in crystal symmetry accompanies this transformation. It appears that the metal-metal interactions are retained even in the insulating phase of Cr-doped V(,2)O(,3). A comparison of the structural variation in this series of Cr-doped samples with that of the Ti-doped V(,2)O(,3) system suggests that the change from metallic to insulating behavior cannot be a structural effect. Heating 0.325% Cr-V(,2)O(,3) produces a sigmoidal change in structure similar to that seen in pure V(,2)O(,3). For 0.350% Cr-V(,2)O(,3), a discontinuous structural change, accompanying the first-order metal-insulator transition, occurs with increasing temperature. The crystal structure of these two alloys are similar not only at 23(DEGREES)C, but also at 550(DEGREES)C in spite of their structural and electronic differences at intermediate temperatures. The changes in atomic positions, upon doping with chromium or upon changing the temperature, are consistent with the schematic model by Honig et al.. The change in resistivity as a function of the change in metal-metal distances appears different with chromium-coping than with heating. The distribution of valence electron density in metallic V(,2)O(,3) and insulating 1.0% Cr-V(,2)O(,3) are similar. Both show significant bonding interaction between near-neighbouring metal atoms. The interaction between metal atoms along the c direction appears to have direct, or (sigma), bonding characteristics, while those along the basal plane direction have indirect, or (pi), bonding characteristics.
Degree
Ph.D.
Subject Area
Chemistry
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