METAL-INSULATOR TRANSITIONS IN NONSTOICHIOMETRIC AND TITANIUM-DOPED VANADIUM-TRIOXIDE
Abstract
Single crystals of V(,2)O(,3) of controlled stoichiometry have been prepared by using the technique of crucibleless rf induction melting, followed by subsolidus annealing at fixed oxygen fugacities obtained by the use of appropriate gas mixtures. Extensive electrical, magnetic, and calorimetric measurements on nonstoichiometric V(,2)O(,3) and stoichiometric Ti-doped V(,2)O(,3) are reported. New features in the physical properties of nonstoichiometric V(,2)O(,3) have been observed. It has been found that the temperature of the metal-insulator transition in V(,2)O(,3) may be lowered to 14 K by achieving progressively greater departures from stoichiometry. As the transition temperature is lowered, the conductivity discontinuity at the transition increases, in contrast to earlier findings. In addition, the temperature hysteresis of the metal-insulator transition increases with excess oxygen content. In agreement with previous reports, a low temperature transition from paramagnetic metal to antiferromagnetic metal has been observed in highly nonstoichiometric V(,2)O(,3). The associated critical behavior of resistivity has been observed. The influence of nonstoichiometry and of Ti-doping are contrasted by studying the manner in which metal-insulator transition in V(,2)O(,3) is suppressed by Ti-doping. The different effects of the two are accounted for by invoking the mechanism of ionized impurity scattering in nonstoichiometric V(,2)O(,3) and of conduction in a nondegenerate band semi-conductor for Ti-doped V(,2)O(,3). The enthalpy and the entropy of the metal-insulator transition in nonstoichiometric and Ti-doped V(,2)O(,3) have been measured in a relaxation calorimeter of a new design. The construction and operation of this calorimeter has been described. The entropy change of the transition approaches zero as the transition temperature approaches zero. These findings are analyzed by evaluating the various contributions to the entropy change at the transition. The electronic term in the low temperature heat capacity of nonstoichiometric and Ti-doped V(,2)O(,3) has been found to be larger than previously reported and to depend on sample composition. In agreement with earlier work, the entropy of the low temperature paramagnetic metal-antiferromagnetic metal has been found to be much smaller than expected on the basis of localized moments at the minority V('4+) sites. The recently developed Moriya theory of itinerant electron antiferromagnetism has been invoked to provide a tentative explanation of these findings.
Degree
Ph.D.
Subject Area
Chemistry
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