Date of Award
Doctor of Philosophy (PhD)
Professor John E. Greedan
The compound BaV10O15 exhibits a first-order structural phase transition in the temperature range 105K-135K, depending upon the sample. The variation in transition temperature has been correlated with a variation in the lattice parameters and degree of sample oxidation. Crystal symmetry decreases from Cmca in the high temperature phase, to Pbca in the low temperature phase. Single crystal work has shown a decrease in certain V-V distances, which has been attributed to metal-metal bond formation. A cooperative magnetic transition occurs near 42K that contains features of both spin glass behavior and long range magnetic order. Powder neutron diffraction has shown that the moment is much lower than the expected value of 2.2μB. Furthermore, the magnetic entropy released below 70K is less than 15% of the entropy release expected for this moment value.
The series BaV10-xTixO15 (x=0-8) is isostructural with the high temperature phase of BaV10O15. Spin glass-like behavior is observed up to a doping level of ~50% titanium, above which there is no apparent collective magnetic transition. Two models have been proposed to explain the thermopower and resistivity data for this series. The first model is based upon small polaron hopping of localized carriers. The second involves hole doping of instrinsic band semiconductor.
A new preparation route has been developed for 5H BaVO2.8 using the induction furnace. It was determined that addition of 4% to 8% molar excess of vanadium in the reaction produced a pure material. In a study of BaVO2.8 using power neutron diffraction, peaks due to long-range magnetic order were not detected. The possibilities of either a spin-glass or superparamagnetic ground state remain. Transport property measurements on BaVO2.8 suggest that two-dimensional variable range hopping is occurring at low temperatures. With oxidation, the dominant carrier type changes from holes (BaVO2.8) to electrons (BaVO3.0).
Bridges, Craig Allan, "Structural and Electronic Properties of BaV10O15, BaV10-xTixO15, and BaVO3-x" (2002). Open Access Dissertations and Theses. Paper 771.