ELECTRICAL AND OPTICAL PROPERTIES OF SELECTED OXIDES (VANADIUM, TITANIUM, CHROMIUM)

The high-temperature resistivity anomaly encountered in pure V(,2)O(,3) has been studied as a function of oxygen stoichiometry. Deviation from stoichiometry results in generation of singly ionized cation vacancies, leading to the suppression of the high-temperature transition, which feature supports the conclusions based on earlier magnetic susceptibility results. The (Ti(,y)Cr(,1-y))(,2)O(,3) solid solution series has been studied with respect to its electrical properties and lattice parameters a(,H) and c(,H). While no anomalous behavior is encountered, the most drastic changes in the electrical resistivity occur near the end members. a(,H) varies smoothly over the whole composition range, whereas c(,H) shows a minimum near y = 0.5. The physical properties of (Ti(,y)Cr(,1-y))(,2)O(,3) alloys are discussed in the context of the double-doped V(,2)O(,3) system. Electrical resistivity and magnetic susceptibility experiments have been performed on V(,3)O(,5) single crystal to investigate the metal-insulator transition. The MIT is accompanied by a kink in magnetic susceptibility. The results are compared with earlier findings. Charge localization appears to be the primary mechanism driving the MIT. A first systematic study of optical properties of reduced LiNbO(,3) is reported. An absorption band is found which is interpreted in terms of small-polaron formation; this assignment is consonant with the conclusions drawn from the defect structure and the transport results. The band is analyzed in the context of small-polaron theories. Single crystals of pure and doped potassium tantalate were grown by the flux technique. This is a first study of the optical properties of doped KT. A power-law wavelength dependence of the absorption coefficient is found which may be ascribed to scattering by large polarons. The electrical resistivity is observed to be an increasing function of temperature, due to temperature dependence of mobility; the charge carrier concentration remains constant and no carrier freeze-out effects were observed down to 80 K. Similar optical and electrical characteristics were obtained for reduced SrTiO(,3) and analyzed in the same manner. In contrast to results reported in the literature, our reduced SrTiO(,3) samples in the low carrier concentration range do not exhibit carrier freeze-out effects