Defects and Orientation Dependent Transport in Orthosilicates

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An optical spectroscopy study is proposed into the defect structure and the diffusion of defects in non-cubic orthosilicates of the type A2SiO4 with A = Co, Ni, Mn, which crystallize in the olivine structure. The spectroscopic experiments performed at high temperatures and defined oxygen activities will provide information on electronic defects (holes) and on the chemical diffusion responsible for redox processes in the material. Due to the orthorhombic structure of olivines, the kinetics for oxidation and reduction exhibits a pronounced dependence on crystal orientation. By comparison of chemical diffusion data with tracer diffusion data obtained by the American counterpart, a novel possibility is opened to determine the concentration of cation vacancies from transport data, which can then be compared with the direct data from thermogravimetry to elucidate details of the complex behavior of cation diffusion. The electrical conductivity of the transition metal orthosilicates will be measured as a function of temperature and oxygen activity for the different orientations of the crystals. In the framework of the small-polaron model, electrical conductivity is intimately related to the defect-induced contribution to optical spectra. From a comparison of these data detailed information is available on the microscopic mechanism and geometry of small-polaron hopping.