Intervalence charge transfer and electronic transport in molten salts containing tantalum and niobium complexes of mixed valency

We report measurements of the electronic conductivity and partly the electronic mobility and optical absorption spectra in alkali halide melts containing tantalum or niobium chloride and oxychloride complexes in different oxidation states. Both electrochemical impedance spectroscopy and the Wagner-Hebb polarization method have been employed at various redox potentials and temperatures up to 1000 K. In all systems a nearly parabolic potential dependence of the electronic conductivity around the equilibrium redox potential is observed with a maximum of sigma(c)similar to 0.1 Omega(-1) cm(-1) near the Ta(iv)-Ta(v) equilibrium and lower values for the respective redox equilibria in the niobium containing systems. Results from impedance and polarization experiments agree within a factor of 2. The electronic diffusion coefficients in the tantalum containing melts have been determined from the current transients in the polarization experiments and typical values of similar to 10(-2) cm(2) s(-1) are found at 830 K with an activation energy of about 0.7 eV for 830 less than or equal to T/K less than or equal to 980. A simple two site model for electron hopping and intervalence charge transfer in these melts is discussed which qualitatively explains the observed parabolic potential dependence of the electronic conductivity and its temperature dependence.