Preparation and mobile ion transport studies of Ta and Nb doped Li6Zr2O7 Li-fast ion conductors

Pure and doped Li6−x(Zr2−xMx)O7, M = Nb and Ta; x = 0, 0.15 compounds have been prepared by the urea combustion method followed by annealing at 950 °C for 8 h. The samples are characterized by X-ray diffraction and impedance spectroscopy. Ionic conductivities, σionic, were determined in the temperature range of 60–360 °C by impedance spectroscopy. We observe that the Ta doped Li6Zr2O7 has a measurable σionic at ∼160 °C, and at 300 °C exhibits a conductivity value of 1 × 10−3 S/cm. The temperature dependence of the conductivity in the range 100–360 °C obeys an Arrhenius relation, yielding an activation energy of Ea = 0.95 eV (for M = Ta and x = 0.15). nd valence approach has been used to visualise Li+ ion migration pathways and the conductivity mechanism in these compounds. The lowest energy pathway is found to extend along the [0 1 2] direction. The Bond valence analysis also indicates a significantly anisotropic Li-ion conductivity in compounds with Li6Zr2O7 type structure, predicting activation energies of 1.1 and 0.9 eV for the low energy pathway in undoped and doped Li6Zr2O7.