Correlation of Lithium Ionic Diffusion with Nb Concentration in Li_7?xLa₃Zr_2?xNb_xO₁₂ Evaluated by an Internal Friction Method

Solid lithium-ion conductors Li_7?xLa₃Zr_2?xNb_xO₁₂ (x=0.25, 0.5, 1, 1.5) with cubic garnet structure are successfully prepared by a solid state reaction method, and the effects of Nb concentration on lithium ion diffusion are investigated by means of internal friction (IF) technique. A prominent relaxation-type IF peak (actually composed of two components) is observed in each Nb doped Li₇La₃Zr₂O₁₂ compound: with apeak P_L at lower temperature and a peak P_H at higher temperature. The mechanisms of the two components are suggested to be associated with two diffusion processes of lithium ions via vacancies: 48g?48g and 48g?24 d. The relaxational strength of the IF peak gradually decreases, which is accompanied by the activation energy increasing from 0.45 eV to 0.64 eV with the increasing Nb doping level. The corresponding mechanism is ascribed to originate from lattice contraction as well as the lower concentration of diffusion ions induced by the substitution of Zr⁴⁺ by Nb⁵⁺.