Features of electronic states in the vicinity of band gap and atomic structure of Ta- and Nb-doped Li7La3Zr2O12

Li7La3Zr2O12 is one the most promising materials for Li-conducting solid electrolytes. The incorporation of Ta5+ and Nb5+ into the Zr4+ sites stabilizes its cubic structure and significantly enhances Li-conductivity, due to the formation of Li vacancies. In this research, we have studied the band gap features of Ta and Nd-doped Li7La3Zr2O12. Our findings indicate that Nb ions are present not only in the +5 valence state, but also in the +4 state, leading to the formation of oxygen vacancies. In the case of the Ta-doping, such an effect was not observed. This could be the reason for the approximately one order of magnitude higher lithium conductivity observed in the case of the Ta doping, in comparison to the Nb doping.