Influence of Cation Vacancies on Li Conductivity of La 1/2 Li 1/2-2 x Sr x TiO 3 Perovskites (0 < x ≤ 0.25): The Role of Nominal and Effective Vacancies

The Li Sr La TiO series (0 ≤ ≤ 0.25) is investigated with X-ray diffraction, nuclear magnetic resonance, and impedance spectroscopy techniques. The substitution of two Li by one Sr in Li La TiO perovskite generates cation vacancies that, when ordered in alternating planes along the -axis, confer a two-dimensional character to Li mobility. In previous works, it was shown that Li ions partially occupy the center of the six faces of the cubic perovskite, resulting in the associated A-sites to participate like a vacancy in the definition of the percolation vacancy threshold. The results obtained in the Li Sr La TiO series are compared with those obtained in the Li La TiO series, and other Sr-doped solid solutions (Li Sr La TiO and Li Sr La TiO ), to highlight the importance of the effective vacancies with respect to the nominal ones in conductivity. The analysis of four series, belonging to the ternary SrTiO -La TiO -Li TiO phase diagram, permits a better understanding of the ionic conduction mechanism in perovskites. The results show that the vacancy percolation model is more adequate to explain Li conductivity than the conventional hopping probability model. In the analyzed series, Li conductivity is maximum when a small amount of Sr is incorporated into the pseudo-cubic La Li TiO end member, while it decreases as the amount of strontium increases.