Li mobility in Nasicon-type materials LiM2(PO4)3, M = Ge, Ti, Sn, Zr and Hf, followed by 7Li NMR spectroscopy

Abstract

Lithium mobility in LiM₂(PO₄)₃ compounds, M = Ge and Sn, has been investigated by ⁷Li Nuclear Magnetic Resonance (NMR) spectroscopy, and deduced information compared with that reported previously in Ti, Zr and Hf members of the series in the temperature range 100–500 K. From the analysis of ⁷Li NMR quadrupole interactions (C_Q and η parameters), spin–spin T₂⁻¹ and spin–lattice T₁⁻¹ relaxation rates, structural sites occupancy and mobility of lithium have been deduced. Below 250 K, Li ions are preferentially located at M₁ sites in rhombohedral phases, but occupy intermediate M₁₂ sites between M₁ and M₂ sites in triclinic ones. In high-temperature rhombohedral phases, a superionic state is achieved when residence times at M₁ and M₁₂ sites become similar and correlation effects on Li motion decrease. This state can be obtained by large order–disorder transformations in rhombohedral phases or by sharp first order transitions in triclinic ones. The presence of two relaxation mechanisms in T₁⁻¹ plots of rhombohedral phases has been associated with departures of conductivity from the Arrhenius behavior. Long term mobility of lithium is discussed in terms of the cation vacancy distribution along conduction paths.