La3Li3W2O12: Ionic Diffusion in a Perovskite with Lithium on both A- and 6-Sites

The structure and Li+ ion dynamics of a new class of ABO3 perovskite with Li on both the A- and B-sites are described. La3Li3W2O12 is synthesized by solid state reaction at 900 degrees C and shown by powder X-ray diffraction to adopt the structure of a monoclinic double perovskite (A2)BB'O6, (La1.5Li0.5)WLiO6, with rock salt order of W6(+) and Li+ on the B-site. High resolution powder neutron diffraction locates A-site Li in a distorted tetrahedron displaced from the conventional perovskite A-site, which differs considerably from the sites occupied by Li in the well studied La2/3xLi3xTiO3 family. This is confirmed by the observation of a lower coordinated Li+ ion in the 6Li magic angle spinning nuclear magnetic resonance (NMR) spectra, in addition to the B-site LiO6, and supported computationally by density functional theory (DFT), which also suggests local order of A-site La3+ and Li+. DFT shows that the vacancies necessary for transport can arise from Frenkel or La excess defects, with an energetic cost of similar to 0.4 eV/vacancy in both cases. Ab initio molecular dynamics establishes that the Li+ ion dynamics occur by a pathway involving a series of multiple localized Li hops between two neighboring A-sites with an overall energy barrier of similar to 0.25 eV, with additional possible pathways involving Li exchange between the A- and B-sites. A similar activation energy for Li+ ion mobility (similar to 0.3 eV) was obtained from variable temperature 6Li and 7Li line narrowing and relaxometry NMR experiments, suggesting that the barrier to Li hopping between sites in La3Li3W2O12 is comparable to the best oxide Li+ ion conductors. AC impedance-derived conductivities confirm that Li+ ions are mobile but that the long-range Li+ diffusion has a higher barrier (similar to 0.5 eV) which may be associated with blocking of transport by A-site La3+ ions.