Synthesis and characterisation of Li11RE18M4O39-δ: RE = Nd or Sm; M = Al, Co or Fe

Four new phases of general formula, Li11RE18M4O39-δ: REM = NdAl, NdCo, SmCo, SmFe, have been synthesised and characterised. The NdAl phase, and probably the others, is isostructural with the NdFe analogue, but some cation disorder and partial site occupancies prevent full structural refinement of powder neutron diffraction data. The NdCo phase also forms a solid solution with variable Li content (and charge compensation by either oxygen vacancies or variable transition metal oxidation state). The NdAl phase is a modest conductor of Li(+) ions whereas the other three phases are electronic conductors, attributed to mixed valence of the transition metal ions. Subsolidus phase diagrams for the systems Li2O-Nd2O3-Al2O3, 'CoO' have been determined and an additional new phase, LiCoNd4O8, which appears to have a K2NiF4-related superstructure, identified.