A neutron diffraction study of the d{sup 0} and d{sup 10} lithium garnets Li{sub 3}Nd{sub 3}W{sub 2}O{sub 12} and Li{sub 5}La{sub 3}Sb{sub 2}O{sub 12}

The garnets Li{sub 3}Nd{sub 3}W{sub 2}O{sub 12} and Li{sub 5}La{sub 3}Sb{sub 2}O{sub 12} have been prepared by heating the component oxides and hydroxides in air at temperatures up to 950deg. C. Neutron powder diffraction has been used to examine the lithium distribution in these phases. Both compounds crystallise in the space group Ia3-bard with lattice parameters a=12.46869(9)A (Li{sub 3}Nd{sub 3}W{sub 2}O{sub 12}) and a=12.8518(3)A (Li{sub 5}La{sub 3}Sb{sub 2}O{sub 12}). Li{sub 3}Nd{sub 3}W{sub 2}O{sub 12} contains lithium on a filled, tetrahedrally coordinated 24d site that is occupied in the conventional garnet structure. Li{sub 5}La{sub 3}Sb{sub 2}O{sub 12} contains partial occupation of lithium over two crystallographic sites. The conventional tetrahedrally coordinated 24d site is 79.3(8)% occupied. The remaining lithium is found in oxide octahedra which are linked via a shared face to the tetrahedron. This lithium shows positional disorder and is split over two positions within the octahedron and occupies 43.6(4)% of the octahedra. Comparison of these compounds with related d{sup 0} and d{sup 10} phases shows that replacement of a d{sup 0} cation with d{sup 10} cation of the same charge leads to an increase in the lattice parameter due to polarisation effects.