Chemically stable solid lithium ion conductors

Chemically stable solid Li ion conductor of garnet-like crystal structure and Li ion conductivity greater than 3.4.10 ->6 S/cm and stoichiometric composition L(5+x)AxG(3-x)M2O12, where L = Li ion, M = Nb or Ta. An independent claim is included for a method of preparing the Li ion conductor by reaction of salts and/or oxides of L, A, G, and M, where L = Li, A = monovalent metal cation, G = trivalent metal cation, M= pentavalent metal cation.