We have examined by neutron powder diffraction (NPD) a Li-poor member of the Li3xLa⅔
−
xTiO3 series, with x
= 0.04, which exhibits one of the highest reported values of ionic conductivity. The unit cell deduced, a 2ao
× 2ao
× 2ao superstructure of the elementary perovskite unit (ao
≈ 3.8 Å), results from two superimposed effects: the out-of-phase tilting of the TiO6 octahedra along the b crystallographic axis, and the ordering of La vacancies, preferentially occupying the z/c
= 0.5 over the z/c
= 0 layers. In this superstructure, defined in the Cm2m space group, a bi-dimensional conductivity is favored by the alternating disposition of lanthanum and cation vacancies in consecutive planes along the c-axis.
