The structure of the high-temperature solid electrolyte lithium sulphate at 908K

Abstract

The structure of the high-temperature, plastic form of lithium sulphate has been determined from neutron powder diffraction data collected at 908K. The structure is face-centred cubic, a=7.07 AA with the SO₄^2- ion situated at the origin and the oxygen atoms rotationally disordered about the sulphur atom. The Li⁺ ions occupy the +or-(¹/₄,¹/₄,¹/₄) positions. The large isotropic temperature factors (B(SO₄)=17.5 AA² and B(Li)=33 AA²) suggest that the lithium ions occupy a statistical distribution of sites instantaneously displaced from +or-(¹/₄,¹/₄,¹/₄), in short-range correlation with the instantaneous orientations of the surrounding SO₄^2- ions. This is the first example of a fast ionic conductor where ionic motion is shown to be enhanced by rotational motion of the translationally static counter ions, although closer study of simple alkali salts may reveal several other examples.