Neutron diffraction structural study of the nasicon-related phases LixMIIxMIII2 –x(SO4)3 –y(SeO4)y(MII= Mg, Ni, Zn; MIII= Al, Cr)

Abstract

The structures of a range of lithium-containing nasicon-type phases, Li_xM^II_xMM^III_{2 –x}(SO₄)_{3 –y}(SeO₄)_y(M^II= Mg, Ni, Zn; M^III= Al, Cr), have been examined by powder neutron diffraction. The refined structural data indicate a rhombohedral structure similar to that of NaZr₂(PO₄)₃ and LiZr₂(PO₄)₃ but with a reduced symmetry, R compared to Rc. Partial ordering of the M^II and M^III cations in the two octahedral sites is observed in the majority of samples, and the Li ions do not occupy the octahedral cavity sites, but unexpectedly seem to prefer the larger 8–10 coordinate sites. However, in these sites the Li ions are shifted off-centre to give four short (ca. 2 Å) Li—O bonds, arranged approximately tetrahedrally. The results are compared with the structure of LiZr₂(PO₄)₃.