NEUTRON-DIFFRACTION STRUCTURAL STUDY OF THE NASICON-RELATED PHASES LI(X)M(X)(II)M(2-X)(III)(SO4)(3-Y)(SEO4)(Y) (M(II)=MG, NI, ZN M(III)=AL,CR)

The structures of a range of lithium-containing nasicon-type phases, L i(x)M(x)(II)MM(2-x)(III)(SO4)(3-y)(SeO4)(y) (M(II) = Mg, Ni, Zn; M(III ) = Al, Cr), have been examined by powder neutron diffraction. The ref ined structural data indicate a rhombohedral structure similar to that of NaZr2(PO4)(3) and LiZr2(PO4)(3) but with a reduced symmetry, R $$( $) over bar 3 compared to R $($) over bar$$ 3c. Partial ordering of th e 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 coordina te sites. However, in these sites the Li ions are shifted off-centre t o give four short (ca. 2 Angstrom) Li-O bonds, arranged approximately tetrahedrally. The results are compared with the structure of LiZr2(PO 4)(3).