A mixed alpha/beta superstructure in NASICON ionic conductors: Neutron diffraction study of Li2FeTi(PO4)(3) and Li2FeZr(PO4)(3)

The lithium conductors Li2FeTi(PO4)(3) and Li2FeZr(PO4)(3), synthesized by solid-state reaction and characterized by X-ray powder diffractometry, were studied structurally at room temperature by neutron powder diffraction at high resolution (HRPD, ISIS Facility, U.K.). By trial-and-error and Rietvel d refinements (Rp = 0.111, R(F-2)= 0.112), the first compound (orthorhombic Pbca, Z = 8; a = 8.5515(1), b = 8.6229(1), 23.9116(3) PI) was shown to hav e a complex superstructure sharing features of both the alpha and beta NASI CON-type phases of LiZr2(PO4)(3). Four (001) layers of PO4 and (Fe, Ti)O-6 polyhedra are present per unit-cell, and they are related both by I inversi on centers (a structure) and by a glide planes (beta structure). Ti4+ and F e3+ order in the two interlayer regions, respectively. Owing to the structu re complexity, only half of the lithium atoms could be refined in tetrahedr al coordination with [Li-O]=1.99 Angstrom. Li2FeZr(PO4)(3) (orthorhombic Pb na, Z = 4; a = 8.70559(8), b = 8.78572(9), c = 12.2202(1) Angstrom) proved to be similar to beta -LiZr2(PO4)(3); however, by Fourier synthesis and Rie tveld refinement (R-p = 0.0618, R(F-2)= 0.0574) Li was located in a fully o rdered tetrahedral configuration with [Li-O] = 2.01 Angstrom, instead of be ing disordered as in the beta phase of LiZr2(PO4)(3). (C) 2001 Academic Pre ss.