Determination and Rietveld refinement of the crystal structure of Li0.50Ni0.25TiO(PO4) from powder X-ray and neutron diffraction

The structure of the oxyphosphate Li0.50Ni0.25TiO(PO4) has been determined from conventional X-ray and neutron powder diffraction data. The parameters of the monoclinic cell (space group P2(1)/c, Z=4), obtained from X-ray results, are: a = 6.3954(6) Angstrom, b = 7.2599(6) Angstrom, c = 7.3700(5) and beta = 90.266(6)degrees; those resulting from neutron study are: a = 6.3906(7) b = 7.2568(7) Angstrom c = 7.3673(9) Angstrom, and beta = 90.234(7)degrees. Refinement by the Rietveld method using whole profile, leads to satisfactory reliability factors: cR(wp) = 0.128, cR(p) = 0.100, and R-B = 0.038 for X-ray and cR(wp) = 0.110, cR(P) = 0.120, and R-B = 0.060 for neutrons. The structure of Li0.50Ni0.25TiO(PO4) can be described as a TiOPO4 framework constituted by chains of tilted corner-sharing TiO6 octahedra running parallel to the c axis and cross linked by phosphate tetrahedra. In this framework, there are octahedral cavities occupied by Li and Ni atoms: Li occupies the totality of the 2a sites and Ni occupies statistically half of the 2b sites. Ti atoms are displaced from the center of octahedra units in alternating long (2.242 Angstrom) and short (1.711 Angstrom) Ti-O bonds along chains. (C) 2002 International Centre for Diffraction Data.