<i>Ab initio</i> determination and Rietveld refinement of the crystal structure of <i>Ni</i> _0.50 <i>TiO</i> ( <i>PO</i> ₄ )

Abstract

The structure of the oxyphosphate Ni 0.50 TiO ( PO 4 ) has been determined ab initio from conventional X-ray powder diffraction data by the “heavy atom” method. The cell is monoclinic (space group P 2 1 / c , Z =4) with a =7.3830(5) Å, b =7.3226(5) Å, c =7.3444(5) Å, and β=120.233(6)°. Refinement of 46 parameters by the Rietveld method, using 645 reflexions, leads to cR wp =0.152, cR p =0.120, and R B =0.043. The structure of Ni 0.50 TiO ( PO 4 ) can be described as a TiOPO 4 framework constituted by chains of tilted corner-sharing TiO 6 octahedra running parallel to the c axis, crosslinked by phosphate tetrahedra and in which one-half of octahedral cavities created are occupied by Ni atoms. Ti atoms are displaced from the center of octahedra units in alternating long (2.231) and short (1.703 Å) Ti–O bonds along chains.