Crystal structure and colour of SrNiP2O7 and SrNi3(P2O7)(2)

The crystal structures of SrNiP2O7 (1) and SrNi3(P2O7)(2) (II) have been re fined from single crystal data [(I): P2(1)/n, Z = 4. a = 5.2630(16), b = 8. 2605(10), c = 12.6018(15) Angstrom, beta = 90.224(19)degrees, 101 parameter s, 1143 independent reflections, R/wR(2) = 0.029/0.070; (II): P2(1)/c, Z = 2, a = 7.4092(9), b = 7.6594(8), c = 9.4474(10) Angstrom, beta = 112.216(9) degrees, 104 parameters, 1484 independent reflections, R/wR(2) = 0.027/0.06 3]. SrNiP2O7 belongs to the alpha -Ca2P2O7 structure family with Ni2+ ions occupying isolated square-pyramidal sites, (d) over bar (Ni-O) = 2.032 Angs trom. SrNi3(P2O7)(2) is isostructural to AM(3)(P2O7)(2) (A = Ca, Ph and M = Fe, Co, Ni). Two crystallographic ally independent, slightly distorted [Ni O6] octahedra ((d) over bar (Ni-O) = 2.083 Angstrom) share edges, thus for- Ming chains along the b-axis (d(Ni-Ni) = 3.143 and 3.226 Angstrom). The col ours of SrNiP2O7 (orange-red) and SrNi3(P2O7)(2) (greenish-yellow) are sign ificantly different. Reflectance spectra in the UV/VIS/NIR region are repor ted for SrNiP2O7, SrNi3(P2O7)(2), and the isotypic diphosphates CaNi3(P2O7) (2) and BaNi3(P2O7)(2). Angular overlap parameters for the Ni-O interaction are derived. The shift in d-electron energies of Ni2+ caused by the change in coordination from [NiO5] (C-4v) to [NiO6] (O-h) is reproduced nicely by the model calculations. (C) 2001 Editions scientifiques et medicales Elsev ier,SAS. All rights reserved.