Crystal structure and colour of SrNiP 2 O 7 and SrNi 3 (P 2 O 7 ) 2

The crystal structures of SrNiP2O7 (I) and SrNi3(P2O7)2 (II) have been refined from single crystal data [(I): P21/n, Z=4, a=5.2630(16), b=8.2605(10), c=12.6018(15) Å, β=90.224(19)°, 101 parameters, 1143 independent reflections, R/wR2=0.029/0.070; (II): P21/c, Z=2, a=7.4092(9), b=7.6594(8), c=9.4474(10) Å, β=112.216(9)°, 104 parameters, 1484 independent reflections, R/wR2=0.027/0.063]. SrNiP2O7 belongs to the α-Ca2P2O7 structure family with Ni2+ ions occupying isolated square-pyramidal sites, d(Ni O) = 2.032 Å. SrNi3(P2O7)2 is isostructural to AM3(P2O7)2 (A = Ca, Pb and M = Fe, Co, Ni). Two crystallographically independent, slightly distorted [NiO6] octahedra (d(NiO) = 2.083 Å) share edges, thus forming chains along the b-axis (d(NiNi) = 3.143 and 3.226 Å). The colours of SrNiP2O7 (orange-red) and SrNi3(P2O7)2 (greenish-yellow) are significantly different. Reflectance spectra in the UV/VIS/NIR region are reported 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] (C4v) to [NiO6] (Oh) is reproduced nicely by the model calculations.