Crystal structure and colour of SrNiP_2O_7 and SrNi_3(P_2O_7)_2

摘要

The crystal structures of SrNiP_2O_7 (I) and SrNi_3(P_2O_7)_2 (II) have been refined from single crystal data ((I): P2_1, Z = 4, a = 5.2630(16), b = 8.2605(10), c = 12.6018(15) A, #beta# = 90.224(19) deg, 101 parameters, 1143 independent reflections, R/#omega#R_2 = 0.029/0.070; (II): P2_1/c, Z = 2, a = 7.4092(9), b = 7.6594(8), c = 9.4474(10) A, #beta# = 112.216(9) deg, 104 parameters, 1484 independent reflections, R/#omega#R_2 = 0.027/0.063). SrNiP_2O_7 belongs to the #alpha#-Ca_2P_2O_7 structure family with Ni~(2+) ions occupying isolated square-pyramidal sites, d-bar(Ni-O) = 2.032 A. SrNi_3(P_2O_7)_2 is isostructural to AM_3(P_2O_7)_2 (A = Ca, Pb and M = Fe, Co, Ni). Two crystallographically independent, slightly distorted (NiO_6) octahedra (d-bar (Ni-O) = 2.083 A) share edges, thus forming chains along the b-axis (d(Ni-Ni) = 3.143 and 3.226 A). The colours of SrNiP_2O_7 (orange-red) and SrNi_3(P_2O_7)_2 (greenish-yellow) are significantly different. Reflectance spectra in the UV/VIS/NIR region are reported for SrNiP_2O_7, SrNi_3(P_2O_7)_2, and the isotypic diphosphates CaNi_3(P_2O_7)_2 and BaNi_3(P_2O_7)_2. Angular overlap parameters for the Ni-O interaction are derived. The shift in d-electron energies of Ni~(2+) caused by the change in coordination from (NiO_5) (C_(4#upsilon#)) to (NiO_6) (O_h) is reproduced nicely by the model calculations.