Spectroscopic properties of five-coordinated Co2+ in phosphates
Résumé
Co3(PO4)2, SrCo2(PO4)2, Co2P2O7, BaCoP2O7 and SrCoP2O7 present different geometries of five-coordinated Co2+ ([5]Co2+) sites, coexisting with [6]Co2+ in Co3(PO4)2 and Co2P2O7, and [4]Co2+ in SrCo2(PO4)2. [5]Co Kedge XANES spectra show that the intensity of the pre-edge and main-edge is intermediate between those of [6]- and [4]Co. Diffuse reflectance spectra show the contributions of Co2+ in (D3h) symmetry for SrCo2(PO4)2, and (C4v) symmetry for BaCoP2O7 and SrCoP2O7. In Co3(PO4)2 and Co2P2O7 the multiple transitions observed arise from energy level splitting and may be labeled in (C2v) symmetry. Spectroscopic data confirm that (D3h) and (C4v) symmetries may be distinguished upon the intensity of the optical absorption bands and crystal field splitting values. We discuss the influence of the geometrical distortion and of the nature of the next nearest neighbors.