Dopant concentration effect on NiO-doped sodium metaphosphate glasses: A combined X-ray absorption fine structure (XAFS) and UVNIS/NIR spectroscopic investigation

NiO-doped sodium metaphosphate glasses (NaPO3)(1-x),(NiO)(x), (0.008 <= x <= 0.30) show a color shift from yellow to orange-brown with increasing NiO concentration. XANES and EXAFS spectra of these glasses suggest the presence of [(NiO6)-O-II] groups as chromophores. EXAFS (Ni K-edge) analysis of the NiO-doped phosphate glasses including an evaluation of higher coordination shells leads to d(av)(Ni-O) = 2.06(2) angstrom. Evidence is provided for an increased connectivity of [(NiO6)-O-II] chromophores at higher NiO concentration in the glasses. A decrease in the intensity of the main absorption edge with increasing nickel oxide concentration is observed. This systematic decrease is attributed to a change in the bonding characteristics between nickel(II) and the coordinating phosphate groups from mainly ionic to a small but significant contribution of covalent bonding. A similar effect is observed in the electronic absorption spectra of glasses showing a decrease of the Racah parameter B for the Ni2+ ions.