Dopant Concentration Effect on NiO-Doped Sodium Metaphosphate Glasses: A Combined X-Ray Absorption Fine Structure (XAFS) and UV/VIS/NIR Spectroscopic Investigation

Abstract

NiO-doped sodium metaphosphate glasses (NaPO3)1−χ(NiO)χ (0.008 ≤ χ ≤ 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 [NiIIO6] groups as chromophores. EXAFS (Ni K-edge) analysis of the NiO-doped phosphate glasses including an evaluation of higher coordination shells leads to dav(Ni-O) = 2.06(2) Å. Evidence is provided for an increased connectivity of [NiIIO6] 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.