Towards enhanced structural and optical properties of NaF-added oxyfluoride borate glass activated by nickel ions

This article reports on the impact of sodium fluoride (NaF) on the structure and properties of oxyfluoride borate glass activated by small portions of nickel ions and prepared by the melt-quenching method. The structural and optical properties were investigated using mass density, infrared spectra, optical spectra, ligand field environment and non-linear optical behaviors. Structural variations were reported by the increase in experimental density by 19 % and empirical density by 5 %, while the molar volume decreased by 32 % for experimental molar volume and 24 % for empirical molar volume. Furthermore, adding sodium fluoride enhanced the network conversion from trigonal to tetrahedral structural units ( N 4 ratio increased from 31.66 % to 56.12 %), forming new ones such as BO 3 F, and BF 4 , with observed decrease in non-bridging bonds ( N B O + F decreased from 18.20 % to 4.04 %). Regarding optical activity, a set of absorption bands of nickel ions within visible and near infrared regions were observed. The glass color confirmed the existence of Ni 2 + ions in octahedral coordination form. However, the change in the glass color from yellow to yellow-orange is due to the intensity of ligand field around octahedral Ni 2 + ions. Furthermore, the ligand field strength (10Dq) augmented (10650–10905 cm − 1 ), while the Racah B and C parameters decreased (483-445 cm − 1 ) and (2327-2221 cm − 1 ), respectively. Such reversed trends are due to the intensive electron delocalization at Ni 2 + ions and confirm the covalent character in the Ni 2 + -ligand bonds. The increase in sodium fluoride content leads to a decrease in optical band gaps from 3.50 to 3.21 eV and an increase in Urbach energy from 0.14 eV to 0.19 eV due to the disorder enhancement. Overall, the optical non-linear characteristics of these oxyfluoride borate glasses are enhanced with more addition of NaF, stemming from an observed enhancement in refractive indices. The current article affords good structural-optical relations, offering a decent nickel doped oxyfluoride borate glass host for optical applications.