The role of NiO on the structural and optical properties of sodium zinc borate glasses

Nickel-doped sodium zinc borate (NZB) glasses of composition [15 ZnO - (65 - x) B2O3-x NiO - 20 Na2O], where (x = 0.0, 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 mol %), were prepared following the melt quenching technique. The effect of NiO additives on the structural and optical properties of NZB glasses has been studied by a set of comple-mentary techniques. The amorphous nature of the prepared samples was verified by both X-ray diffraction pattern analysis (XRD) and scanning electron microscopy (SEM). Additional structural information and physically relevant parameters such as density, molar volume, inter ionic distance, the average Boron-Boron separation, and ion concentration, have been calculated. Furthermore, the optical basicity and electronic polarizability of the system were empirically determined. Boron structural units and both Ni2+ and Zn2+ specific vibrations were identified using FTIR spectroscopy. The direct comparison of the vibrational spectra of all samples showed a slight reduction in N-4 ratio, from 32% to 29%, with increasing NiO content. The detailed deconvolution process of optical spectra revealed specific absorption bands characteristic for Ni2+ ions, in the visible and NIR regions, preferentially occupying octahedral sites. Ligand field parameters such as crystal field and Racah parameters were estimated, and the covalent nature between Ni2+ ions and the surrounding ligand is indicated from the nephelauxetic effect. The optical band gap energy as calculated from Tauc's model was observed to decrease from 3.88 to 3.49 eV with increasing NiO content, while the refractive index follows the opposite behavior. Urbach energy values and metallization criterion factor confirmed the semiconductor character of the present Ni-doped NZB glasses.