Environmental impacts of La2O3 on the optical and ligand field parameters of Ni ions inside Na2O-B2O3 glass

Abstract

In this work, a glass system of La2O3, NiO, and Na2O borate glass was prepared by changing the concentrations of La2O3/B2O3 contents. Further, the internal structure, optical absorbance, and ligand field parameters of Ni ions were studied. The internal structure was investigated using the Fourier transform infrared technique. The optical absorbance was measured, and the optical band gaps were obtained using Tauc's plot. According to the internal structure studies, BO4 contents increase with increasing La2O3/B2O3 concentrations, while BO3 contents decrease. Furthermore, the brown color and the absorption band at 424–428 nm of the prepared glass samples confirm the existence of Ni2+ cations in the form of NiO4 species and they act as glass formers. Additionally, the observed bands at 424–428, 490–500 and 798–806 nm are associated with the spin-allowed electron transitions of Ni2+ in octahedral coordination. Furthermore, the observed bands at 710–718 and 940–950 nm are related to the electronic transitions for Ni2+ in tetrahedral coordination. Moreover, the positions of all absorption bands shift toward higher wavelengths (redshift) with higher La2O3 concentrations. Additionally, the fundamental glass absorption edges move toward lower energy values as La2O3/B2O3 contents increase, confirming the band gaps behavior. Also, with increasing La2O3/B2O3 contents, the linear and nonlinear refractive indices increase. These behaviors were justified by the optical basicity and the electronic polarizability increase. On the other hand, the ligand field parameters of Ni ions; the crystal field splitting and Racah parameters (B and C), show opposite behaviors with further La2O3 additions. In detail, the crystal field splitting shows decreasing values and Racah parameters show increasing values. These results reflect the tendency of the bonds between the Ni cations and their surroundings towards a higher ionic nature (lower covalent nature).