Impact of SiO2/B2O3 substitution on the structural polymerization and ligand field parameters of Ni2+ ions in Na2O-B2O3-SiO2 glasses

The present work investigates the influence of borosilicate glass network polymerization on the structural, magnetic, and optical behavior of Ni 2+ ions. Borosilicate glasses containing a fixed concentration of NiO were synthesized using the melt-quenching technique with systematic substitution of B 2 O 3 by SiO 2 . XRD confirmed the amorphous nature of all samples, while FTIR spectroscopy revealed a progressive transformation from a borate-rich to a silicate-rich network, characterized by an increase in BO 4 units and a marked reduction in non-bridging oxygens (NBOs). Density and molar volume analyses supported the formation of a more compact and polymerized glass network with increasing SiO 2 content. EDX results verified the homogeneous incorporation of constituent elements within the glass matrix. Electron paramagnetic resonance (EPR) spectra exhibited broad and asymmetric signals characteristic of Ni 2+ ions in distorted octahedral coordination. A gradual decrease in EPR signal intensity and a slight narrowing of linewidth were observed with increasing SiO 2 concentration, attributed to reduced dipole–dipole interactions and enhanced spin–lattice relaxation in a more polymerized network. Optical absorption spectra showed characteristic spin-allowed transitions of Ni 2+ ions, accompanied by systematic redshifts of absorption bands with increasing SiO 2 content. Ligand field analysis revealed a decrease in the Racah parameter (B) and nephelauxetic ratio (β), indicating enhanced covalency of the Ni–O bonds due to increased glass network connectivity and reduced NBO concentration. The strong correlation between FTIR, EPR, and optical results demonstrates that the local electronic structure of Ni 2+ ions is governed by glass network polymerization, highlighting the tunability of borosilicate glasses for optical and photonic applications.