Influence of La2O3 on the structural, mechanical and optical features of cobalt doped heavy metal borate glasses

Abstract

The impact of rare-earth oxide (La2O3) on the structural, mechanical and optical properties of cobalt-doped heavy metal borate glassy system is studied. XRD assured the amorphous nature of the prepared samples. The density and average boron-boron separation were found to increase with La2O3 content, thereby enforcing the compactness of the glass network. FT-IR analysis revealed the growth of NBO's ratio due to the gradual conversion of the tetrahedral (BO4) to trigonal (BO3) units with the development of a certain boroxol group (B3O6) fraction. A noticeable enhancement of the elasticity properties, such as the elastic, bulk, and Young's moduli, as well as the microhardness, was confirmed by measuring the ultrasound velocities within prepared samples. The absorption and photoluminescence emission spectra showed a progressive enhancement of Co3+/Co2+ ions in the tetrahedral and/or octahedral positions with La2O3 content. The ligand field strength and Racah parameters were estimated, in which the ionicity nature between Co2+ ions and the nearby ligands is inferred from the nephelauxetic effect. Absorption edge analysis, here performed through Tauc's model, showed a decrease in the band gap and a rise in Urbach energy; the values of each, together with the metallization parameter, reflect the semiconducting character of the prepared samples. The dispersion of the refractive index was determined using the Wemple-DiDomenico single oscillator model and compared to the optical gap energy. Additionally, the nonlinear optical coefficients, here determined within the lower energy spectral range using a third-order nonlinear susceptibility, exhibited progressively increasing value, suggesting a possible integration of the current system in potential nonlinear optical applications. Furthermore, the absorption and photoluminescence spectral tunability here offered through La2O3 additives qualifies the current glasses as optically active materials for many possible applications in multifunctional optical devices operating in visible and near-infrared spectral regimes.