Environmental influences of La3+ ions on the structural, ligand field, and optical characteristics of Mn2+/Mn3+ ions inside heavy metal borate glass

In this work, the heavy metal borate glass system (B2O3-PbO) was doped with a hybrid of transition metal and rare earth ions via a fixed ratio of MnO2 and a variable La2O3 ratio. X-ray diffraction spectra confirmed the amorphous nature of the manufactured glassy system. The effect of La2O3 ions in causing modifications to the internal structure was examined through density measurements and Fourier-transform infrared spectroscopy. According to structural studies, the density increases gradually with increasing La2O3/B2O3 concentrations, in addition to an increase in the ratio of non-bridging oxygen resulting from structural transformations between the BO4 and BO3 units. The optical absorption bands were deconvoluted and studied, where the samples acquired a light brown color resulting from the absorption band at ~466nm and the electronic transition 6A1g(S) → 4A1g(G). The detected absorption bands verified the existence of Mn2+/Mn3+ in octahedral/tetrahedral sites. However, photoluminescence spectra indicated the tendency of Mn2+ ions to occupy more octahedral coordination. The shift of the fundamental absorption edges to lower energy with La2O3 addition confirmed the successive narrowing of the optical band gap. In addition, the entry of La3+ ions improved the optical basicity and electronic polarization, raising linear/nonlinear refractive indices. The ligand field effect showed the role of La3+ ions in reducing the crystal field intensity around Mn2+ ions and increasing the nephelauxetic ratio, indicating a gradual increase in the ionic bonding nature between the Mn2+ cations and the adjacent ions.