Understanding of distribution model and coordination structure of Dy3+ in B2O3-Bi2O3-SiO2 glass based on Raman, infrared and XPS analysis

Bismuth glasses doped with rare earth ions, characterized by low phonon energy, high refractive index, and broad infrared transmission range, hold significant promise for photonic devices. The spectral behavior of bismuth glasses is critically influenced by the distribution and coordination structure of rare earth ions. This study employed Dy3+ ions, known for their high sensitivity to ligand surroundings, as structural probes to elucidate the chemical bonding characteristics of rare earth ions in bismuth glasses. The investigation revealed that the excessive doping of Dy3+ ions leads to composition fluctuations and the initiation of B2O3 crystalline phases within the mother glass network. Dy3+ ions preferentially occupied Bi ion sites owing to similar ionic radii and identical chemical valences, demonstrating the high solubility of rare earth ions in bismuth glass. Raman and FTIR analyses indicated two transformations of structural units, namely from octahedral [BiO6] to tetrahedral [BiO3] and from triangular [BO3] to tetrahedral [BO4]. The decrease in both the Ω2 parameter and yellow/blue intensity ratio signified an increase in the ionic degree of Dy-O− bonds and a decrease in the asymmetry of ligand surroundings of Dy3+ ions in glass. This was attributed to the strong tendency of distortion in octahedral [BiO6], influenced by the chemically active 6s2 lone pair electrons of Bi3+. XPS analyses of Dy 3d, Bi 4f, and O 1 s corroborated these findings. The proposed model for the structural evolution of highly doped bismuthate glasses with rare earth ions provides insights for designing new glasses with desired properties or functions.