Survival of Fragmented BO4 Units in Highly Modified Rare-Earth-Rich Borate Glasses

Highly modified La₂O₃–Y₂O₃–B₂O₃ ternary glasses were fabricated by using a levitation technique. The thermal and structural properties of (50 – x)La₂O₃–xY₂O₃–50B₂O₃ glasses and (60 – y)La₂O₃–yY₂O₃–40B₂O₃ glasses were investigated. Raman scattering spectra indicated that B atoms mainly formed isolated planar BO₃ triangles similar to those of crystalline LaBO₃. This process was independent of the ratio of La₂O₃ and Y₂O₃. ¹¹B magic angle spinning nuclear magnetic resonance spectra confirmed that the BO₄ units that should have disappeared in the glass with highly modified compositions remained as fragmented species. Approximately 4% of the B atoms formed BO₄ in the 50La₂O₃–50B₂O₃ glass. This ratio increased with an increase in the Y₂O₃ content, and it reached its maximum value (15%) in the 50Y₂O₃–50B₂O₃ glass. Comparison of the electron density distribution was conducted using ab initio calculations of the LaBO₃ and YBO₃ crystals and indicated that more electrons localize near the atomic nuclei in the Y–O bond than in the La–O bonds. Comparison of the electron density distribution was conducted using ab initio calculations of the LaBO₃ and YBO₃ crystals and indicated that electrons in the Y–O bond localize near the atomic nuclei compared to those in the La–O bonds. Thus, the unconventional existence of BO₄ in a highly modified glass is attributed to the increase in the level of Y³⁺, which causes the localization of electrons near the atomic nuclei. Thus, the ratio of BO₄ and BO₃ in highly modified glass can be controlled by tuning the glass content of modifier rare-earth oxides, which opens a new door to glass science.