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 - <i>x</i>)La₂O₃-<i>x</i>Y₂O₃-50B₂O₃ glasses and (60 - <i>y</i>)La₂O₃-<i>y</i>Y₂O₃-40B₂O₃ 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 <i>ab initio</i> 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 <i>ab initio</i> 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.