Survival of fragmentated BO4 units in highly modified rare-earth-rich borate glasses

Highly modified La2O3−Y2O3−B2O3 ternary glasses were fabricated 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 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 the increase in the Y2O3 content, and it reached its maximum value (14%) in the 50Y2O3–50B2O3 glass. Comparison of the electron density distribution was conducted based on Ab initio calculations of the LaBO3 and YBO3 crystals and indicated that Y–O was more ionically bonded than La–O. Thus, the unconventional existence of BO4 in highly modified glass is attributed to the presence of highly ionic modifier ions that can efficiently compensate for the charge of the BO4 unit, which is more negatively charged than BO3. 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 in glass science.