Structural Origin of Additional Infrared Transparency and Enhanced Glass-Forming Ability in Rare-Earth-Rich Borate Glasses without B–O Networks

R2O3–B2O3 binary glasses (R denotes rare-earth elements or Y) were fabricated in a very wide composition region using a levitation technique. The maximum R2O3 content of light rare-earth compounds reached 63 mol % and decreased with a decrease in the ionic radius of R3+. The thermal, optical, vibrational, and structural properties were investigated, particularly for 50R2O3–50B2O3 glasses. The glass transition temperature increased with a decrease in the ionic radius of R3+, while the thermal stability was not affected by the glass composition. The packing density increased with a decrease in the ionic radius of R3+ due to lanthanoid contraction. Raman scattering and Fourier transform infrared spectra revealed that, in the rare-earth-rich glasses, no conventional three-dimensional networks consisting of corner-sharing BOn (n = 3 or 4) units existed because all B atoms were formed as isolated BO3 units. The simple environment around B atoms in the glasses led to additional IR transmittance regions, irrespective of the kinds of R. The total correlation functions obtained from high-energy X-ray diffraction measurements were analyzed using the pair-function method and compared with those of various RBO3 crystalline phases. It was suggested that the local structure around R resembles the ν-NdBO3-type crystal structure, and the O coordination number of R ranged from 6.5 to 7.7, smaller than that of the crystalline phase. The glass-forming ability depending on R was discussed based on the structural similarities between the melt, glass, and crystalline phases.