The percentages of boron belonging to each structural unit (monomer, dimer, trimer, and polymer) in the four glass systems are shown in Fig. . Boron atoms in polymer units are the majority in the a-La30 glass, and the percentage of boron atoms in monomer and dimer units increased with increasing RE content. This trend implies that RE atoms in the glass play the role of network modifiers to break the borate glass network. The percentage of boron in monomer or dimer form in the a-La50 glass was found to be greater than that in the a-Y50 glass. The degree to which La 3+ induces structural modification was found to be higher than that caused by Y 3+ . Changes in the glass structure depending on the trivalent ionic species (La or Y) will be further discussed in the analysis of the atomic coordination and electronic structure of these glasses in the subsection "RE coordination structure." To elucidate the origin of the variety of boron units in the solid glass, a reaction analysis during the quenching process was undertaken with a view to providing deep insights into the formation mechanisms of these diverse boron structural units.