A reaction-event analysis was performed for all data within the temperature range from 1500 to 300 K; the numbers of units recognized within this temperature range are summarized in Table . The structural units consisting of isolated 3-rings (B3O6-ring, B3O7-ring, and B3O9-ring) were observed rarely in the glass melts. Note that these structures are not present in the solid glasses. Isolated boron units missing oxygen in monomers, dimers, and trimers (B1O2, B2O4, and B3O5) were also observed only in the melts and are predicted to be the transition units when structural recombination occurs. Dimers with two linked B 4 units (B2O7) were found primarily in the a-La60 glass melt. This structure has a formal charge of -8e, as calculated from the number of O 1 and the valence electrons of boron. Such an excess negative charge in a dimer unit will lead to instability, except in glass systems with abundant REs. Although double-quantum 11 B NMR has revealed the BO 4 -BO 4 linkage in alkali borosilicate glass, 66,67 the B2O7 dimer with two BO 4 units is unlikely to be present in RE-rich borate glass. Nonetheless, a trimer consisting only of B 4 (B3O10) was present in the a-La60 glass; this unit can become more stable by forming a trimer.