Evidence for Self-Organization in Lithium and Sodium Borate Glasses

Modulated DSC experiments reveal that glass transitions in Li- and Na- borates become thermally reversing respectively in the 14% < x < 27% range, and the 20% < x < 40% range of the modifier, henceforth called reversibility window (RW). We identify these square-well like RWs with stress-free Intermediate Phases (IPs) formed. The RW is sharper in Li- than in Na- borates because of the higher mobility of Li than Na. In Raman Scattering a triad of modes (705 cm-1, 740 cm-1, 770 cm-1) next to the Boroxyl ring mode (808 cm-1) are observed, and are identified with tetrahedral B replacing triangular B in Boroxyl rings, mixed rings that define select structural groupings (SGs). Specifically, scattering strength of the 770 cm-1 mode shows a principal maximum near 20% and a secondary one near 16.6% of Lithia, which we identify respectively with isostatic tetra- and penta-borate SGs percolating in the IP. The same mode, on the other hand, shows a principal maximum near 25%, and a secondary one near 20% of soda which is signature of tri- and tetraborate SGs percolating in the IP because of Na-borates. The larger width and shift of the IP to higher soda content compared to Lithia content is traced to isostatic tri-pentaborate and diborate SGs percolating in the Na- but not in the Li-borates. These data provide for the first time the medium range structural origin of the IP in network glasses.