TOWARDS A STRUCTURAL INTERPRETATION OF FRAGILITY AND DECOUPLING TRENDS IN BORATE SYSTEMS

Qualitative correlations are established between the fragility, decoup ling index, glass transition temperature and structure of xMO .(1 - x) B2O3 systems (M = Li-2, Na-2, Be, Sr, Ca). Despite the similarities be tween the cationic field strengths of alkali and alkaline earth cation s, the latter have a higher cross-linking efficiency than the former, Therefore, they 'freeze' the liquid structure at higher glass transiti on temperatures and arrest a less polymerized berate network. The comp etition between the cross-linking induced by the B-O and the M-O bonds leaves the fragility index relatively independent of M. The enhanced participation of the bivalent ions in network formation results in a d ecrease of the decoupling index at T-g.