Temperature dependent structural changes in borate, borosilicate and boroaluminate liquids:: high-resolution 11B, 29Si and 27Al NMR studies

High-resolution B-11,Si-29 and Al-27 nuclear magnetic resonance spectroscopy is used to quantitatively determine temperature dependent structural changes in Na-borate, Na-borosilicate and Li-boroaluminate liquids. Such changes are found to involve transformation of (a) BO3 units from boroxyl ring to non-ring configuration in the borate and of (b) BO4 units into asymmetric BO3 units in the borosilicate liquid with increasing temperature. No temperature dependent speciation of the silicate units was observed in the borosilicate liquid. In boroaluminate liquids, only the peraluminous composition shows transformation of higher-coordinated Al-V and Al-VI species into Al-IV with increasing temperature. No temperature dependent borate speciation is detectable in these compositions. It is shown that although such structural changes may be the principal source of configurational entropy in the lower-alkali borate liquids, other sources must be sought for the higher-alkali borate, borosilicate and boroaluminate liquids. (C) 1998 Elsevier Science B.V. All rights reserved.