TEMPERATURE-DEPENDENT STRUCTURAL-CHANGES IN BORATE, BOROSILICATE AND BOROALUMINATE LIQUIDS - HIGH-RESOLUTION B-11, SI-29 AND AL-27 NMR-STUDIES

High-resolution B-11,Si- 29 and Al-27 nuclear magnetic resonance spect roscopy is used to quantitatively determine temperature dependent stru ctural changes in Na-borate, Na-borosilicate and Li-boroaluminate liqu ids. Such changes are found to involve transformation of (a) BO3 units from boroxyl ring to non-ring configuration in the berate and of (b) BO4 units into asymmetric BO3 units in the borosilicate liquid with in creasing temperature. No temperature dependent speciation of the silic ate units was observed in the borosilicate liquid. In boroaluminate li quids, only the peraluminous composition shows transformation of highe r-coordinated Al-V and Al-VI species into Al-IV with increasing temper ature, No temperature dependent berate speciation is detectable in the se compositions. It is shown that although such structural changes may be the principal source of configurational entropy in the lower-alkal i berate liquids, other sources must be sought for the higher-alkali b erate, borosilicate and boroaluminate liquids. (C) 1998 Elsevier Scien ce B.V. All rights reserved.