Al coordination in sol-gel and conventional calcium aluminate glasses

We present the results of a study in which we compared one aspect of structure for a conventionally prepared glass and sol-gel material in the calcium aluminate (CA) family of composition 50CaO . 50Al(2)O(3) (wt%). The work was directed towards a comparison of the aluminum coordination in the two materials, which was determined via (27)Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. Also, infra red (IR) spectroscopy was utilized to compare the hydroxyl content and the structural features of the materials. The Al coordination was examined in gels which were subjected to various heat treatments, some of which produced partial or nearly complete crystallization. We observed, as found previously by others, that in the conventionally prepared glass all the Al atoms are tetrahedrally (4-fold) coordinated. On the other hand, in all the sol-gel materials, regardless of heat treatment schedule, a fraction of the Al is found to be in octahedral (G-fold) coordination. We found that. as the hydroxyl content of the gel increases, the ratio of 6-fold to 4-fold Al increased. Hence, the presence of octahedral coordination in the gel is largely, and most likely totally, due to the water content of this material. (C) 1998 Published by Elsevier Science B.V. All rights reserved.