AL COORDINATION IN SOL-GEL AND CONVENTIONAL CALCIUM ALUMINATE GLASSES

We present the results of a study in which we compared one aspect of s tructure for a conventionally prepared glass and sol-gel material in t he calcium aluminate (CA) family of composition 50CaO . 50Al(2)O(3) (w t%). The work was directed towards a comparison of the aluminum coordi nation in the two materials, which was determined via Al-27 magic angl e spinning nuclear magnetic resonance (MAS NMR) spectroscopy. Also, in fra 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, som e of which produced partial or nearly complete crystallization. We obs erved, as found previously by others, that in the conventionally prepa red glass all the Al atoms are tetrahedrally (4-fold) coordinated. On the other hand, in all the sol-gel materials, regardless of heat treat ment schedule, a fraction of the Al is found to be in octahedral (G-fo ld) coordination. We found that. as the hydroxyl content of the gel in creases, the ratio of 6-fold to 4-fold Al increased. Hence, the presen ce of octahedral coordination in the gel is largely, and most likely t otally, due to the water content of this material. (C) 1998 Published by Elsevier Science B.V. All rights reserved.