Evolution of crystalline aluminates from hybrid gel-derived precursors studied by XRD and multinuclear solid-state MAS NMR; II. Yttrium-aluminium garnet, Y3Al5O12

The thermal evolution of structure in a hybrid gel of YAG composition (Y3Al5O12) prepared from aluminum alkoxide and yttrium acetate was studied up to 1250C. XRD indicates that the gels are amorphous <900C, when they abruptly crystallize to YAG and hexagonal YAlO3. The latter exists only briefly, being fully converted to YAG by 1000C. 27Al MAS NMR shows that after the loss of volatiles at 375C, the Al occupies three types of sites in the amorphous gel (tetrahedral, octahedral and a site characterized by an NMR resonance at 38 ppm). The distribution of Al over these sites progressively changes during heating <980C but 89Y MAS NMR suggests that the Y is comparatively less mobile within the gel, achieving its final YAG environment only after the YAG crystal structure has been established. (Copyright (c) 1999 Elsevier Science B.V., Amsterdam. All rights reserved.)