CRYSTAL-STRUCTURE STUDIES ON THE GARNET Y3AL5O12 BY AL-27 SINGLE-CRYSTAL NMR-SPECTROSCOPY

The capability of single-crystal NMR to extract crystal structure info rmation is demonstrated by a Al-27 single-crystal NMR study of the gar net Y3Al5O12 (YAG), By rotation of a YAG single crystal about three pe rpendicular axes 93 Al-27 spectra have been acquired, resulting in the observation of 3132 resonance frequencies; These have been assigned t o three tetrahedral and four octahedral aluminum Sites. A thorough; an alysis of the magnitudes and-relative orientations of the quadrupole c oupling tensors for these seven aluminum; sites indicates the presence of a combination of symmetry elements that is unique to cubic crystal structures. Upper limits for the magnitudes of the anisotropic chemic al shielding interaction are also determined for the seven aluminum si tes and show that the geometrical arrangements of oxygen,ions in the f irst coordination sphere of all aluminum sites-must be nearly regular tetrahedra and octahedra, respectively.