MULTIPLE-QUANTUM AL-27 MAGIC-ANGLE-SPINNING NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPIC STUDY OF SRAL12O19 - IDENTIFICATION OF A AL-27 RESONANCE FROM A WELL-DEFINED ALO5 SITE

SrAl12O19, a material with the magnetoplumbite-type structure, has bee n characterized by field-dependent Al-27 MAS and MQMAS solid-state NMR . In this material five different Al sites are present: one AlO4, one AlO5, and three AlO6 sites, which all could be revealed by a quintuple quantum (5Q) MAS spectrum recorded at 7.05 T. A distribution is measu red in the NMR parameters that is ascribed to statistical disorder of aluminum in the AlO5 polyhedron. Quantitative simulations, using distr ibutions in quadrupole coupling constant (C-q), of the conventional MA S spectra were successful in fitting the spectra. The Al-27 resonance of the AlO5 polyhedron has a relatively high shielding (delta(iso): 18 ppm) and a C-q of 2.1 MHz. The high shielding is explained by the dis tortion of the 5-fold-coordinated Al because relation seems to exist b etween the isotropic chemical shift and the distortion of 5-fold-coord inated aluminum in various materials.