Multiple Quantum 27Al Magic-Angle-Spinning Nuclear Magnetic Resonance Spectroscopic Study of SrAl12O19: Identification of a 27Al Resonance from a Well-Defined AlO5 Site

SrAl 12 O 19, a material with the magnetoplumbite-type structure, has been characterized by field-dependent 27 Al MAS and MQMAS solid-state NMR. In this material five different Al sites are present: one AlO 4, one AlO 5, and three AlO 6 sites, which all could be revealed by a quintuple quantum (5Q) MAS spectrum recorded at 7.05 T. A distribution is measured in the NMR parameters that is ascribed to statistical disorder of aluminum in the AlO 5 polyhedron. Quantitative simulations, using distributions in quadrupole coupling constant ( C q ), of the conventional MAS spectra were successful in fitting the spectra. The 27 Al resonance of the AlO 5 polyhedron has a relatively high shielding (δ iso: 18 ppm) and a C q of 2.1 MHz. The high shielding is explained by the distortion of the 5-fold-coordinated Al because a relation seems to exist between the isotropic chemical shift and the distortion of 5-fold-coordinated aluminum in various materials.