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

Abstract

SrAl12O19, a material with the magnetoplumbite-type structure, has been characterized by field-dependent 27Al 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 measured in the NMR parameters that is ascribed to statistical disorder of aluminum in the AlO5 polyhedron. Quantitative simulations, using distributions in quadrupole coupling constant (Cq), of the conventional MAS spectra were successful in fitting the spectra. The 27Al resonance of the AlO5 polyhedron has a relatively high shielding (δiso: 18 ppm) and a Cq 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.