Multiple Quantum ↑(27)A1 Magic-Angle-Spinning Nuclear Magnetic Resonance Spectroscopic Study of SrAl↓(12)O↓(19): Identification of a ↑(27)A1 Resonance from a Well-Defined AlO↓(5) Site

摘要

SrAl↓(12)O↓(19), a material with the magnetoplumbite-type structure, has been characterized by field-dependent ↑(27)A1 MAS and MQMAS solid-state NMR. In this material five different A1 sites are present: one A104, one A1O↓(5), and three A1O↓(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 A1O↓(5) polyhedron. Quantitative simulations, using distributions in quadrupole coupling constant (C↓(q)), of the conventional MAS spectra were successful in fitting the spectra. The 27A1 resonance of the A1O↓(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 A1 because a relation seems to exist between the isotropic chemical shift and the distortion of 5-fold-coordinated aluminum in various materials.