Oxygen-17 NMR in solids by dynamic-angle spinning and double rotation

IT is widely lamented that despite its unqualified success with spin-1/2 nuclei such as ¹³C, ²⁹Si and³¹P, the popular NMR technique of magic-angle spinning (MAS) has experienced a somewhat restricted applicability among quadrupolar nuclei such as ¹⁷O, ²³Na and ²⁷A1 (refs 1-3). The resolution in the central (1/2 ↔-1/2) transition of these non-integer quadrupolar spins under MAS is thought to be limited primarily by second-order quadrupolar broadening. Such effects of second-order spatial anisotropy cannot be eliminated by rotation about a fixed axis or by multiple-pulse techniques^4,5. More general mechanisms of sample reorientation (refs 6-8 and A. Samoson and A. Pines, manuscript in preparation) can, however, make high-resolution NMR of quadrupolar nuclei feasible. MAS is implemented by spinning a sample about a single axis so that second-rank spherical harmonics (which give rise to first-order broadening through anisotropy of electrical and magnetic interactions) are averaged away. But dynamic-angle-spinning (DAS) and double-rotation (DOR) NMR involve spinning around two axes, averaging away both the second- and fourth-rank spherical harmonics, which are responsible for second-order broadening. Here we present the application of these new techniques to ¹⁷O in two minerals, cristobalite (SiO₂) and diopside (CaMgSi₂O₆). This work goes beyond previous results on ²³Na (ref. 8) by showing the first experimental results using DAS and by demonstrating the application of DOR to the resolution of distinct oxygen sites in an important class of oxide materials.