27Al and 23Na double-rotation NMR of sodalites

The 27Al NMR spectra of calcium tungstate aluminate sodalite (CAW), Ca8[Al12O24](WO4)2, and the 23Na NMR spectra of sodium aluminosilicate sodalites of general composition Na9[Si6Al6O24]A2 with A = B(OH)4- (SBS), SCN- (SRS) and A2 = SO4(2-) (SSS), MoO4(2-) (SMS) have been measured using magic-angle spinning (MAS) and double-rotation (DOR) techniques. Rotor synchronized pulse excitation is applied in the DOR experiments. Dramatic line narrowing is observed in the DOR spectra of all samples. The 27Al DOR NMR spectra of CAW measured at 9.4 and 11.7 T and spinning rates of 800-1150 Hz of the outer and 5 kHz of the inner rotor show seven sharp central lines accompanied by a manifold of spinning sidebands. These lines correspond to the seven crystallographically inequivalent Al sites of the CAW framework derived from X-ray structure analysis. From the difference of the line positions in the 9.4 and 11.7 T spectra the quadrupole coupling constant, QCC, quadrupole induced shift, sigma qs, and isotropic chemical shift, delta cs, of each Al site have been calculated. QCC values in the range of 5 to 9 MHz are obtained which reflect the strong tetragonal distortion of the AlO4 tetrahedra in CAW. delta cs shows only small changes in the range between 74.4 and 77.2 ppm. A tentative assignment of all lines to the distinct Al sites is derived from the correlation between QCC and a "shear strain parameter" describing quantitatively the distortion of the AlO4 tetrahedra.(ABSTRACT TRUNCATED AT 250 WORDS)