Niobium-93 MQMAS NMR spectroscopic study of alkali and lead niobates

Nb-93 (I = 9/2) multiple-quantum magic-angle spinning (MQMAS) NMR spectra o f a series of inorganic niobates have been measured. Nb-93 MQMAS spectrosco py yields spectra with typically an order of magnitude higher resolution th an that obtainable with Nb-93 MAS spectroscopy and Nb-93 dynamic-angle spin ning (DAS) spectroscopy. For example, the full-width at half-maximums of th e Nb-93 resonances of LiNbO3 were 9 (MAS), 5.8 (DAS), and 0.7 kHz (MQMAS). Broadening of the Nb-93 MAS and DAS spectra is due to the second-order quad rupolar and homonuclear dipolar interactions, respectively. The quadrupolar products (P-Q) and isotropic chemical shifts (delta (iso)) of the Nb-93 re sonances of LiNbO3, NaNbO3, PbNb2O6, Pb2Nb2O7, Pb3Nb2O8, Pb5Nb4O15, Pb3Nb4O 13, and Pb1.83Nb1.71Mg0.29O6.39 were determined from MQMAS spectra and rang e from 13.6 to 26.8 MHz and from -951 to -1113 ppm, respectively. Resonance s with relatively large quadrupolar coupling constants (> 30 MHz) were not observed using MQMAS spectroscopy, but were detected using nutation spectro scopy. The applicability and limitations of MQMAS spectroscopy in studying inorganic niobates containing multiple Nb-93 resonances are addressed and c ompared with those of MAS, nutation, and DAS spectroscopies, (C) 2001 Acade mic Press.