NUCLEAR-SPIN-LATTICE RELAXATION MECHANISMS IN KAOLINITE CONFIRMED BY MAGIC-ANGLE-SPINNING

Spin-lattice relaxation mechanisms in kaolinite have been reinvestigat ed by magic-angle spinning (MAS) of the sample. MAS is useful to disti nguish between relaxation mechanisms: the direct relaxation rate cause d by the dipole-dipole interaction with electron spins is not affected by spinning while the spin diffusion-assisted relaxation rate is. Spi n diffusion plays a dominant role in H-1 relaxation. MAS causes only a slight change in the relaxation behavior, because the dipolar couplin g between H-1 spins is strong. Si-29 relaxes directly through the dipo le-dipole interaction with electron spins under spinning conditions hi gher than 2 kHz. A spin diffusion effect has been clearly observed in the Si-29 relaxation of relatively pure samples under static and slow- spinning conditions. Al-27 relaxes through three mechanisms: phonon-co upled quadrupole interaction, spin diffusion and dipole-dipole interac tion with electron spins. The first mechanism is dominant, while the l ast is negligibly small. Spin diffusion between Al-27 spins is suppres sed completely at a spinning rate of 2.5 kHz. We have analyzed the rel axation behavior theoretically and discussed quantitatively. Concentra tions of paramagnetic impurities, electron spin-lattice relaxation tim es and spin diffusion rates have been estimated.