title: Nuclear spin-lattice relaxation mechanisms in kaolinite confirmed by magic-angle spinning.
abstract: Spin-lattice relaxation mechanisms in kaolinite have been reinvestigated by magic-angle spinning (MAS) of the sample. MAS is useful to distinguish between relaxation mechanisms: the direct relaxation rate caused by the dipole-dipole interaction with electron spins is not affected by spinning while the spin diffusion-assisted relaxation rate is. Spin diffusion plays a dominant role in 1H relaxation. MAS causes only a slight change in the relaxation behavior, because the dipolar coupling between 1H spins is strong. 29Si relaxes directly through the dipole-dipole interaction with electron spins under spinning conditions higher than 2 kHz. A spin diffusion effect has been clearly observed in the 29Si relaxation of relatively pure samples under static and slow-spinning conditions. 27Al relaxes through three mechanisms: phonon-coupled quadrupole interaction, spin diffusion and dipole-dipole interaction with electron spins. The first mechanism is dominant, while the last is negligibly small. Spin diffusion between 27Al spins is suppressed completely at a spinning rate of 2.5 kHz. We have analyzed the relaxation behavior theoretically and discussed quantitatively. Concentrations of paramagnetic impurities, electron spin-lattice relaxation times and spin diffusion rates have been estimated.
authors.affiliation: National Institute of Materials and Chemical Research, Ibaraki, Japan.
authors.forename: S
authors.initials: S
authors.lastname: Hayashi
authors.forename: E
authors.initials: E
authors.lastname: Akiba
mesh_terms: D000535:Aluminum; D007554:Isotopes; D007616:Kaolin / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008280:Magnetics; D011522:Protons; D015203:Reproducibility of Results; D012825:Silicon
chemical_list: D007554:Isotopes; D011522:Protons; D007616:Kaolin; D000535:Aluminum; D012825:Silicon