AL-27 NUCLEAR-MAGNETIC-RESONANCE STUDY OF SYNTHETIC AND NATURAL CORUNDUM (ALPHA-AL2O3) - SOME EXPERIMENTAL ASPECTS OF QUANTITATIVE AL-27 NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY

The Al-27 nuclear magnetic resonance (NMR) response of a series of nat ural and synthetic corundum (alpha-Al2O3) samples is studied quantitat ively by short-pulse excitation and frequency-stepped adiabatic half-p assage (FSAHP). Using on- and off-resonance nutation NMR, it was estab lished that the quadrupole coupling parameters of visible Al is identi cal in all samples. Remarkably, the relaxation behavior for the alumin um is very different in the various samples and has a marked effect on the quantitative response. In natural corundum samples the Al-27 spin -lattice relaxation is very efficient as these samples contain paramag netic impurities. As a result, however, the full signal could not be r ecovered, which is attributed to relaxation broadening of spins in the vicinity of these impurities. In synthetic samples, containing no imp urities, the full signal could be recovered, although the relaxation b ehaviour appeared to depend strongly on the preparation method. We obs erved differences in the spin-lattice relaxation by a factor 20; the l ongest T1 was observed in a crushed single crystal. This implies that alpha-Al2O3 can only be used as a standard in quantitative analyses if it has been characterized thoroughly. Furthermore, the effective rela xation behaviour for different types of excitation is studied. Finally , a method to measure the spin-lattice relaxation of half-integer quad rupole nuclei is introduced, using a frequency-stepped adiabatic passa ge (FSAP) to invert the spin system.