Kentgens A P, Bos A, Dirken P J
NSR Center for Molecular Structure, Design and Synthesis, National HF-NMR Facility, University of Nijmegen, Netherlands.
Solid State Nucl Magn Reson. 1994 Nov;3(6):315-22. doi: 10.1016/0926-2040(94)90015-9.
The 27Al nuclear magnetic resonance (NMR) response of a series of natural and synthetic corundum (alpha-Al2O3) samples is studied quantitatively by short-pulse excitation and frequency-stepped adiabatic half-passage (FSAHP). Using on- and off-resonance nutation NMR, it was established that the quadrupole coupling parameters of visible Al is identical in all samples. Remarkably, the relaxation behavior for the aluminum is very different in the various samples and has a marked effect on the quantitative response. In natural corundum samples the 27Al spin-lattice relaxation is very efficient as these samples contain paramagnetic impurities. As a result, however, the full signal could not be recovered, which is attributed to relaxation broadening of spins in the vicinity of these impurities. In synthetic samples, containing no impurities, the full signal could be recovered, although the relaxation behaviour appeared to depend strongly on the preparation method. We observed differences in the spin-lattice relaxation by a factor 20; the longest 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 relaxation behaviour for different types of excitation is studied. Finally, a method to measure the spin-lattice relaxation of half-integer quadrupole nuclei is introduced, using a frequency-stepped adiabatic passage (FSAP) to invert the spin system.
通过短脉冲激发和频率步进绝热半通道（FSAHP）对一系列天然和合成刚玉（α-Al2O3）样品的27Al核磁共振（NMR）响应进行了定量研究。利用共振和非共振章动NMR，确定了所有样品中可见Al的四极耦合参数是相同的。值得注意的是，不同样品中铝的弛豫行为差异很大，并且对定量响应有显著影响。在天然刚玉样品中，27Al自旋-晶格弛豫非常有效，因为这些样品含有顺磁性杂质。然而，结果是无法恢复完整信号，这归因于这些杂质附近自旋的弛豫展宽。在不含杂质的合成样品中，尽管弛豫行为似乎强烈依赖于制备方法，但仍可恢复完整信号。我们观察到自旋-晶格弛豫的差异高达20倍；在粉碎的单晶中观察到最长的T1。这意味着α-Al2O3只有在经过充分表征后才能用作定量分析的标准。此外，还研究了不同类型激发的有效弛豫行为。最后，介绍了一种使用频率步进绝热通道（FSAP）反转自旋系统来测量半整数四极核自旋-晶格弛豫的方法。
