Experimental study of nuclear electric quadrupole splittings of 27Al in Topaz

Abstract

The nuclear magnetic resonance spectrum of 27Al in single crystals of natural topaz (Al 2[F,OH] 2SiO 4) has been investigated at room temperature at 10.500 MHz. Observed quadrupolar perturbations have been analyzed by the method of Volkoff, for three orthogonal planes, to yield eight nuclear quadrupole coupling and electric field gradient tensors. Satellite splitting indicated that the crystallographic class of topaz is holohedral. Eigenvalues and electric field gradient asymmetry parameters of the tensors fall into two groups of four, the members of each group having identical parameters. Eigenvectors of all tensors are different. The eight equivalent Al 3+ sites given by the X-ray space group are thus divided into two types of magnetically nonequivalent sites, each type consisting of four magnetically equivalent positions and the term magnetic being used to describe site symmetries as revealed by magnetic resonance. Quadrupole coupling constants and field asymmetry parameters for the two types of sites are: Type α sites Type β sites C z= eQφ zz h 1.5918±0.008MHz 1.6506±0.008MHz ν=(φ χχ−φ yy)φ! zz 0.704±0.007 0.501±0.005