Die elektrische Quadrupolaufspaltung von Al27in Spinell MgAl2O4und Korund Al2O3

Abstract

The nuclear resonance of Al 27 in natural and synthetic spinel single-crystals was examined. In natural spinel the Al-ions predominantly occupy octahedrally coordinated sites with non-cubic point symmetry in the anion packing. The quadrupole splitting was measured and the maximum gradient of the electric field in the crystal at the position of the Al-nucleus was determined. In heated natural and synthetic spinel crystals the frequency bands of the satellites are so broadened, that the quadrupole splitting can only be recognised by the displacement of the central lines. This broadening of the signal–present even in weakened form in natural crystals– is considered to be caused by a departure of the local symmetry of the electric fields at the position of the separate Al-nuclei from the crystallographic point symmetry of the point positions occupied by Al in the structure. Those departures may be best explained by a disordered Mg/Al distribution with slight disorder in natural spinel and higher disorder in heated natural and synthetic spinels. Comparisons of the intensity of the signals of a natural spinel and of a synthetic corundum give a measure of the amount of Al situated in fields which are deformed by the type and position of the nearest neighbour ions.