Knowledge Bank

A previous investigation of the optical spectra of first-series transition metal ions in corundum $(Al2O3)$ led to the conclusion that the trigonal component of the crystal field has a positive sign for all $ions.1$ This sign is opposite from that calculated by the point-charge model of the crystal field if the impurity ion resides at the A/ site. If the impurity is displaced by 0.1 {\AA} toward the empty octahedral site, however, the calculated sign becomes positive, as observed. In order to see if the calculations were meaningful, the spectrum of $Cr2O3$ was compared with the results of calculations similar to those done for $Al2O3:Cr$. The $Cr2O3$ spectrum was obtained in crystals five microns thick, cut with the c-axis in the plane of the sample, so that polarized spectra could be obtained. The dichroism of such samples is opposite from that of ruby, and in agreement with the calculations by the point-charge model. The implication is that the ions in solid solution are actually displaced, as was suspected on the basis of the earlier work. The possible influence of the antiferromagnetism of $Cr2O3$ on its $spectrum,2$ and on this conclusion will be discussed.