Optical Transitions in Ruby across the Corundum toRh2O3(II) Phase Transformation

First principles calculations of a substitutional chromium impurity in ${\mathrm{Al}}_2{\mathrm{O}}_3$ and analysis of its multiplet structure reveal that the major effect of the corundum to ${\mathrm{Rh}}_2{\mathrm{O}}_3$ (II) phase transformation in ruby's optical transitions should be a redshift of the broad $U$ and $Y$ absorption lines across the transformation. A similar phenomenon is experimentally detected above 30 GPa, which is too low a pressure to attribute this phenomenon to a phase transition in the host. However, an anomalous relaxation around the impurity in corundum could explain the observed frequency shifts. The underlying cause of such abnormal relaxation could be a similar transformation in ${\mathrm{Cr}}_2{\mathrm{O}}_3$ taking place below 30 GPa. The centroids of the $R$, $R^{\prime }$, and $B$ lines are quite insensitive to the transformation.