Optical transitions in ruby across the Corundum to Rh₂O₃(II) phase transformation

First principles calculations of a substitutional chromium impurity in Al₂O₃ and analysis of its multiplet structure reveal that the major effect of the corundum to Rh₂O₃ (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 Cr₂O₃ taking place below 30 GPa. The centroids of the R, R′ and B lines are quite insensitive to the transformation.