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First principles calculations of a substitutional chromium impurity in Al 2 O 3 and analysis of its multiplet structure reveal that the major effect of the corundum to Rh 2 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 Cr 2 O 3 taking place below 30 GPa. The centroids of the R, R 0 , and B lines are quite insensitive to the transformation. [S0031-9007(98) 07269-X]

No abstract