Comparison of the Crystal Fields and Optical Spectra of Cr2O3 and Ruby

Abstract

The polarized optical absorption spectrum of Cr₂O₃ in thin single‐crystal plates has been obtained at temperatures from 4.2°K upward. The crystal field at the Cr ions in Cr₂O₃ has been calculated in the same way as for Cr:Al₂O₃ since their crystal structures are very similar. The experimental results for Cr₂O₃ were used to derive empirical values for the quantities 〈r²〉 and 〈r⁴〉 which appear in the expressions for the cubic and trigonal fields of Cr₂O₃, and these were substituted into the expression for the trigonal field of Cr:Al₂O₃. The result is a calculated trigonal field much smaller than is observed. The Cr ion in Al₂O₃ may therefore be displaced from the Al position as was suggested in a previous study. The details of the Cr₂O₃ spectrum, particularly the lines, are illustrated and commented upon, but no detailed analysis is presented. The spectrum does show many evidences of exchange coupling, however.