Strain-Induced Effects on the Degenerate Spectral Line of Chromium in MgO Crystals

When uniaxial pressure is applied along [100], [110], and [111] directions to MgO crystals with chromium impurities, both a splitting and a shift of the purely cubic field fluorescence line at 14 319 ${\mathrm{cm}}^{-1\mathrm{}}$ are observed. The splitting is ascribed to the removal of the degeneracy associated with the ${t_2}^3{}_{}{}^2{}_{}{}^{}E$ excited state by the strain-induced low-symmetry crystal fields. A theoretical calculation of the splitting, assuming a point-charge model, gives a surprisingly good agreement with the experiment. The shift is clearly due to the isotropic part of the strain-induced crystal fields, and a simple consideration shows that the observed red shift is caused by the strain-induced change of the Coulomb interaction between the $t_2$ electrons. Finally, limitations of the point-charge model adopted here are discussed.