JAHN-TELLER THEORY OF CR2+ IN CORUNDUM

The dynamic Jahn-Teller effect is used in a theoretical study of the properties of the Cr2+ ion in corundum. The ion is produced in ruby crystals by X-irradiation. The trigonal distortion in the crystal is taken into account in setting up the Jahn-Teller interaction, in choosing zeroth-order orbital states for the ion and in the magnetic interactions. Using the cluster approximation, the effective Hamiltonian is thus derived. The effects of trigonal distortion are found to be small and thus the problem closely resembles that of Cr2+: MgO discussed previously in which there is a very strong orbit-lattice interaction. Theoretical values for the parameters needed to describe the low-lying energy levels of Cr2+: Al2O 3 are deduced. They are in agreement with values calculated from preliminary acoustic resonance results when the applied magnetic field is along the trigonal axis. It is hoped that the theory given will serve as a satisfactory basis for the discussion of more detailed acoustic resonance results and measurements of the magneto-thermal resistance in irradiated rubies.