EPR ofCr2+in II-VI lattices

The EPR spectra of ${\mathrm{Cr}}^{2+}$ in cubic ZnS, ZnSe, ZnTe, and CdTe, and in hexagonal ZnS and CdS are reported. For each, $\Delta M=\pm 4, \pm 2, \pm 1$ transitions are observed allowing complete analysis of the $S=2\mathrm{}$ spin Hamiltonian. A static tetragonal Jahn-Teller distortion is observed in all cases. Stress-alignment studies are described which allow a direct estimate of the Jahn-Teller coupling coefficients. From these, the magnitudes of the Jahn-Teller energies are estimated to be ∼500 ${\mathrm{cm}}^{-1\mathrm{}}$. A ligand-field model is developed which satisfactorily accounts for the fine-structure parameters and their large variation from one crystal to another. In this treatment, ligand contributions to the spin-orbit interaction with excited $d^4$ terms are also included and found to be important. Strain-coupling coefficients, describing the changes in the fine-structure terms under externally applied stress, have been experimentally determined for CdTe (${\mathrm{Cr}}^{2+}$). A simple theory for this effect is developed which includes ligand contributions to the spin-orbit interactions but relies on point-ion estimates for the strain matrix elements. Agreement with experiment is reasonable for the tetragonal coefficients, but the wrong sign is predicted for the trigonal coefficients. The calculated trigonal coefficients for CdS do, however, satisfactorily account for the small departures from tetragonal symmetry observed in the spin Hamiltonian for ${\mathrm{Cr}}^{2+}$ in this hexagonal wurtzite lattice.