Electron-Paramagnetic-Resonance Investigation of the Dynamic Jahn—Teller Effect in Sr${\mathrm{Cl}}_2$:${\mathrm{Y}}^{2+}$ and Sr${\mathrm{Cl}}_2$:${\mathrm{Sc}}^{2+}$

EPR spectra have been observed for Sr${\mathrm{Cl}}_2$:${\mathrm{Y}}^{2+}$ and Sr${\mathrm{Cl}}_2$:${\mathrm{Sc}}^{2+}$ at liquid-helium temperatures. At 1.2 K the spectra were dominated by anisotropic hyperfine patterns whose line shapes and angular dependences were explained using second-order solutions of the effective Hamiltonian for an isolated ${}_{}{}^2{}_{}{}^{}E_g^{}{}_{}{}^{}$ state split by large random internal strains. Pronounced asymmetries in some of the strain-produced line shapes for Sr${\mathrm{Cl}}_2$:${\mathrm{Sc}}^{2+}$ are shown to result from second-order terms in the solution of the effective Hamiltonian. Coexisting with the anisotropic hyperfine patterns are weak nearly isotropic hyperfine patterns with typical line shapes. Variations in the apparent intensity of lines in these weak hyperfine patterns as functions of the applied-magnetic-field direction and temperature imply that these lines result from averaging by vibronic relaxation of a portion of the anisotropic pattern. This interpretation is further strengthened in the case of Sr${\mathrm{Cl}}_2$:${\mathrm{Sc}}^{2+}$ by the observation of a predicted anisotropy in the "averaged" spectrum. The effective-Hamiltonian parameters for Sr${\mathrm{Cl}}_2$:${\mathrm{La}}^{2+}$, Sr${\mathrm{Cl}}_2$:${\mathrm{Y}}^{2+}$, and Sr${\mathrm{Cl}}_2$:${\mathrm{Sc}}^{2+}$ are analyzed in terms of crystal field theory modified to include a weak to moderate vibronic interaction, i.e., a dynamic Jahn-Teller effect.