EPR investigations of intermediate Jahn-Teller coupling effects forCu2+in MgO and CaO

The electron-paramagnetic-resonance (EPR) spectra of divalent copper in CaO and MgO single crystals have been observed at 1.2 and 77 K. Although the basic characteristics of the low-temperature (1.2 K) ${\mathrm{Cu}}^{2+}$ spectra in these hosts were somewhat similar to those resulting from a dynamic Jahn-Teller effect, significant deviations in the line shapes and angular variations were found. For ${\mathrm{C}\mathrm{a}\mathrm{O}:\mathrm{C}\mathrm{u}}^{2+}$, the angular variations in the {110} and {001} planes due to both the Zeeman and hyperfine interactions were fully consistent with the properties of a ${}_{}{}^2{}_{}{}^{}E$ vibronic ground state with appreciable random-strain coupling to an excited $A_1$ vibronic singlet (i.e., an intermediate Jahn-Teller effect). A numerical diagonalization of the matrix of random strain and tunneling within the $A_1$ and ${}_{}{}^2{}_{}{}^{}E$ manifold was performed for various ratios of average random-strain splitting $\overline{\delta }$ to tunneling splitting $3\Gamma$ and the corresponding EPR spectra were computed. The calculated line shapes and angular variations were then compared with the experimental results. For ${\mathrm{Cu}}^{2+}$ in CaO, a value of $\frac{\overline{\delta }}{3\Gamma }=0.67\mathrm{}$ was obtained; and this value together with the effective spin-Hamiltonian parameters for the isolated ${}_{}{}^2{}_{}{}^{}E$ state describes both the complete angular variation and the details of the asymmetric line shapes found for $\overrightarrow{H}\parallel \mathrm{}\left[111\right]\mathrm{}$. From this value of $\frac{\overline{\delta }}{3\Gamma }$ and a recently published value for $3\Gamma$, we find that $\overline{\delta }=2.7\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$ for ${\mathrm{C}\mathrm{a}\mathrm{O}:\mathrm{C}\mathrm{u}}^{2+}$. In contrast to the ${\mathrm{C}\mathrm{a}\mathrm{O}:\mathrm{C}\mathrm{u}}^{2+}$ results, the ${\mathrm{M}\mathrm{g}\mathrm{O}:\mathrm{C}\mathrm{u}}^{2+}$ EPR spectrum exhibited the effects of random-strain coupling to an excited $A_2$ vibronic singlet. Although a value of $\frac{\overline{\delta }}{3\Gamma }$ could be determined which described the angular variation of the Zeeman interaction, the angular dependence of the hyperfine structure and...