Electron Paramagnetic Resonance of Copper in Beryllium Oxide

Paramagnetic-resonance absorption of copper-doped single crystals of beryllium oxide has been investigated at 9 GHz. The best-fit values to a spin Hamiltonian with $S=\frac{1}{2}$ are $\left|g_{\parallel }\right|=1.709\mathrm{}\pm 0.002$, $\left|g_{\perp }\right|=2.379\mathrm{}\pm 0.001$, $\left|A_{\parallel }\right|=(50\mathrm{}\pm 1)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $\left|A_{\perp }\right|=(108\mathrm{}\pm 1)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, and $\mathrm{}\left|Q\right|=(11\mathrm{}\pm 1)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, where the hyperfine values refer to the ${\mathrm{Cu}}^{63}$ isotope, for which $I=\frac{3}{2}$. The observations are interpreted as due to ${\mathrm{Cu}}^{2+}$ substitutional for beryllium at normal lattice sites. Crystal-field theory which incorporates covalent bonding has been used to interpret these results in terms of the $3d^9$ configuration of ${\mathrm{Cu}}^{2+}$ in $C_{3v}$ symmetry. The experimental results are shown to be consistent with the theory, which yields an orbital reduction factor of 0.6, and values for $〈r^{-3\mathrm{}}〉$ and the contact hyperfine interaction close to those for the free ion. The results are also compared with some other cases of $3d^9$ ions in tetrahedral coordination.