Electron paramagnetic resonance ofCu2+ion paris in CaO:Cu

Electron paramagnetic resonance of copper pairs was observed in CaO:Cu. The spectra are typical of $S=1\mathrm{}$ in a tetragonal symmetry with the pair axes parallel to the [100], [010], or [001] directions. The $g$ factor is isotropic, while the hyperfine parameters are highly anisotropic. This behavior is explained by assigning the spectra to linear ${\mathrm{Cu}}^{2+}$-${\mathrm{O}}^{2-}$-${\mathrm{Cu}}^{2+}$ bonds, where the two Cu ions of each pair are coupled through the bridging oxygen ion with an antiferrodistortive-ferromagnetic interaction. The ground state of the pair is composed of a $\theta$ state ($3z^2-r^2$) of one ${\mathrm{Cu}}^{2+}$ ion and an $\epsilon$ state ($x^2-y^2$) of the other with an exchange constant $J=-2\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$.