Electron Paramagnetic Resonance of Trivalent-Rare-Earth-Monovalent-Alkaline-Earth Ion Pairs in CaF2

Paramagnetic resonance from rare-earth ions in sites having rhombic symmetry have been observed from calcium fluoride crystals doped with rare-earth and sodium fluorides. The ions investigated were ${\mathrm{Ce}}^{3+}$, ${\mathrm{Nd}}^{3+}$, ${\mathrm{Dy}}^{3+}$, and ${\mathrm{Yb}}^{3+}$, and for each the axes of the $g$ tensor were found to lie along the [110], [$\overline{1}10$], and [001] directions. In the case of the ytterbium-doped crystals, replacing the sodium fluoride by lithium or potassium fluoride generated new, but similar, rhombic spectra. It is proposed that all these spectra arise from trivalent-rare-earth-monovalent-alkaline-earth ion pairs, the ions occupying adjacent ${\mathrm{Ca}}^{2+}$ ion sites. New tetragonal spectra have also been observed from ${\mathrm{Ce}}^{3+}$ and ${\mathrm{Nd}}^{3+}$ ions in crystals heavily doped with NaF, and it is suggested that these might arise from the association of the rare-earth ions with sodium ions on the next nearest ${\mathrm{Ca}}^{2+}$ ion sites, though this is not firmly established.