Electron Spin Resonance of Rare-Earth Ions in Ce${\mathrm{O}}_2$: ${\mathrm{Yb}}^{3+}$ and ${\mathrm{Er}}^{3+}$

The paramagnetic resonance spectrum of ${\mathrm{Yb}}^{3+}$ and ${\mathrm{Er}}^{3+}$ in single crystals of Ce${\mathrm{O}}_2$ has been measured at 10 Gc/sec and 4°K. In sites of cubic symmetry, the ground states in both cases are found to be isotropic ${\Gamma }_7$ doublets with $g=3.424\mathrm{}\pm 0.001$ for ${\mathrm{Yb}}^{3+}$ and $g=6.747\mathrm{}\pm 0.006$ for ${\mathrm{Er}}^{3+}$. Hyperfine structures for the odd isotopes were resolved with $A\left({\mathrm{Yb}}^{171}\right)=(877.1\mathrm{}\pm 1.0)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $A\left({\mathrm{Yb}}^{173}\right)=(242.0\mathrm{}\pm 0.3)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, and $A\left({\mathrm{Er}}^{167}\right)=(232.1\mathrm{}\pm 0.3)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$. Nearby charge compensation produces sites of axial symmetry. For ${\mathrm{Yb}}^{3+}$, one set of axes is along the four crystal directions with $g_{\parallel }=4.733\mathrm{}\pm 0.004$ and $g_{\perp }=2.744\mathrm{}\pm 0.002$. For ${\mathrm{Er}}^{3+}$ two different axial spectra were measured, also with trigonal symmetry about the directions. For one set $g_{\parallel }=10.25\mathrm{}\pm 0.05$ and $g_{\perp }=4.847\mathrm{}\pm 0.005$, and for the other set $g_{\parallel }=4.539\mathrm{}\pm 0.005$ and $g_{\perp }=7.399\mathrm{}\pm 0.007$. In most crystals the linewidths were 20 Oe; however, in some crystals in which no cubic sites were found, the axial linewidths were greater than 50 Oe. Comparisons of the axial $g$-tensor traces with the cubic $g$ tensor and the magnitude and sign of $\Delta g=g_{\parallel }-g_{\perp }$ are the basis for a discussion of the possible nature of the charge compensation producing...