The paramagnetic resonance spectrum of ${\mathrm{Yb}}^{3+}$ and ${\mathrm{Er}}^{3+}$ in Th${\mathrm{O}}_2$ has been investigated at 10.5 kMc/sec over the temperature range of 4 to 2°K. Sites of cubic symmetry as well as axial sites due to nearby charge compensation were observed. Both cubic spectra gave isotropic $g$ values with $g=3.423\mathrm{}\pm 0.001$ for ${\mathrm{Yb}}^{3+}$ and $g=6.753\mathrm{}\pm 0.005$ for ${\mathrm{Er}}^{3+}$, and these values are in good agreement with the predicted values for ${\Gamma }_7$ doublets. Temperature variation showed that these were the lowest states. For both ions in dilute crystals the linewidth was ≤0.5 Oe, and in heavily doped crystals the linewidth was 3 to 4 G. Nearby charge compensation in the ${\mathrm{Yb}}^{3+}$ case produces four sites of trigonal symmetry with [111] axes, while in the ${\mathrm{Er}}^{3+}$ case the preponderant axial site had tetragonal symmetry with three [100] axes. For ${\mathrm{Yb}}^{3+}$, $g_{\mathrm{II}}=4.772\mathrm{}\pm 0.002$ and $g_{\perp }=2.724\mathrm{}\pm 0.001$, and one-third the trace of the $g$ tensor was 3.407. For ${\mathrm{Er}}^{3+}$, $g_{\mathrm{II}}=3.462\mathrm{}\pm 0.003$ and $g_{\perp }=7.624\mathrm{}\pm 0.005$, and one-third the trace of the $g$ tensor was 6.240. The linewidths obtained for the axial sites were broader than those for cubic sites and varied with angle. Evidence for macroscopic crystalline imperfections was also found.

• Received 3 August 1964

DOI:https://doi.org/10.1103/PhysRev.137.A138