Evidence for Covalent Bonding from Electron Spin Resonance Spectra of Some Rare-Earth Ions in Single Crystals of Calcium Oxide

Abstract

Investigations of electron spin resonance (ESR) spectra of ${\mathrm{Yb}}^{3+}$ and ${\mathrm{Dy}}^{3+}$ in the cubic crystal field of calcium oxide show $g$ factors which are smaller than the calculated $g$ factors. It is shown that covalent bonding through the effect of the orbital reduction factor can explain these discrepancies. It is found that ($1-k$) is about 0.04, much smaller than found in the iron group, but larger than that found for rare earth ions in the crystal field of Ca${\mathrm{F}}_2$. From the ESR spectra of ${\mathrm{Dy}}^{3+}$ and ${\mathrm{Er}}^{3+}$ it is inferred that the fourth-order potential $B_4$ is at least 200 times larger than the sixth-order potential $B_6$.