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

Investigations of electron spin resonance (ESR) spectra of Yb³⁺ and Dy³⁺ 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 CaF₂. From the ESR spectra of Dy³⁺ and Er³⁺ it is inferred that the fourth-order potential B₄ is at least 200 times larger than the sixth-order potential B₆.