Evidence for covalency in Tm2+ and Yb3+ in calcium fluoride

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Abstract
Axe and Burns have discussed the effects of covalency in Tm2+ in calcium fluoride, with particular emphasis on its contribution to the crystalline-field splittings of the energy levels. This theory has been developed to give a consistent interpretation of the transferred hyperfine interaction with the neighbouring fluorine ions and the orbital reduction factor, as well as the crystal-field splittings. The contribution to the transferred hyperfine structure from polarization of the 5p electrons of the rare earth ion is extrapolated from the configuration 4f7 and is shown to be fairly small. The treatment is extended to the isoelectronic ion Yb3+ where the covalency is larger. The larger crystal-field splitting is predicted by the theory, as well as larger transferred hyperfine structure and orbital reduction factor, and the quantitative estimates of the crystal field splittings are as good as those of Axe and Burns for Tm2+. No attempt is made at exact calculation, but it is demonstrated that the fairly crude covalent calculation gives a consistent interpretation of all of the available experimental information for the two isoelectronic ions.