Crystal-field states of theCe3+ion inCeF3: A demonstration of vibronic interaction in ionic rare-earth compounds

We have investigated the Raman, infrared, and EPR spectra of ${\mathrm{CeF}}_3$ in a search for the excited crystal-field levels of the 4$f^1$ configuration in this compound. While this can be achieved routinely for the ${}_{}{}^2{}_{}{}^{}\mathrm{F}_{7/2}^{}{}_{}{}^{}$ levels between 2100 and 2900 ${\mathrm{cm}}^{\mathrm{-}1}$, problems arise for the ${}_{}{}^2{}_{}{}^{}\mathrm{F}_{5/2}^{}{}_{}{}^{}$ levels because of a strong interaction between the electronic states and the near-resonant optical phonons. Pure electronic crystal-field states obviously do not exist in this range, but are hybridized with optical phonons and additional, new vibronic states are found in the wave-number regions of the expected crystal-field levels. A tentative model of a local Jahn-Teller–type 4f-electron–phonon interaction is studied, but only qualitative agreement with the experimental results can be obtained. Approximate values for the wave numbers of the uncoupled crystal-field states are given. An extrapolation of existing values of crystal-field parameters for the trigonal tysonite structure to ${\mathrm{CeF}}_3$ gives reasonable agreement between calculated crystal-field levels and the findings from experiment for these uncoupled states.