Susceptibility of Interconfiguration-Fluctuation Compounds

We present a simple model for the magnetic susceptibility of rare-earth compounds which exhibit interconfiguration fluctuations (ICF). It is shown that the temperature dependence of the magnetic susceptibility of several ICF Yb compounds can be fitted quantitatively, assuming an energy separation $E_{\mathrm{ex}}$ between the two configurations and an intrinsic lifetime $\tau$ due to the $4f$-shell-conduction-electron interaction. With the parameters $E_{\mathrm{ex}}$ and $\tau$ obtained from such fits it is possible to predict the temperature dependence of the valence.