Electronic properties of4fsubstances at configuration crossover

In a system of concentrated $4f$ ions with two configurations $4f^n$ and $4f^{n-1\mathrm{}}$ competing for stability, a narrow resonance in the electronic density of states near the Fermi energy is expected. Using an explicitly configuration-based approach with appropriate intraconfigurational splittings included, we obtain a sum rule on the integrated density of states and a microscopic estimate of the resonance width. The width, identifiable with an interconfiguration fluctuation rate, is proportional to the nominal mixing width $\Delta \left({\epsilon }_F\right)=\pi {\left|V_{\mathrm{mix}}\right|}^2{\rho }_{\mathrm{cond}}\left({\epsilon }_F\right)$, but with a proportionality factor depending rather strongly upon the specific ionic states involved. Results are obtained for electronic specific heats, x-ray photoemission linewidths, and Mössbauer spectra in such systems.