Large low-energy oscillator strength for Ce 4felectrons in the solid state

The conductivity sum rule applied to LaSn{sub 3}, CeSn{sub 3}, LaB{sub 6}, and CeB{sub 6} between 1.5 and 4.35 eV shows that there is approximately one more electron contributing to the optical conductivity of the Ce compounds than to that of the analogous La compounds. From the data and band calculations we conclude that about 0.5 Ce 4{ital f} electron per atom contributes as initial states for CeSn{sub 3} in the above energy range, and about half that number of 4{ital f} electrons serve as initial states in LaSn{sub 3}. This shift of oscillator strength from higher energies for atoms to lower energies for solids is attributed to hybridization. Much of the remaining Ce-La difference is from transitions to {ital p}-like final states, which are reduced in the La compounds due to hybridization with unoccupied 4{ital f} states.