  A new kind of phase transition is proposed for lattice fermion systems with
simplified f^2 configurations at each site. The free energy of the model is
computed in the mean-field approximation for both the itinerant state with the
Kondo screening, and a localized state with the crystalline electric field
(CEF) singlet at each site. The presence of a first-order phase transition is
demonstrated in which the itinerant state changes into the localized state
toward lower temperatures. In the half-filled case, the insulating state at
high temperatures changes into a metallic state, in marked contrast with the
Mott transition in the Hubbard model. For comparison, corresponding states are
discussed for the two-impurity Kondo system with f^1 configuration at each
site.
