PHASE-TRANSITIONS IN MIXED-VALENCE SYSTEMS

The experimental properties of URu2Si2 are briefly discussed to demons trate that the lattice degrees of freedom may be tightly involved into the mixed valence phenomenon and the heavy fermion behavior. Instead of describing the f-electrons in terms of rigid single ion orbitals, w e emphasize the importance of the lattice effects in localization of a n f-electron near the given site. In view that so many pertinent inter actions and parameters are involved, a phenomenological approach to th e problem is discussed in terms of the generalized Anderson model for single centers. Implications of the picture to the symmetry considerat ions are discussed in some more details for URu2Si2. Some features of the Kondo insulator state are also discussed to indicate that they may also be compatible with the developed views.