Polarization catastrophe at low densities of polarons: from Cuprates to Metal-Ammonia Solutions

We review some results on the role played by dielectric polarons at the metal-insulator transition in polarizable materials, taking into account the long-range nature of the Coulomb interactions. The occurrence of a polarization catastrophe is examined in a model describing a Wigner crystal of polarons. The possible relevance of this scenario in the cuprates and in metal-ammonia solutions is discussed.