Copper and nickel mixed valence in oxides and chalcogenides: The key for the discovery of exotic metals

Attempts to induce delocalisation of n and p-type carriers in iono-covalent systems represent a very important challenge in order to discover exotic metals. This idea is strongly supported by the concept of Cu2+-Cu3+ mixed valence that we developed in the 80′s for the realisation of numerous metallic cuprates. We show here that K.A. Muller's discovery of high Tc superconductivity in one of these exotic metals has boosted the research in a vast field of high Tc superconducting cuprates. Importantly, we demonstrate that the Cu+-Cu2+ mixed valence concept also allows numerous thermoelectric copper sulphides with exotic p-type metallic properties to be generated. We discuss the role of the Ni+-Ni2+ mixed valence in the metallic properties and thin film superconductivity of nickelates.