Copper Mixed Valence Concept: “Cu(I)–Cu(II)” in Thermoelectric Copper Sulfides—an Alternative to “Cu(II)–Cu(III)” in Superconducting Cuprates

The present essay is focused on the role of the Cu(I)–Cu(II) mixed valence for designing thermoelectric sulfides with p-type metal–like properties. A comparison is made with the concept of Cu(II)–Cu(III) mixed valence used for the synthesis of high Tc superconducting cuprates. Attention is drawn on the structural and metal-like conductivity analogy of these thermoelectric sulfides with CuCl, which was previously investigated as a potential superconductor by T. Geballe et al.