Valence competition in high-T c superconductors

Abstract

We presente a description of copper valence in superconductors based on the La 2O 3 crystal structure. It is supposed that the two outermost valence electrons of La lead to more ionic bonds than those obtained from the third deep valence electron. Consequently the two oxygen atoms of the tetrahedral site form covalent bonds with the deep valence electron of La. The oxygen of the octahedral site is found to have bonds with just the outermost valence electron of La. Thus it provides an explanation for oxygen holes found in all the new High-Tc superconductors. The above distinction between valence electrons leads to covalent bonds between Cu and O for the CuO 2 planes of La 2CuO 4. For La 2 − xSr xCuO 4 compounds, copper has been supposed to be trivalent, but spectroscopic studies point out bivalent copper. We propose that the krypton shell of Sr is responsible for the lack of one unit of valence, an hypothesis supported by the existence of rare gas compounds. We further suppose that the competition of valence between the contribution from Cu valence electrons and those of Sr-Kr shell might explain the conductivity in these materials.