ABSTRACT
The formula La2Cu04, on which the first class of superconductors is based, is created by adding an extra layer of 0 atoms to the perovskite formula, CaTi03. Some 10% of the La ions are replaced with Ba ions, and to compensate for this loss of charge, a corresponding number of Cu atoms change from Cu(II) to Cu(III). It is thought that the superconductivity is due to electron transfer between the two copper states, mediated by the oxygens, perhaps also involving copper (I), and assisted by the small oxygen deficiency. From general copper chemistry, we recall that Cu(II) has a tetragonally distorted coordination sphere.
