It was recalled that the negative-U centre model for superconductivity in low-coordination materials described well the high critical-current superconducting state in metal oxides. Within the framework of the model, the superconducting current arose from the Bose condensation of bipolarons in a band which was constructed from negative-U centre states. A quantitative description was provided for the present material. Account was taken of low-coordination Cu-O chains and of the disproportionation reaction: 2Cu2+ ↔ Cu+ + Cu3+, which occurred with increasing O content and with a consequent increase in the degree of Cu-O bond covalence. The variously charged states, D+,0,- of the negative-U centre corresponded to Cu3+,2+,+ ion states.

Negative-U Centres Model of High-Tc Superconductivity in Metal Oxides. K.D.Tsendin, B.P.Popov: Superconductor Science and Technology, 1999, 12[5], 255-8