Cluster calculations on the electronic structures of the high-temperature superconductors Y1Ba2Cu4OX (x=7.5, 8.0, 8.5) and Y1Ba2Cu5O9

Conclusions Cluster calculations show that there is oxygen stabilization in the copper-oxygen chains in Y₁Ba₂Cu₄O₈ on account of the lower excess negative charge on the oxygen and the presence of an additional Cu–O bond together with geminal O...O and Cu...Cu interactions. No basis is found for possible high T_C for YBa₂Cu₄O_3.5 and YBa₂Cu₅O₉. The mechanisms that raise T_C on compressing YBa₂Cu₄O₈ or doping it with calcium are identical and mount to incrase in the state density at the Fermi level because there is stronger interaction between the copper-oxygen plane and the chain on account of the strengthening of the bonds joining them and the bonding involving barium or calcium. Lattice compression also favors this.This study has been performed within the framework of the ANOKONDA project under the State High-Temperature Superconductivity program.Novomoscow Branch, Mendeleev Moscow Chemical Engineering Institute. Translated from Zhurnal Strukturnoi Khimii, Vol. 33, No. 6, pp. 36–44, November–December, 1992