Electronic Structure and Properties of High-Tc Superconductor Y-Ba-Cu-O. 1. Oxygen-deficiency in the YBa2Cu3Ox Superconductor (6??x??7)

摘要

The effect of oxygen-deficiency on the charge distributions and orbital energies for small copper oxide clusters representing the superconducting materials YBa2Cu3Ox (6÷x÷7) were investigated by the extended Huckel molecular orbital (EHMO) method with the tight-binding model. Our calculations show +3 oxidation state of Cu(1) in the CuO3 chain and +2 or +1 of Cu(2) in the CuO2 layers for YBa2Cu3O7 with the nominal charge of Cu3 = +7 (or +5), while for YBa2Cu3O6 +1 oxidation state of Cu(1) and +3 (or +2) of Cu(2) in the CuO2 layers with the nominal charge of Cu3 = +7 (or +5). For Cu3O12 cluster representing YBa2Cu3O7 with the nominal charge of Cu3 = +7 the Cu(2) dx2-y2 orbitals in the CuO2 layers is a typical Jahn-Teller d9 system with the partial hole and the Cu(1) dz2-y2 orbital in the CuO3 chain contains hole occupancy. For Cu3O10 cluster representing YBa2Cu3O6 with the nominal charge of Cu = +5 the orbital character of the highest partially occupied MO (HPOMO) and the lowest completely unoccupied MO (LCUMO) of Cu3O12 representing YBa2Cu3O7 with the nominal charge of Cu3 = +7 is reversed, and the character of Cu(1) dx2-y2 orbital of LCUMO of the Cu3O12 cluster is vanished. It is suggested that the local crystal field environment of Cu(1) by the oxygens in the Cu(1) chain may play a vital role in conductivity and superconductivity, either alone or through cooperative electronic coupling with the Cu(2) layers in YBa2Cu3O7.