Effect of hole localization on Madelung potential of YBa2Cu3O7

The Madelung potential and formation energy of the superconducting compound YBa2Cu3O7 have been computed for hole localization at different sites in the crystal. The cases considered include Cu3+ ion at Cu(1) and Cu(2) sites, O− ion at O(1), O(2), O(3) and O(4) sites and combinations of O− and Cu3+ ions at O(4) and Cu(1) and O(2,3) and Cu(2) sites. The two lowest-energy configurations correspond to Cu3+ ion at Cu(1) site and O− ion at O(4) site. The difference in formation energy between those configurations is relatively small. The next preferred configuration corresponds to simultaneous partial localization of the hole at Cu (1) site and O(1) site. Other configurations are much less stable. The results suggest a resonating or fluctuating valence model for YBa2Cu3O7.