Distortions on a metal-oxide framework: Local pairs in oxide superconductors

Electronic structure and Madelung potential calculations are used to examine the effects of local changes in coordination geometry in various metal-oxide frameworks. In particular, the authors are investigating the function of pairs of charged defects and their potential role in the superconducting oxides. They have specifically studied the transition metal oxide networks in La2-xSrxCuO4, YBa2Cu3O7-y, and Li1±xTi2O4. In the cuprates, the Fermi level lies in a band which has Cu-O σ-antibonding character. For Cu2+, this band is half-filled, and one-electron theory always predicts a structural distortion for such cases. In the metal oxides, this manifests as charge disproportionation. They have performed numerous semi-empirical and Hubbard-type calculations as one- and two-dimensional models of distorted Cu-O frameworks. They find for electron counts near the half-filled band that certain distorted configurations are electronically stable and lead to interesting charge distributions throughout the framework. Finally, they extend this model to systems in which the Fermi level crosses metal-oxygen π-antibonding levels as in numerous ruthenate perovskites and pyrochlores