Disorder in oxides

In this paper the intimate correlation among structure, defect thermodynamics and physical properties in oxides is briefly reviewed through few examples concerning high Tc superconductors and ionic conductors. In some cases the desired physical properties appear only when disorder is introduced in the structures, like in neodymium cuprate and doped ceria. Disorder can be even expressly introduced in order to tune the physical properties already present in the undoped compound, which is the case of Al-doped REBa2Cu3O6+d. It will be demonstrated that not only the point defects and the local distortions induced by them, but also the correlation lengths of disorder are to be taken into account in order to correctly understand the physical properties of interesting materials.