Defects in oxide superconductors: The key to synthesis and superconductivity

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Abstract
The oxide superconductors are doped insulators. The doping, or creation of carriers in a system that would otherwise be insulating, results from the formation of defects. The range of defects formed includes metal-site substitution, metal anti-site defects, metal-site vacancies, oxygen vacancies, interstitial oxygen, and the incorpora-tion of excess oxygen made possible by the modulation of local atom displacements. In some systems, the defect itself is the doping mechanism; in others, defect formation plays a key role during synthesis and, in fact, permits the synthesis of compounds that would otherwise not be attainable. Understanding these defects is the goal of numerous research efforts and is an area where neutron diffraction has contributed unique information.