After the discovery of superconductivity at 40 K in cuprates by Georg Bednorz and Alex Müller in 1986, many researchers became involved in the synthesis of new higher‐temperature superconductors. I believe that many of them failed in this quest simply because they did not realize that the crystal chemistry of such materials is extremely complicated. By the following spring, of course, groups all over the world were fabricating new superconducting cuprates of yttrium,bismuth and thallium that allowed critical temperatures to exceed the all‐important 77‐K boiling point of liquid nitrogen. But because of the complex crystal chemistry involved, it will take a long time to optimize the superconductingproperties of these cuprates for the development of useful superconductingwires. Thin films of superconducting cuprates, by contrast, are already quite close to being commercially available for SQUIDS and microwave applications.
Stacking faults, ionic vacancies, columnar defects, grain boundaries and other crystal defects play crucial roles in determining the superconducting properties of high‐T c layered cuprates. Such defects can be exploited to optimize superconductivity.