Control and Use of Defects in Materials

Defects bring to mind imperfection and blemish, but for materials, what might be considered a defect may lead to dramatic performance improvements over the “ideal” material. Certainly, defects can limit what materials can do—for example, the theoretical strength calculated for crystalline solids is almost never observed in the laboratory because real materials generally contain a certain number of dislocations that can provide sites for the material to crack and fail. However, whereas pure corundum (alumina) has its uses as a hard grinding material, it is the presence of defects—trace metal impurities such as iron or chromium—that produces the electronic transitions that create the beautiful colors of rubies and sapphires. This special issue of Science focuses on the ways that scientists and engineers work around, and with, defects to get the most out of materials.