It is impossible to find a substance in the world that is completely free of defects. In fact, our understanding of materials begins with the knowledge of their defects. It is the defects that create the variety of materials, and the exceptional properties of materials are largely related to these defects. Therefore, understanding the structures of defects in materials is crucial for developing new materials or for selecting appropriate materials for applications. Crystal defects can be defined as deviations from perfect atomic arrangements, and the greater the deviation, the higher the energy of the defect.