Defect engineering using crystal symmetry

Defects in solids can be broadly classified based on their dimensionality (1). Zero-dimensional defects (point defects) arise as a consequence of entropy considerations (configurational entropy) and thus are thermodynamically required in any material. One-dimensional defects such as dislocations are not required by thermodynamics but arise anyway as a consequence of imperfections in the synthetic environment or due to structural constraints imposed by applied stresses from a variety of sources. Dislocations arise as a consequence of stresses imposed through thermal history, processing, or structural mismatch, as in the case of heteroepitaxial thin films, which are so prevalent in modern technology. Extended defects are known to have a profound influence in achieving or impacting the desired material performance. Complex networks of dislocations are designed into alloys to enhance their mechanical properties, such as the tensile strength in steels and other technologically relevant alloys (2). In many cases, defects, such as dislocations, are harmful to the desired performance of the device (3). Volumetric defects (such as second-phase inclusions, some of which are deliberately designed into materials such as the Guinier–Preston zones in Al–Cu alloys) (4, 5) can arise as a consequence of improper processing or through deliberate materials engineering. The characteristic length scale of such defects will depend on the physical phenomenon in question (e.g., pinning of the movement of dislocations would require second phases that are of a certain size and spacing). We now come to 2D, surface-type defects, the designing of which is the focus of the paper by Wang et al. (6). Two-dimensional defects, such as antiphase boundaries (APBs) and domain walls in certain ferroic materials, arise primarily as a consequence of symmetry mismatch in spatial coordinates, … [↵][1]1Email: rramesh{at}berkeley.edu. [1]: #xref-corresp-1-1