What is a substitutional defect?

• A. A missing atom in the lattice
• B. An extra atom occupying an interstitial site
• C. A foreign atom substitutes for a host atom in the lattice, forming a solid solution
• D. A dislocation line representing a misfit at the lattice edge

Answer: (C)

Substitutional defect is when a foreign atom takes the place of a host atom in the crystal lattice, forming a substitutional solid solution. In this defect, solute atoms occupy lattice sites that would normally be occupied by host atoms, typically when the atoms are similar in size and structure. An example is brass, where zinc atoms substitute for copper in the copper lattice. This differs from vacancies, where a lattice site is empty, or interstitial defects, where an extra atom sits in a between-atom site, and from dislocations, which are line defects representing misfits along a row of atoms. Substitutional defects can alter lattice spacing, density, and diffusion behavior due to the size and chemical differences of the substituting atoms.