Which statement distinguishes interdiffusion from self-diffusion?

• A. Interdiffusion involves diffusion between different materials; self-diffusion involves atoms of the same type
• B. Interdiffusion occurs only at high temperatures; self-diffusion occurs at any temperature
• C. Interdiffusion requires interstitial atoms; self-diffusion requires vacancies
• D. Interdiffusion occurs only in liquids; self-diffusion only in solids

Answer: (A)

The main idea is that interdiffusion involves atoms moving between two different materials, changing the composition across their interface, while self-diffusion is diffusion of atoms of the same element within a single material, so the overall composition stays the same. For example, copper atoms diffusing into a nickel lattice at their boundary is interdiffusion, whereas copper atoms moving around inside a pure copper crystal is self-diffusion. Both processes can occur via vacancies or interstitials and both are temperature dependent, but the distinguishing feature is whether different materials are involved in the diffusion path. The other statements are not correct because interdiffusion is not restricted to high temperatures only, diffusion mechanisms can vary and are not strictly limited to interstitials for interdiffusion or vacancies for self-diffusion, and interdiffusion certainly can occur in solids, not only in liquids.