(a) Compare interstitial and vacancy atomic mechanisms for diffusion. (b) Cite two reasons why interstitial diffusion is normally more rapid than vacancy diffusion.

Interstitial diffusion and vacancy diffusion are two mechanisms for atomic diffusion in solid materials. 1. Interstitial diffusion: In this mechanism, diffusion occurs when atoms or molecules move through the interstitial sites of the crystal lattice structure. It means that the atoms or molecules pass through the empty spaces between the already existing atoms in the lattice. 2. Vacancy diffusion: In vacancy diffusion, atoms move from one lattice site to another vacant site by relocating vacancies. A vacancy is an empty lattice site within the crystal structure where an atom should be present but is missing.

Interstitial and vacancy diffusion mechanisms can be compared in terms of their atomic movement and diffusion rates. 1. Atomic movement: In interstitial diffusion, an atom or molecule moves through the empty spaces within the crystal lattice, while in vacancy diffusion, the atom moves from one lattice site to another vacant site by swapping positions with the vacancy. 2. Diffusion rates: The diffusion rate for interstitial diffusion is generally faster than that of vacancy diffusion. This difference in diffusion rate is due to the atomic movement and the availability of spaces for atomic motion.

There are two main reasons why interstitial diffusion is often more rapid than vacancy diffusion: 1. Interstitial atoms/ions are smaller in size: The interstitial atoms or molecules are generally of smaller size compared to the atoms in the crystal lattice. The smaller atom/ion size allows them to easily move through the interstitial spaces compared to the vacancies, which need to be large enough to accommodate an atom/ion to initiate vacancy diffusion. Thus, interstitial diffusion requires less energy for atomic migration. 2. Greater availability of interstitial spaces: There are generally more interstitial spaces available within a crystal lattice compared to the number of vacancies. This greater availability of sites for diffusion contributes to the higher diffusion rate for interstitial diffusion.