(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 occurs when atoms move from one interstitial site (spaces between regular lattice positions) to another within the crystal structure of a material. These atoms are typically very small in size and can easily fit into the interstitial spaces without much disruption of the crystal lattice. Vacancy diffusion, on the other hand, occurs when atoms move from one lattice position to another via lattice vacancies. A vacancy is a vacant lattice site, where an atom has been removed from its original lattice position. In vacancy diffusion, atoms move by jumping into a nearby vacant site, effectively exchanging positions with the vacancy.

While both interstitial and vacancy diffusion mechanisms involve the movement of atoms, they differ in the following ways: 1. In interstitial diffusion, atoms occupy the interstitial spaces, while in vacancy diffusion, they occupy the lattice sites. 2. Interstitial diffusion typically involves the movement of small atoms or ions within a crystal lattice, whereas vacancy diffusion involves the movement of lattice atoms themselves. 3. Interstitial diffusion generally does not cause any significant distortion of the crystal lattice, while vacancy diffusion may result in a slight change in the lattice arrangement.

There are two main reasons why interstitial diffusion is generally more rapid than vacancy diffusion: 1. Small atom size: Interstitial diffusing atoms are typically much smaller than the lattice atoms, and therefore, they require less energy to move through the interstitial spaces of the lattice. This results in a higher diffusion coefficient for interstitial diffusion as compared to vacancy diffusion. 2. Higher availability of interstitial sites: Interstitial sites are often more abundant than vacancies in a crystal lattice, which allows interstitial diffusing atoms to have more paths available for diffusion. This results in a higher rate of diffusion for interstitial atoms compared to vacancy diffusion, where the movement relies on the limited availability of vacant lattice sites.