In a crystalline solid, the diffusivity $D$ that appears in Fick's law is set by thermally activated atomic motion through the structure.

An atom cannot usually move continuously through occupied lattice sites. Instead it changes position through mechanisms such as:

• vacancy diffusion: an atom jumps into a neighboring vacant lattice site;
• interstitial diffusion: a sufficiently small atom jumps between interstitial sites.

Interstitial diffusion is often faster because it does not require a neighboring vacancy and small interstitial atoms encounter lower geometric barriers.

For a regime dominated by one activated mechanism, the diffusivity commonly has the Arrhenius form

$$D=D_0\exp!\left(-\frac{Q_D}{RT}\right),$$

where $D_0$ is the diffusion prefactor and $Q_D$ is the activation energy for the relevant diffusion process. Measuring $D$ at several temperatures can therefore reveal an apparent diffusion activation energy.

Diffusion can also be unusually fast along grain boundaries, dislocation cores and surfaces because these regions are less densely constrained than the bulk lattice. Different paths can have different prefactors and activation energies, so the dominant path can change with temperature and microstructure.

The atomic mechanism therefore determines the mobility encoded in $D$, while concentration gradients or other thermodynamic driving forces determine the direction and magnitude of net transport.