The Debye-Waller factor is a term (in scattering equations) which accounts for how thermal fluctuations extinguish scattering intensity (especially high-q peaks). This scattering intensity then appears as diffuse scattering. Conceptually, thermal fluctuations create disorder, because the atoms/particles oscillate about their equilibrium positions and thus the lattice is never (instantaneously) perfect.
The same conceptual framework can be used to describe static disorder. A defective lattice where many particles are displaced from their idealized positions will cause the structural scattering to be weakened, with diffuse scattering appearing instead.
For a lattice-size a, the constituent entities (atoms, particles, etc.) will oscillate about their equilibrium positions with an rms width , attenuating structural peaks like:
Where is the root-mean-square displacement of the lattice-spacing a (such that the spacing at time t is ), and is the relative displacement.
Thus, the intensity of the structural peaks is multiplied by , which attenuates the higher-order (high-q) peaks, and redistributes this intensity into a diffuse scattering term, which appears in the structure factor () as:
And thus appears in the overall intensity as:
where is the form factor.
In the high-q limit, form factors frequently exhibit a scaling (c.f. sphere form factor), in which case one expects (since ):
(Which reproduces the scaling of the Porod law.)