# Crystal

A material is a crystal if it has **essentially** a sharp diffraction pattern. The word **essentially** means that most of the intensity of the diffraction is concentrated in relatively sharp **Bragg peaks**, besides the always present diffuse scattering. In all cases, the positions of the diffraction peaks can be expressed by

\[H = \sum _{i=1}^n h_i a_i^* \, (n \ge 3)\]

Here and *h*_{i} are the basis vectors of the *n* is the minimum for which the positions of the peaks can be described with integer *h*_{i}.

The conventional crystals are a *n* = 3.

### See also

*Acta Cryst.* (1992), **A48**, 928 where the definition of a crystal appears in the **Terms of reference** of the IUCr commission on aperiodic crystals