# Crystal

### From Online Dictionary of Crystallography

##### Revision as of 12:20, 3 April 2009 by GervaisChapuis (talk | contribs)

Revision as of 12:20, 3 April 2009 by GervaisChapuis (talk | contribs)

## Definition

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

[math]\textbf{H}=\sum_{i=1}^nh_{i}\textbf{a}_{i}^{*}~~(n\ge 3)[/math]

Here [math]\textbf{a}_{i}[/math] and [math]h_{i}[/math] are the reciprocal lattice vectors and integer coefficients respectively.

The conventional crystals are a special class, though very large, for which *n* = 3.