# Crystal

### From Online Dictionary of Crystallography

Cristal (*Fr*); Kristall (*Ge*); Cristal (*Sp*); Кристалл (*Ru*); Cristallo (*It*); 結晶 (*Ja*); بلور (*Ar*).

## 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 basis vectors of the reciprocal lattice and integer coefficients respectively and the number *n* is the minimum for which the positions of the peaks can be described with integer coefficient [math]h_{i}[/math].

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

## See also

*Acta Cryst.*(1992), A**48**, 928 where the definition of a crystal appears in the**Terms of reference**of the IUCr Commission on Aperiodic Crystals.