# Difference between revisions of "Crystal"

### From Online Dictionary of Crystallography

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− | Here <math>\textbf{a}_{i}^{*}</math> and <math>h_{i}</math> are the reciprocal lattice | + | 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. | The conventional crystals are a special class, though very large, for which ''n'' = 3. |

## Revision as of 10:40, 13 February 2012

## 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), **A48**, 928 where the definition of a crystal appears in the **Terms of reference** of the IUCr commission on aperiodic crystals