What Is a Crystal Structure?

Many solids and some crystalline liquids have a regular, repeating, three-dimensional arrangement of atoms known as a crystal structure or crystal lattice. In contrast, an amorphous solid is a type of solid material, such as glass, that lacks such a long-range repeating structure. Many of the physical, optical, and electrical properties of crystalline solids or liquids are closely related to crystal structure. The repeating units of a crystalline structure, which are made up of small boxes or other three-dimensional shapes, are referred to as "cells." Many of these cells are grouped together in a repeating, orderly structure to make up the overall structure.

The crystal structure of a give crystalline material can affect many of that material's overall properties. It is one of the major defining factors affecting the optical properties of the material, for instance. Crystal structure also significantly affects the reactivity of the crystalline material, as it determines the arrangement of reactive atoms on the outside edges and faces of the crystalline solid or liquid. Other important material traits, including electrical and magnetic properties of some materials, are also determined largely by crystal structure.

Mineralogists, crystallographers, chemists, and physicists often study crystalline materials in laboratory settings. Some simple aspects of crystal structures can be determined through simple geometric measurements, but various methods based on the diffraction of x-rays, neutrons, electrons, or other particles allow for much easier and more accurate determinations of structure. Some researchers are only concerned with determining the structure of a given crystalline material while others are more interested in determining how that structure connects to the material's other properties. Still other researchers are interested in finding useful applications for various materials based on their structures, and some even try to synthesize new crystalline solids and liquids based on the expected properties of their desired structures.

It should be noted that, though theoretical crystalline materials are composed of a perfect and consistent series of repeated units, real crystals tend to have flaws. These flaws are, in most cases, simply irregularities in the otherwise regular crystal structure. In some cases, this occurs when one atom takes a different place in a given crystal structure than it normally would. The different properties of this atom can have substantial impacts on how the crystal's structural units arrange themselves around it. Likewise, the defects or irregularities of real crystals can have substantial impacts on the overall properties of the crystalline material.