Crystal structure

The structure of crystals is described by the two concepts and basic grid.

Grid

The crystal lattice, also called point grid is a three-dimensional arrangement of (mathematical) point. Subunit of the grid is called the unit cell. It contains all the information needed for describing the crystal. These unit cells are enhanced by translational symmetry to a three-dimensional network. In three-dimensional space, the 14 Bravais lattices describe all possibilities of translational symmetry. From each grid point of the cell of the ( infinitely extended ) crystal must look exactly the same, no matter which direction you look. Because the crystal lattice is constructed only of points, it is always centrosymmetric.

Base

The basis of a crystal structure composed of atoms, ions or molecules. It represents the smallest group of these elements is repeated periodically in three dimensions congruent. The base comprises at least one atom, but also may include several thousand atoms (protein crystals). For sodium chloride, the base is, for example, an Na - and Cl - ion.

Each base is then assigned to a reference point ( in the illustration the upper left corner of the rectangle ). These reference points constituting the crystal lattice when viewed ( hereinafter referred to as image gratings ), only the points. They span the so-called basic vectors from a lattice point to its neighbors have ( in 2D: two, 3D: three pieces). The plane spanned by these basis vectors is called parallelepiped unit or unit cell. This has at its corners, therefore each one grid point, but need not be drawn between directly neighboring points, but can be chosen arbitrarily large.

In the literature it is often spoken of the structure type, or of the lattice structure. One then speaks of the sodium chloride lattice, cesium chloride lattice, etc.. However, because the crystal lattice contains only points and no ions, this expression is misleading. More precisely, it is called sodium chloride, cesium chloride, diamond or zinc-blende structure. These structures are used for the typing of a number of other compounds which correspond in terms of the crystal structure with the examples. So you can also use the terms of sodium chloride structure type, cesium chloride structure type, etc..

Polymorphism

Chemically identical solids may nevertheless occur in different crystal modifications, which differ in their physical properties, for example, have different melting points. This is called polymorphism. To investigate the polymorphism, which has a special meaning in the pharmacy for characterization of some drugs, the differential thermal analysis (DTA ) is a method commonly used. DME allows to detect this complex phenomenon, and to interpret, especially when the sample to be analyzed is a mixture of several crystalline modifications.

Periodic table of elements

The structures of metallic elements at standard conditions are shown color coded with mercury as the only exception, where the low-temperature form is specified for the otherwise liquid element. Non-metals such as noble gases are non- crystalline at standard conditions, while others may have different allotropes such as carbon and therefore can not be enumerated.