Cubic crystal system

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The cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in metallic crystals.

1 Bravais lattices and point/space groups
2 Atomic packing factors and examples
3 Single-element compounds
4 Multi-element compounds
5 See also
6 References

[edit] Bravais lattices and point/space groups

The three Bravais lattices that form the cubic crystal system are:

Simple cubic

Body-centered cubic

Face-centered cubic

The simple cubic system consists of one lattice point on each corner of the cube. Each lattice point is then shared equally between eight adjacent cubes, and the unit cell therefore contains in total one lattice point (1/8 * 8). The body centered cubic system has one lattice point in the center of the unit cell in addition to the eight corner points. It has in total 2 lattice points per cell ((1/8)*8 + 1). Finally, the face centered cubic lattice has lattice points on the faces of the cube, giving a total of 4 lattice points (1/8 * 8 + 1/2*6).

The point groups and space groups that fall under this crystal system are listed below, using the international notation.

Point group	#	Cubic space groups
$23\,\!$	195-199	P23	F23	I23	P2₁3	I2₁3
$m\bar3\,\!$	200-206	Pm $\bar3$	Pn $\bar3$	Fm $\bar3$	Fd $\bar3$	I $\bar3$	Pa $\bar3$	Ia $\bar3$
$432\,\!$	207-214	P432	P4₂32	F432	F4₁32	I432	P4₃32	P4₁32	I4₁32
$\bar4 3m\,\!$	215-220	P $\bar4$ 3m	F $\bar4$ 3m	I $\bar4$ 3m	P $\bar4$ 3n	F $\bar4$ 3c	I $\bar4$ 3d
$m\bar3 m\,\!$	221-230	Pm $\bar3$ m	Pn $\bar3$ n	Pm $\bar3$ n	Pn $\bar3$ m	Fm $\bar3$ m	Fm $\bar3$ c	Fd $\bar3$ m	Fd $\bar3$ c
$m\bar3 m\,\!$	221-230	Im $\bar3$ m	Ia $\bar3$ d

There are 36 cubic space groups, of which 10 are hexoctahedral: Fd3c, Fd3m, Fm3c, Fm3m, Ia3d, Im3m, Pm3m, Pm3n, Pn3m, and Pn3n. Other terms for hexoctahedral are normal class, holohedral, ditesseral central class, galena type.

[edit] Atomic packing factors and examples

The cubic crystal system is one of the most common crystal systems found in elemental metals, and naturally occurring crystals and minerals. One very useful way to analyse a crystal is to consider the atomic packing factor. In this approach, the amount of space which is filled by the atoms is calculated under the assumption that they are spherical.

[edit] Single-element compounds

Assuming one atom per lattice point, the atomic packing factor of the simple cubic system is only 0.524. Due to its low density, this is a high energy structure and is rare in nature. Similarly, the body centered structure has a density of 0.680. The higher density makes this a low energy structure which is fairly common in nature. Examples include iron, chromium, and tungsten.

Finally, the face centered cubic crystals have a density of 0.741, a ratio that it shares with several other systems, including hexagonal close packed and one version of tetrahedral BCC. This is the most tightly packed crystal possible with spherical atoms. Due to its low energy, FCC is extremely common, examples include lead (for example in lead(II) nitrate), aluminum, copper, and gold.

[edit] Multi-element compounds

The Sodium Chloride Crystal Structure of type fcc. Each atom has six nearest neighbors, with octahedral geometry. This arrangement is known as cubic close packed (ccp).
Light blue = Na⁺ (Sodium ion)
Dark green = Cl⁻ (Chlorine ion)

When the compound is formed of two elements whose ions are of roughly the same size, they have what is called the interpenetrating simple cubic, where two atoms of a different type have individual simple cubic crystals. However, the unit cell consists of the atom of one being in the middle of the 8 vertices, structurally resembling body centered cubic. The most common example is caesium chloride CsCl.

However, if the cation is slightly smaller than the anion (a cation/anion radius ratio of 0.414 to 0.732), it forms a different structure, interpenetrating FCC. When drawn separately, both atoms are arranged in an FCC structure. The unit cell for this is shown to the left.