Cubic crystal system

An example of the cubic crystals, halite

The cubic crystal system (or isometric) is a crystal system where the unit cell is in the shape of a cube. Halite is the Mineral form of Sodium chloride, Na[[chlorine Cl]] commonly known as rock salt. A crystal system is a category of Space groups which characterize Symmetry of structures in three dimensions with Translational symmetry in three directions In Mineralogy and Crystallography, a crystal structure is a unique arrangement of Atoms in a Crystal. A cube is a three-dimensional solid object bounded by six square faces facets or sides with three meeting at each vertex. 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 lattices
4 Multi-element compounds
5 See also
6 References

Bravais lattices and point/space groups

The three Bravais lattices which 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. In Geometry and Crystallography, a Bravais lattice, named after Auguste Bravais, is an infinite set of points generated by a set of discrete translation Each atom at the lattice points is then shared equally between eight adjacent cubes, and the unit cell therefore contains in total one atom (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 a contribution of 2 lattice points per unit cell ((1/8)*8 + 1). Finally, the face centered cubic has lattice points on the faces of the cube of which each unit cube gets exactly one half contribution, in addition to the corner lattice points, giving a total of 4 atoms per unit cell ((1/8 for each corner) * 8 corners + (1/2 for each face) * 6 faces). Attempting to create a C-centered cubic crystal system would result in a simple tetragonal Bravais lattice. There are 8 lattice points on a simple cubic for each corner of the shape. There are 9 lattice points for a body centered because of the extra point in the center of the unit. There are 14 lattice points on a face centered cubic.

The point groups and space groups that fall under this crystal system are listed below, using the international notation. In Crystallography, a crystallographic point group is a set of Symmetry operations like rotations or reflections that leave a point fixed while moving each atom The space group of a Crystal or crystallographic group is a mathematical description of the Symmetry inherent in the structure

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.

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 Crystallography, atomic packing factor (APF or packing fraction is the fraction of volume in a Crystal structure that is occupied by Atoms In this approach, the amount of space which is filled by the atoms is calculated under the assumption that they are spherical.

Single-element lattices

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, but is found in Polonium ^[1]. The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different Polonium (pəˈloʊniəm is a Chemical element with the symbol Po and Atomic number 84 discovered in 1898 by Marie and Pierre Curie 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 Fe-iron, Cr-chromium, W-tungsten, and Nb-niobium. Iron (ˈаɪɚn is a Chemical element with the symbol Fe (ferrum and Atomic number 26 Chromium (ˈkroʊmiəm is a Chemical element which has the symbol Cr and Atomic number 24 Tungsten (ˈtʌŋstən also known as wolfram (/ˈwʊlfrəm/ is a Chemical element that has the symbol W and Atomic number 74 Niobium (naɪˈoʊbiəm or columbium (/kəˈlʌmbiəm/ is a Chemical element that has the symbol Nb and Atomic number 41

Finally, the face centered cubic crystals have a density of approximately 0. 740, 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), Al- aluminium, Cu- copper, Au- gold and Ag- silver. Characteristics Lead has a dull luster and is a dense, Ductile, very soft highly Lead(II nitrate is an Inorganic compound with the Chemical formula Pb ( NO 32 WikipediaNaming Copper (ˈkɒpɚ is a Chemical element with the symbol Cu (cuprum and Atomic number 29 Gold (ˈɡoʊld is a Chemical element with the symbol Au (from its Latin name aurum) and Atomic number 79 Silver (ˈsɪlvɚ is a Chemical element with the symbol " Ag " (argentum from the Ancient Greek: ἀργήντος - argēntos gen

Multi-element compounds

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 structure, 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. Cesium chloride is the Chemical compound with the formula Cs[[Chlorine Cl]] This structure actually has a simple cubic lattice with a two atom basis, the atom positions being atom A at (0,0,0) and and atom B at(0. 5,0. 5. 0. 5)

However, if the cation is slightly smaller than the anion (a cation/anion radius ratio of 0. 414 to 0. 732), the crystal forms a different structure, interpenetrating FCC. When drawn separately, both atoms are arranged in an FCC structure. This structure has an FCC lattice, with a two atom basis, the atom positions being atom A at (0,0,0) and atom B at (0. 5,0. 5,0. 5). Sodium Chloride is a common example of this type of structure

References

Hurlbut, Cornelius S. In Materials science, a dislocation is a Crystallographic defect, or irregularity within a Crystal structure. In Mineralogy and Crystallography, a crystal structure is a unique arrangement of Atoms in a Crystal. ; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed. , Wiley, ISBN 0-471-80580-7

^ Greenwood, N. N. ; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.

© 2009 citizendia.org; parts available under the terms of GNU Free Documentation License, from http://en.wikipedia.org
network: | |

Contents

Bravais lattices and point/space groups

Atomic packing factors and examples

Single-element lattices

Multi-element compounds

See also

References