Cubic crystal system
In crystallography, 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 crystals and minerals.
There are three main varieties of these crystals:
Each is subdivided into other variants listed below. Although the unit cells in these crystals are conventionally taken to be cubes, the primitive unit cells often are not.
Bravais lattices
The three Bravais latices in the cubic crystal system are:| Bravais lattice | Primitive cubic | Body-centered cubic | Face-centered cubic |
| Pearson symbol | cP | cI | cF |
| Unit cell |
The primitive cubic lattice consists of one lattice point on each corner of the cube; this means each simple cubic unit cell has in total one lattice point. Each atom at a lattice point is then shared equally between eight adjacent cubes, and the unit cell therefore contains in total one atom.
The body-centered cubic lattice has one lattice point in the center of the unit cell in addition to the eight corner points. It has a net total of two lattice points per unit cell.
The face-centered cubic lattice has lattice points on the faces of the cube that each give exactly one half contribution, in addition to the corner lattice points, giving a total of four lattice points per unit cell.
The face-centered cubic lattice is closely related to the hexagonal close packed system, where the two systems differ only in the relative placements of their hexagonal layers. The Miller index| plane of a face-centered cubic lattice is a hexagonal grid.
Attempting to create a base-centered cubic lattice results in a simple tetragonal Bravais lattice.
Coordination number is the number of nearest neighbors of a central atom in the structure. Each sphere in a cP lattice has coordination number 6, in a cI lattice 8, and in a cF lattice 12.
Atomic packing factor is the fraction of volume that is occupied by atoms. The cP lattice has an APF of about 0.524, the cI lattice an APF of about 0.680, and the cF lattice an APF of about 0.740.
Crystal classes
The isometric crystal system class names, point groups, type, examples, international tables for crystallography space group number, and space groups are listed in the table below. There are a total 36 cubic space groups.Other terms for hexoctahedral are: normal class, holohedral, ditesseral central class, galena type.
Single element structures
As a rule, since atoms in a solid attract each other, the more tightly packed arrangements of atoms tend to be more common. Accordingly, the primitive cubic structure, with especially low atomic packing factor, is rare in nature, but is found in polonium. The bcc and fcc, with their higher densities, are both quite common in nature. Examples of bcc include iron, chromium, tungsten, and niobium. Examples of fcc include aluminium, copper, gold and silver.Another important cubic crystal structure is the diamond cubic structure, which can appear in carbon, silicon, germanium, and tin. Unlike fcc and bcc, this structure is not a lattice, since it contains multiple atoms in its primitive cell. Other cubic elemental structures include the A15 structure found in tungsten, and the extremely complicated structure of manganese.
Multi-element structures
Compounds that consist of more than one element often have crystal structures based on the cubic crystal system. Some of the more common ones are listed here. These structures can be viewed as two or more interpenetrating sublattices where each sublattice occupies the interstitial sites of the others.Caesium chloride structure
One structure is the "interpenetrating primitive cubic" structure, also called a "caesium chloride" or B2 structure. This structure is often confused for a body-centered cubic structure because the arrangement of atoms is the same. However, the caesium chloride structure has a basis composed of two different atomic species. In a body-centered cubic structure, there would be translational symmetry along the direction. In the caesium chloride structure, translation along the direction results in a change of species. The structure can also be thought of as two separate simple cubic structures, one of each species, that are superimposed within each other. The corner of the chloride cube is the center of the caesium cube, and vice versa.It works the same way for the NaCl structure described in the next section. If you take out the Cl atoms, the leftover Na atoms still form an FCC structure, not a simple cubic structure.
In the unit cell of CsCl, each ion is at the center of a cube of ions of the opposite kind, so the coordination number is eight. The central cation is coordinated to 8 anions on the corners of a cube as shown, and similarly, the central anion is coordinated to 8 cations on the corners of a cube. Alternately, one could view this lattice as a simple cubic structure with a secondary atom in its cubic void.
In addition to caesium chloride itself, the structure also appears in certain other alkali halides when prepared at low temperatures or high pressures. Generally, this structure is more likely to be formed from two elements whose ions are of roughly the same size.
The space group of the caesium chloride structure is called Pmm, or "221". The Strukturbericht designation is "B2".
There are nearly a hundred rare earth intermetallic compounds that crystallize in the CsCl structure, including many binary compounds of rare earths with magnesium, and with elements in groups 11, 12, and 13. Other compounds showing caesium chloride like structure are CsBr, CsI, high-temperature RbCl, AlCo, AgZn, BeCu, MgCe, RuAl and SrTl.
Rock-salt structure
The space group of the rock-salt or halite structure is denoted as Fmm, or "225". The Strukturbericht designation is "B1".In the rock-salt structure, each of the two atom types forms a separate face-centered cubic lattice, with the two lattices interpenetrating so as to form a 3D checkerboard pattern. The rock-salt structure has octahedral coordination: Each atom's nearest neighbors consist of six atoms of the opposite type, positioned like the six vertices of a regular octahedron. In sodium chloride there is a 1:1 ratio of sodium to chlorine atoms. The structure can also be described as an FCC lattice of sodium with chlorine occupying each octahedral void or vice versa.
Examples of compounds with this structure include sodium chloride itself, along with almost all other alkali halides, and "many divalent metal oxides, sulfides, selenides, and tellurides". According to the radius ratio rule, this structure is more likely to be formed if the cation is somewhat smaller than the anion.
The interatomic distance in some rock-salt-structure crystals are: 2.3 Å for NaF, 2.8 Å for NaCl, and 3.2 Å for SnTe. Most of the alkali metal hydrides and halides have the rock salt structure, though a few have the caesium chloride structure instead.
| Oxides | Sulfides | Selenides | Tellurides | Polonides | |
| Magnesium | Magnesium oxide | Magnesium sulfide | Magnesium selenide | Magnesium telluride | |
| Calcium | Calcium oxide | Calcium sulfide | Calcium selenide | Calcium telluride | Calcium polonide |
| Strontium | Strontium oxide | Strontium sulfide | Strontium selenide | Strontium telluride | Strontium polonide |
| Barium | Barium oxide | Barium sulfide | Barium selenide | Barium telluride | Barium polonide |
| Oxides | Sulfides | Selenides | Tellurides | Polonides | |
| Scandium | Scandium monosulfide | ||||
| Yttrium | Yttrium monosulfide | ||||
| Lanthanum | Lanthanum monosulfide | ||||
| Cerium | Cerium monosulfide | Cerium monoselenide | Cerium monotelluride | ||
| Praseodymium | Praseodymium monosulfide | Praseodymium monoselenide | Praseodymium monotelluride | ||
| Neodymium | Neodymium monosulfide | Neodymium monoselenide | Neodymium monotelluride | ||
| Promethium | ? | ? | ? | ? | |
| Samarium | Samarium monosulfide | Samarium monoselenide | Samarium monotelluride | Samarium monopolonide | |
| Europium | Europium monoxide | Europium monosulfide | Europium monoselenide | Europium monotelluride | Europium monopolonide |
| Gadolinium | Gadolinium monosulfide | ||||
| Terbium | Terbium monosulfide | Terbium monopolonide | |||
| Dysprosium | Dysprosium monosulfide | Dysprosium monopolonide | |||
| Holmium | Holmium monosulfide | Holmium monopolonide | |||
| Erbium | Erbium monosulfide | ||||
| Thulium | Thulium monosulfide | Thulium monopolonide | |||
| Ytterbium | Ytterbium monoxide | Ytterbium monosulfide | Ytterbium monopolonide | ||
| Lutetium | Lutetium monosulfide | Lutetium monopolonide | |||
| Actinium | ? | ? | ? | ? | |
| Thorium | Thorium monosulfide | Thorium monoselenide | |||
| Protactinium | ? | ? | ? | ? | |
| Uranium | Uranium monosulfide | Uranium monoselenide | Uranium monotelluride | ||
| Neptunium | Neptunium monosulfide | Neptunium monoselenide | Neptunium monotelluride | ||
| Plutonium | Plutonium monosulfide | Plutonium monoselenide | Plutonium monotelluride | ||
| Americium | Americium monosulfide | Americium monoselenide | Americium monotelluride | ||
| Curium | Curium monosulfide | Curium monoselenide | Curium monotelluride |
| Carbides | Nitrides | |
| Titanium | Titanium carbide | Titanium nitride |
| Zirconium | Zirconium carbide | Zirconium nitride |
| Hafnium | Hafnium carbide | Hafnium nitride |
| Vanadium | Vanadium carbide | Vanadium nitride |
| Niobium | Niobium carbide | Niobium nitride |
| Tantalum | Tantalum carbide | |
| Chromium | Chromium nitride |
Many transition metal monoxides also have the rock salt structure. The early actinoid monocarbides also have this structure.