Compounds that consist of more than one element (e.g.
binary compounds) 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 unit cell. The two colors of spheres represent the two types of atoms. 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 [111] direction. In the caesium chloride structure, translation along the [111] 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 (for example, ionic radius of Cs+ = 167 pm, and Cl− = 181 pm). The
space group of the
caesium chloride (CsCl) structure is called Pmm (in
Hermann–Mauguin notation), or "221" (in the International Tables for Crystallography). 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 (sodium chloride) structure is denoted as Fmm (in
Hermann–Mauguin notation), or "225" (in the International Tables for Crystallography). 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. 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. Many
transition metal monoxides also have the rock salt structure (
TiO,
VO,
CrO,
MnO,
FeO,
CoO,
NiO,
CdO). The early actinoid monocarbides also have this structure (
ThC,
PaC,
UC,
NpC,
PuC).
Zincblende structure The
space group of the Zincblende structure is called F3m (in
Hermann–Mauguin notation), or 216. The Strukturbericht designation is "B3". The Zincblende structure (also written "zinc blende") is named after the mineral zincblende (
sphalerite), one form of
zinc sulfide (β-ZnS). As in the rock-salt structure, the two atom types form two interpenetrating face-centered cubic lattices. However, it differs from rock-salt structure in how the two lattices are positioned relative to one another. The zincblende structure has
tetrahedral coordination: Each atom's nearest neighbors consist of four atoms of the opposite type, positioned like the four vertices of a
regular tetrahedron. In zinc sulfide the ratio of zinc to sulfur is 1:1. This group is also known as the
II-VI family of compounds, most of which can be made in both the zincblende (cubic) or
wurtzite (hexagonal) form. This group is also known as the
III-V family of compounds. s with formula X2YZ (e. g., Co2MnSi).
Heusler structure The Heusler structure, based on the structure of Cu2MnAl, is a common structure for
ternary compounds involving
transition metals. It has the space group Fmm (No. 225), and the
Strukturbericht designation is L21. Together with the closely related half-Heusler and inverse-Huesler compounds, there are hundreds of examples.
Iron monosilicide structure structure. The space group of the iron monosilicide structure is P213 (No. 198), and the
Strukturbericht designation is B20. This is a
chiral structure, and is sometimes associated with
helimagnetic properties. There are four atoms of each element for a total of eight atoms in the unit cell. Examples occur among the transition metal silicides and germanides, as well as a few other compounds such as
gallium palladide. ==Weaire–Phelan structure==