Crystallographic system where the unit cell is in the shape of a cube
A rock containing three crystals ofpyrite (FeS2). The crystal structure of pyrite is primitive cubic, and this is reflected in the cubic symmetry of its naturalcrystal facets.A network model of a primitive cubic systemThe primitive and cubic close-packed (also known as face-centered cubic) unit cells
Primitive cubic (abbreviatedcP and alternatively calledsimple cubic)
Body-centered cubic (abbreviatedcI orbcc)
Face-centered cubic (abbreviatedcF orfcc)
Note: the termfcc is often used in synonym for thecubic close-packed orccp structure occurring in metals. However,fcc stands for a face-centered cubic Bravais lattice, which is not necessarily close-packed when a motif is set onto the lattice points. E.g. the diamond and the zincblende lattices arefcc but not close-packed.Each is subdivided into other variants listed below. Although theunit cells in these crystals are conventionally taken to be cubes, theprimitive unit cells often are not.
The primitive cubic lattice (cP) consists of onelattice 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 (1⁄8 × 8).[1]
The body-centered cubic lattice (cI) 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 (1⁄8 × 8 + 1).[1]
The face-centered cubic lattice (cF) has lattice points on the faces of the cube, that each gives exactly one half contribution, in addition to the corner lattice points, giving a total of four lattice points per unit cell (1⁄8 × 8 from the corners plus1⁄2 × 6 from the faces).
The face-centered cubic lattice is closely related to thehexagonal close packed (hcp) system, where two systems differ only in the relative placements of their hexagonal layers. The[111] plane of a face-centered cubic lattice is a hexagonal grid.
Attempting to create a base-centered cubic lattice (i.e., putting an extra lattice point in the center of each horizontal face) results in a simpletetragonalBravais lattice.
Coordination number (CN) is the number of nearest neighbors of a central atom in the structure.[1] Each sphere in a cP lattice has coordination number 6, in a cI lattice 8, and in a cF lattice 12.
Atomic packing factor (APF) 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.
As a rule, since atoms in a solid attract each other, the more tightly packed arrangements of atoms tend to be more common. (Loosely packed arrangements do occur, though, for example if theorbital hybridization demands certainbond angles.) Accordingly, the primitive cubic structure, with especially low atomic packing factor, is rare in nature, but is found inpolonium.[4][5] Thebcc andfcc, with their higher densities, are both quite common in nature. Examples ofbcc includeiron,chromium,tungsten, andniobium. Examples offcc includealuminium,copper,gold andsilver.
Another important cubic crystal structure is thediamond cubic structure, which can appear incarbon,silicon,germanium, andtin. Unlike fcc and bcc, this structure is not a lattice, since it contains multiple atoms in itsprimitive cell. Other cubic elemental structures include theA15 structure found intungsten, and the extremely complicated structure ofmanganese.
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 theinterstitial sites of the others.
Acaesium chloride 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.[6]
This graphic shows the interlocking simple cubic lattices of cesium and chlorine. You can see them separately and as they are interlocked in what looks like a body-centered cubic arrangement
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 thecoordination 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 itscubic void.
In addition to caesium chloride itself, the structure also appears in certain otheralkali halides when prepared at low temperatures or high pressures.[7] 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).
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 hasoctahedralcoordination: Each atom's nearest neighbors consist of six atoms of the opposite type, positioned like the six vertices of aregular 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 eachoctahedral void or vice versa.[6]
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".[7] According to theradius ratio rule, this structure is more likely to be formed if the cation is somewhat smaller than the anion (a cation/anion radius ratio of 0.414 to 0.732).
The interatomic distance (distance between cation and anion, or half the unit cell lengtha) in some rock-salt-structure crystals are: 2.3 Å (2.3 × 10−10 m) for NaF,[13] 2.8 Å for NaCl,[14] and 3.2 Å for SnTe.[15] Most of thealkali metalhydrides andhalides have the rock salt structure, though a few have thecaesium chloride structure instead.
Much like the rock salt structure, thefluorite structure (AB2) is also an Fm3m structure but has 1:2 ratio of ions. The anti-fluorite structure is nearly identical, except the positions of the anions and cations are switched in the structure. They are designatedWyckoff positions 4a and 8c whereas the rock-salt structure positions are 4a and 4b.[48][49]
The Zincblende structure (also written "zinc blende") is named after the mineral zincblende (sphalerite), one form ofzinc 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 hastetrahedralcoordination: Each atom's nearest neighbors consist of four atoms of the opposite type, positioned like the four vertices of aregular tetrahedron. In zinc sulfide the ratio of zinc to sulfur is 1:1.[6] Altogether, the arrangement of atoms in zincblende structure is the same asdiamond cubic structure, but with alternating types of atoms at the different lattice sites. The structure can also be described as an FCC lattice of zinc with sulfur atoms occupying half of thetetrahedral voids or vice versa.[6]
The Heusler structure, based on the structure of Cu2MnAl, is a common structure forternary compounds involvingtransition metals. It has the space group Fm3m (No. 225), and theStrukturbericht designation is L21. Together with the closely related half-Heusler and inverse-Huesler compounds, there are hundreds of examples.
The space group of the iron monosilicide structure is P213 (No. 198), and theStrukturbericht designation is B20. This is achiral structure, and is sometimes associated withhelimagnetic 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 asgallium palladide.
Transition metal silicides and germanides with the FeSi structure
It has three orientations of stackedtetradecahedrons withpyritohedral cells in the gaps. It is found as acrystal structure inchemistry where it is usually known as a "type Iclathrate structure".Gas hydrates formed by methane, propane, and carbon dioxide at low temperatures have a structure in which water molecules lie at the nodes of the Weaire–Phelan structure and arehydrogen bonded together, and the larger gas molecules are trapped in the polyhedral cages.
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