Cubic crystal system
In crystallography, the cubic (or isometric) 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:
- Primitive cubic (abbreviated cP and alternatively called simple cubic)
- Body-centered cubic (abbreviated cI or bcc)
- Face-centered cubic (abbreviated cF or fcc)
Note: the term fcc is often used in synonym for the cubic close-packed or ccp 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 are fcc but not close-packed. 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 latticesEdit
Template:Further information 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 | File:Cubic.svg | File:Cubic-body-centered.svg | File:Cubic-face-centered.svg |
The primitive cubic lattice (cP) 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 (Template:Frac × 8).<ref name=IUCnames>Template:Cite journal</ref>
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 (Template:Frac × 8 + 1).<ref name=IUCnames />
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 (Template:Frac × 8 from the corners plus Template:Frac × 6 from the faces).
The face-centered cubic lattice is closely related to the hexagonal 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 simple tetragonal Bravais lattice.
Coordination number (CN) is the number of nearest neighbors of a central atom in the structure.<ref name=IUCnames /> 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.
Crystal classesEdit
Template:Further information The isometric crystal system class names, point groups (in Schönflies notation, Hermann–Mauguin notation, orbifold, and Coxeter notation), type, examples, international tables for crystallography space group number,<ref name="ITC">Template:Cite book</ref> 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 structuresEdit
Template:See also 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 the orbital hybridization demands certain bond angles.) Accordingly, the primitive cubic structure, with especially low atomic packing factor, is rare in nature, but is found in polonium.<ref>Template:Greenwood&Earnshaw</ref><ref>The original discovery was in J. Chem. Phys. 14, 569 (1946).</ref> 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 structuresEdit
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 structureEdit
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.<ref name=":2">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
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.<ref name=Seitz>Seitz, Modern Theory of Solids (1940), p.49</ref> 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 PmTemplate:Overlinem (in Hermann–Mauguin notation), or "221" (in the International Tables for Crystallography). The Strukturbericht designation is "B2".<ref>The CsCl (B2) Structure Template:Webarchive</ref>
There are nearly a hundred rare earth intermetallic compounds that crystallize in the CsCl structure, including many binary compounds of rare earths with magnesium,<ref>Template:Cite journal</ref> and with elements in groups 11, 12,<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> and 13. Other compounds showing caesium chloride like structure are CsBr, CsI, high-temperature RbCl, AlCo, AgZn, BeCu, MgCe, RuAl and SrTl.Template:Citation needed
Rock-salt structureEdit
The space group of the rock-salt or halite (sodium chloride) structure is denoted as FmTemplate:Overlinem (in Hermann–Mauguin notation), or "225" (in the International Tables for Crystallography). The Strukturbericht designation is "B1".<ref>The NaCl (B1) Structure Template:Webarchive</ref>
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.<ref name=":2" />
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".<ref name=Seitz/> According to the radius 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 length a) in some rock-salt-structure crystals are: 2.3 Å (2.3 × 10−10 m) for NaF,<ref>Template:Cite journal</ref> 2.8 Å for NaCl,<ref>Template:Cite journal</ref> and 3.2 Å for SnTe.<ref>Template:Cite journal</ref> Most of the alkali metal hydrides and halides have the rock salt structure, though a few have the caesium chloride structure instead.
| Hydrides | Fluorides | Chlorides | Bromides | Iodides | |
|---|---|---|---|---|---|
| Lithium | Lithium hydride | Lithium fluoride<ref name=Aigs>J. Aigueperse, P. Mollard, D. Devilliers, M. Chemla, R. Faron, R. Romano, J. P. Cuer, "Fluorine Compounds, Inorganic" (section 4) in Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005. {{#invoke:doi|main}}.</ref> | Lithium chloride | Lithium bromide | Lithium iodide |
| Sodium | Sodium hydride | Sodium fluoride<ref name=Aigs/> | Sodium chloride | Sodium bromide | Sodium iodide |
| Potassium | Potassium hydride | Potassium fluoride<ref name=Aigs/> | Potassium chloride | Potassium bromide | Potassium iodide |
| Rubidium | Rubidium hydride | Rubidium fluoride | Rubidium chloride | Rubidium bromide | Rubidium iodide |
| Caesium | Caesium hydride | Caesium fluoride | (CsCl structure) | ||
| Oxides | Sulfides | Selenides | Tellurides | Polonides | |
|---|---|---|---|---|---|
| Magnesium | Magnesium oxide | Magnesium sulfide | Magnesium selenide<ref>Template:Cite journal</ref> | Magnesium telluride<ref>Template:Cite journal</ref> | (NiAs structure) |
| Calcium | Calcium oxide | Calcium sulfide | Calcium selenide<ref>Template:Cite journal</ref> | Calcium telluride | Calcium polonide<ref name=Brown2019>Template:Cite book</ref> |
| Strontium | Strontium oxide | Strontium sulfide | Strontium selenide | Strontium telluride | Strontium polonide<ref name=Brown2019 /> |
| Barium | Barium oxide | Barium sulfide | Barium selenide | Barium telluride | Barium polonide<ref name=Brown2019 /> |
| Oxides | Sulfides | Selenides | Tellurides | Polonides | |
|---|---|---|---|---|---|
| Scandium | (unstable)<ref name=Leger1981>Template:Cite journal</ref> | Scandium monosulfide | |||
| Yttrium | Yttrium monosulfide<ref>Template:Cite journal</ref> | ||||
| Lanthanum | Lanthanum monosulfide<ref name=Didchenko1963>Template:Cite journal</ref> | ||||
| Cerium | Cerium monosulfide<ref name=Didchenko1963 /> | Cerium monoselenide<ref name=Smolensky1968>Template:Cite journal</ref> | Cerium monotelluride<ref name=Smolensky1968 /> | ||
| Praseodymium | Praseodymium monosulfide<ref name=Didchenko1963 /> | Praseodymium monoselenide<ref name=Smolensky1968 /> | Praseodymium monotelluride<ref name=Smolensky1968 /> | ||
| Neodymium | Neodymium monosulfide<ref name=Didchenko1963 /> | Neodymium monoselenide<ref name=Smolensky1968 /> | Neodymium monotelluride<ref name=Smolensky1968 /> | ||
| Promethium | ? | ? | ? | ? | |
| Samarium | Samarium monosulfide<ref name=Didchenko1963 /> | Samarium monoselenide | Samarium monotelluride | Samarium monopolonide<ref name=Kershner1966>Template:Cite journal</ref> | |
| Europium | Europium monoxide | Europium monosulfide<ref name=Didchenko1963 /> | Europium monoselenide<ref name=Wachter1972>Template:Cite journal</ref> | Europium monotelluride<ref name=Wachter1972 /> | Europium monopolonide<ref name=Kershner1966 /> |
| Gadolinium | (unstable)<ref name=Leger1981 /> | Gadolinium monosulfide<ref name=Didchenko1963 /> | |||
| Terbium | Terbium monosulfide<ref name=Didchenko1963 /> | Terbium monopolonide<ref name=Kershner1966 /> | |||
| Dysprosium | Dysprosium monosulfide<ref name=Didchenko1963 /> | Dysprosium monopolonide<ref name=Kershner1966 /> | |||
| Holmium | Holmium monosulfide<ref name=Didchenko1963 /> | Holmium monopolonide<ref name=Kershner1966 /> | |||
| Erbium | Erbium monosulfide<ref name=Didchenko1963 /> | ||||
| Thulium | Thulium monosulfide<ref name=Didchenko1963 /> | Thulium monopolonide<ref name=Kershner1966 /> | |||
| Ytterbium | Ytterbium monoxide | Ytterbium monosulfide<ref name=Didchenko1963 /> | Ytterbium monopolonide<ref name=Kershner1966 /> | ||
| Lutetium | (unstable)<ref name=Leger1981 /><ref>Template:Cite book</ref> | Lutetium monosulfide<ref name=Didchenko1963 /> | Lutetium monopolonide<ref name=Kershner1966 /> | ||
| Actinium | ? | ? | ? | ? | |
| Thorium | Thorium monosulfide<ref name=Kruger1967>Template:Cite journal</ref> | Thorium monoselenide<ref name=Kruger1967 /> | (CsCl structure)<ref>Template:Cite journal</ref> | ||
| Protactinium | ? | ? | ? | ? | |
| Uranium | Uranium monosulfide<ref name=Kruger1967 /> | Uranium monoselenide<ref name=Kruger1967 /> | Uranium monotelluride<ref name=Kruger1967 /> | ||
| Neptunium | Neptunium monosulfide | Neptunium monoselenide | Neptunium monotelluride | ||
| Plutonium | Plutonium monosulfide<ref name=Kruger1967 /> | Plutonium monoselenide<ref name=Kruger1967 /> | Plutonium monotelluride<ref name=Kruger1967 /> | ||
| Americium | Americium monosulfide<ref name=Vogt1995 /> | Americium monoselenide<ref name=Vogt1995 /> | Americium monotelluride<ref name=Vogt1995 /> | ||
| Curium | Curium monosulfide<ref name=Benedict1993 /> | Curium monoselenide<ref name=Benedict1993 /> | Curium monotelluride<ref name=Benedict1993 /> |
| Carbides | Nitrides | |
|---|---|---|
| Titanium | Titanium carbide | Titanium nitride |
| Zirconium | Zirconium carbide | Zirconium nitride |
| Hafnium | Hafnium carbide | Hafnium nitride<ref>Template:Cite journal</ref> |
| Vanadium | Vanadium carbide | Vanadium nitride |
| Niobium | Niobium carbide | Niobium nitride |
| Tantalum | Tantalum carbide | (CoSn structure) |
| Chromium | (unstable)<ref>Template:Cite journal</ref> | Chromium nitride |
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).<ref name=Benedict1993 />
Fluorite structureEdit
{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} Template:Category see also Much like the rock salt structure, the fluorite structure (AB2) is also an FmTemplate:Overlinem 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 designated Wyckoff positions 4a and 8c whereas the rock-salt structure positions are 4a and 4b.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Zincblende structureEdit
The space group of the Zincblende structure is called FTemplate:Overline3m (in Hermann–Mauguin notation), or 216.<ref>Template:Cite book</ref><ref>Birkbeck College, University of London</ref> The Strukturbericht designation is "B3".<ref>The Zincblende (B3) Structure. Naval Research Laboratory, U.S. </ref>
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.<ref name=":2" /> Altogether, the arrangement of atoms in zincblende structure is the same as diamond 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 the tetrahedral voids or vice versa.<ref name=":2" />
Examples of compounds with this structure include zincblende itself, lead(II) nitrate, many compound semiconductors (such as gallium arsenide and cadmium telluride), and a wide array of other binary compounds.Template:Citation needed The boron group pnictogenides usually have a zincblende structure, though the nitrides are more common in the wurtzite structure, and their zincblende forms are less well known polymorphs.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
| Fluorides | Chlorides | Bromides | Iodides | |
|---|---|---|---|---|
| Copper | Copper(I) fluoride | Copper(I) chloride | Copper(I) bromide | Copper(I) iodide |
| Sulfides | Selenides | Tellurides | Polonides | |
|---|---|---|---|---|
| Beryllium | Beryllium sulfide | Beryllium selenide | Beryllium telluride | Beryllium polonide<ref>Template:Greenwood&Earnshaw1st.</ref><ref>Template:Cite report.</ref> |
| Zinc | Zinc sulfide | Zinc selenide | Zinc telluride | Zinc polonide |
| Cadmium | Cadmium sulfide | Cadmium selenide | Cadmium telluride | Cadmium polonide |
| Mercury | Mercury sulfide | Mercury selenide | Mercury telluride | – |
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.
| Nitrides | Phosphides | Arsenides | Antimonides | |
|---|---|---|---|---|
| Boron | Boron nitride* | Boron phosphide | Boron arsenide | Boron antimonide |
| Aluminium | Aluminium nitride* | Aluminium phosphide | Aluminium arsenide | Aluminium antimonide |
| Gallium | Gallium nitride* | Gallium phosphide | Gallium arsenide | Gallium antimonide |
| Indium | Indium nitride* | Indium phosphide | Indium arsenide | Indium antimonide |
This group is also known as the III-V family of compounds.
Heusler structureEdit
{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} The Heusler structure, based on the structure of Cu2MnAl, is a common structure for ternary compounds involving transition metals. It has the space group FmTemplate:Overlinem (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 structureEdit
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.
| Silicides | Germanides | |
|---|---|---|
| Manganese | Manganese monosilicide | Manganese germanide |
| Iron | Iron monosilicide | Iron germanide |
| Cobalt | Cobalt monosilicide | Cobalt germanide |
| Chromium | Chromium(IV) silicide | Chromium(IV) germanide |
Weaire–Phelan structureEdit
A Weaire–Phelan structure has PmTemplate:Overlinen (223) symmetry.
It has three orientations of stacked tetradecahedrons with pyritohedral cells in the gaps. It is found as a crystal structure in chemistry where it is usually known as a "type I clathrate 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 are hydrogen bonded together, and the larger gas molecules are trapped in the polyhedral cages.
See alsoEdit
- Atomium: building which is a model of a bcc unit cell, with vertical body diagonal.
- Close-packing
- Dislocations
- Reciprocal lattice
ReferencesEdit
Further readingEdit
- Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, Template:Isbn
External linksEdit
- JMol simulations by Graz University:
- Making crystal structure with Molview