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In
solid state chemistry Solid-state chemistry, also sometimes referred as materials chemistry, is the study of the synthesis, structure, and properties of solid phase materials, particularly, but not necessarily exclusively of, non-molecular solids. It therefore has a str ...
, the fluorite structure refers to a common motif for compounds with the formula MX2. The X ions occupy the eight tetrahedral
interstitial site In crystallography, interstitial sites, holes or voids are the empty space that exists between the packing of atoms (spheres) in the crystal structure. The holes are easy to see if you try to pack circles together; no matter how close you get ...
s whereas M ions occupy the regular sites of a
face-centered cubic 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 ...
(FCC) structure. Many compounds, notably the common mineral
fluorite Fluorite (also called fluorspar) is the mineral form of calcium fluoride, CaF2. It belongs to the halide minerals. It crystallizes in isometric cubic habit, although octahedral and more complex isometric forms are not uncommon. The Mohs sca ...
(CaF2), adopt this structure. Many compounds with formula M2X have an antifluorite structure. In these the locations of the anions and cations are reversed relative to fluorite (an
anti-structure In crystallography, an anti-structure is obtained from a salt structure by exchanging anion and cation positions. For instance, calcium fluoride, CaF2, crystallizes in a cubic motif called the fluorite structure. The same crystal structure is ...
); the anions occupy the FCC regular sites whereas the cations occupy the tetrahedral interstitial sites. For example,
Magnesium silicide Magnesium silicide, Mg2Si, is an inorganic compound consisting of magnesium and silicon. As-grown Mg2Si usually forms black crystals; they are semiconductors with n-type conductivity and have potential applications in thermoelectric generators. C ...
, Mg2Si, has a lattice parameter of 6.338 Å with magnesium cations occupying the tetrahedral interstitial sites, in which each
silicide A silicide is a type of chemical compound that combines silicon and a (usually) more electropositive element. Silicon is more electropositive than carbon. Silicides are structurally closer to borides than to carbides. Similar to borides and carb ...
anion is surrounded by eight magnesium cations and each magnesium cation is surrounded by four silicide anions in a tetrahedral fashion. File:Fluorite Structure.jpg, The fluorite structure of calcium fluoride CaF2. File:Antifluorite Structure.jpg, The antifluorite structure of magnesium silicide Mg2Si.


Calcium fluoride Example

Crystallography is a powerful tool to investigate the structures of crystalline materials. It is important to understand the crystal structure of materials in order to form structure - property relationships. These relationships can help predict the behavior of crystalline materials, as well as introduce the ability to tune their properties. Calcium fluoride is a classic example of a crystal with the fluorite structure. Crystallographic information can be collected via x-ray diffraction, providing information on the locations of electron density within a crystal structure. Using modern software such as Olex2, one can solve a crystal structure from crystallographic output files. File:calciumfluoritecrystal.jpg, Extended crystal structure of calcium fluoride showing octahedral crystal.


Views of calcium fluoride crystal structure

In calcium fluoride, the calcium cations are surrounded by fluorine anions that occupy the tetrahedral sites, with an 8:4 coordination number, fluorine to calcium. This ration is consistent with the stoichiometry of the compound, where the ratio of fluorine to calcium is 2:1. This relationship can be visualized as a cubic array of anions surrounding the calcium cations. File:CaF2_polyhedra2.jpg, Cubic corner sharing visualization of calcium fluoride. Calcium in blue, fluorine in green. File:CaF2_polyhedra3.jpg, Cubic corner sharing visualized down a separate axis.


Extended fluorite structure

Beyond the until cell, the extended crystal structure of fluorite continues packing in a face centered cubic (fcc) packing structure (also known as cubic close packed or ccp). This pattern of spherical packing follows an ABC pattern, where each successive layer of spheres settles on top of the adjacent hole of the lattice. In contrast, hexagonal close-packed (hcp), are successively layered with an ABAB pattern. These two types of packing are the most closely packed forms of spherical packing. File:CaF2_extenda.jpg, Extended crystal stacking structure of calcium fluoride; unit cell expanded by a unit of 3. File:CaF2_pol1.jpg, View of extended packing structure of calcium fluoride down a separate axis; expanded unit cell.


See also

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Rock-salt structure Halite (), commonly known as rock salt, is a type of salt, the mineral (natural) form of sodium chloride ( Na Cl). Halite forms isometric crystals. The mineral is typically colorless or white, but may also be light blue, dark blue, purple, pi ...


References

{{Authority control Cubic minerals Minerals in space group 225 Fluorine minerals Crystal structure types *