{{Short description|Motif in solid state chemistry}} The '''fluorite structure''' refers to a common motif for compounds with the formula MX<sub>2</sub>.<ref>{{Holleman&Wiberg}}</ref><ref>{{Greenwood&Earnshaw2nd}}</ref> The X ions occupy the eight tetrahedral interstitial sites whereas M ions occupy the regular sites of a face-centered cubic (FCC) structure. Many compounds, notably the common mineral fluorite (CaF<sub>2</sub>), adopt this structure.

Many compounds with formula M<sub>2</sub>X have an '''antifluorite structure'''. In these the locations of the anions and cations are reversed relative to fluorite (an anti-structure); the anions occupy the FCC regular sites whereas the cations occupy the tetrahedral interstitial sites. For example, magnesium silicide, Mg<sub>2</sub>Si, has a lattice parameter of 6.338 Å with magnesium cations occupying the tetrahedral interstitial sites, in which each silicide anion is surrounded by eight magnesium cations and each magnesium cation is surrounded by four silicide anions in a tetrahedral fashion.<ref name="Costel">{{cite book|url=https://shutterwaves.com/product/structure-of-crystalline-solids/|title=Structure of Crystalline Solids, Imperfections and Defects in Crystals|last1=Rizescu|first1=Costel|last2=Rizescu|first2=Mihaela|publisher=Shutter Waves|year=2018|isbn=978-1-947641-17-4|edition=First|location=Parker, TX|access-date=29 January 2020}}</ref> <gallery widths="230px" heights="230px"> File:Fluorite Structure.jpg|The fluorite structure of calcium fluoride CaF<sub>2</sub>.<ref name="Costel"/> File:Antifluorite Structure.jpg|The antifluorite structure of magnesium silicide Mg<sub>2</sub>Si.<ref name="Costel"/> </gallery> {| class="wikitable" |+Lattice constants for fluorite and antifluorite materials at 300&nbsp;K<ref name="Costel" /> ! Material !! Lattice constant (Å) !! Crystal structure |- |BaF<sub>2</sub>||6.196|| Fluorite (FCC) |- |{{nowrap|1=''β''-}}PbF<sub>2</sub>||5.94|| Fluorite (FCC) |- |PuO<sub>2</sub>||5.399|| Fluorite (FCC) |- |SrF<sub>2</sub>||5.7996|| Fluorite (FCC) |- |UO<sub>2</sub>||5.47065|| Fluorite (FCC) |- |CaF<sub>2</sub>||5.463|| Fluorite (FCC) |- |ZrO<sub>2</sub>||5.14|| Fluorite (FCC) |- |K<sub>2</sub>O||6.449|| Antifluorite (FCC) |- |K<sub>2</sub>S||7.406|| Antifluorite (FCC) |- |Li<sub>2</sub>O||4.61|| Antifluorite (FCC) |- |Na<sub>2</sub>O||5.55|| Antifluorite (FCC) |- |Na<sub>2</sub>S||6.54|| Antifluorite (FCC) |- |Rb<sub>2</sub>O||6.74|| Antifluorite (FCC) |- |Mg<sub>2</sub>Si||6.338|| Antifluorite (FCC) |}

== 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 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 a 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,<ref>{{cite web|title=OlexSys |url=https://www.olexsys.org/about/|website=OlexSys}}</ref> one can solve a crystal structure from crystallographic output files.

<gallery widths="300px" heights="300px"> File:calciumfluoritecrystal.jpg|Extended crystal structure of calcium fluoride showing octahedral crystal. </gallery>

=== 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 ratio 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.

<gallery widths="250px" heights="250px"> 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. </gallery>

=== 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).<ref>{{cite book |last1=Shriver |display-authors=etal |title=Inorganic Chemistry |date=January 2014 |publisher=Oxford University Press |isbn=978-1-4292-9906-0 |edition=Sixth}}</ref> 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.<ref>{{cite web |last1=Redwing |first1=Ronald |title=Face Centered Cubic Structure (FCC) |url=https://www.e-education.psu.edu/matse81/node/2133 |website=The Pennsylvania State University}}</ref>

<gallery widths="250px" heights="250px"> 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. </gallery>

==See also== * Rock-salt structure

==References== {{reflist}}

{{Authority control}}

Category:Cubic minerals Category:Minerals in space group 225 Category:Fluorite Category:Fluorine minerals Category:Crystal structure types *