Fluorite structure

In solid state chemistry, the fluorite structure refers to a common motif for compounds with the formula MX₂.^[1]^[2] 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 (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); the anions occupy the FCC regular sites whereas the cations occupy the tetrahedral interstitial sites. For example, magnesium silicide, Mg₂Si, 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.^[3]

The fluorite structure of calcium fluoride CaF₂.^[3]
The antifluorite structure of magnesium silicide Mg₂Si.^[3]

Lattice constants for fluorite and antifluorite materials at 300 K^[3]
Material	Lattice constant (Å)	Crystal structure
BaF₂	6.196	Fluorite (FCC)
β-PbF₂	5.94	Fluorite (FCC)
PuO₂	5.399	Fluorite (FCC)
SrF₂	5.7996	Fluorite (FCC)
UO₂	5.47065	Fluorite (FCC)
CaF₂	5.463	Fluorite (FCC)
ZrO₂	5.14	Fluorite (FCC)
K₂O	6.449	Antifluorite (FCC)
K₂S	7.406	Antifluorite (FCC)
Li₂O	4.61	Antifluorite (FCC)
Na₂O	5.55	Antifluorite (FCC)
Na₂S	6.54	Antifluorite (FCC)
Rb₂O	6.74	Antifluorite (FCC)
Mg₂Si	6.338	Antifluorite (FCC)

^ Holleman, Arnold Frederik; Wiberg, Egon (2001), Wiberg, Nils (ed.), Inorganic Chemistry, translated by Eagleson, Mary; Brewer, William, San Diego/Berlin: Academic Press/De Gruyter, ISBN 0-12-352651-5
^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
^ ^a ^b ^c ^d Rizescu, Costel; Rizescu, Mihaela (2018). Structure of Crystalline Solids, Imperfections and Defects in Crystals (First ed.). Parker, TX: Shutter Waves. ISBN 978-1-947641-17-4. Retrieved 29 January 2020.

[1] Holleman, Arnold Frederik; Wiberg, Egon (2001), Wiberg, Nils (ed.), Inorganic Chemistry, translated by Eagleson, Mary; Brewer, William, San Diego/Berlin: Academic Press/De Gruyter, ISBN 0-12-352651-5

[2] Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.

[Costel-3] Rizescu, Costel; Rizescu, Mihaela (2018). Structure of Crystalline Solids, Imperfections and Defects in Crystals (First ed.). Parker, TX: Shutter Waves. ISBN 978-1-947641-17-4. Retrieved 29 January 2020.

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