Antifluorite

Antifluorite is a crystal-structure type representing the inverse of the fluorite structure. In the fluorite structure (CaF2), calcium ions form a cubic lattice and fluoride ions occupy all tetrahedral voids. In antifluorite, the arrangement is reversed: the anion sublattice forms a face-centered cubic (FCC) lattice, and the cations occupy the tetrahedral holes. The general formula for antifluorite-type compounds is A2X, where A is a 1+ cation and X is a 2− anion; the archetype is Li2O, with oxide anions in the FCC lattice and lithium ions in the tetrahedral sites.

Coordination and symmetry: In antifluorite, each X2− anion is eight-coordinated by A+, while each A+ ion is four-coordinated by X2−. The structure is cubic and is typically described by the space group Fm-3m.

Examples and occurrence: The best-known antifluorite compound is lithium oxide, Li2O. Related compounds with the same structure include lithium sulfide Li2S and lithium selenide Li2Se, among others, when adopting the antifluorite arrangement. The antifluorite structure is common for A2X compositions where a small, highly charged cation occupies tetrahedral holes in an FCC anion lattice.