Anti-structure

In crystallography, an anti-structure is obtained from a salt structure by exchanging anion and cation positions.

Unit cell of CaF₂, known as fluorite structure, from two equivalent perspectives.[1] In the antifluorite structure, the blue positions are taken by the anion, the red positions by the cation.

For instance, calcium fluorite, CaF₂, crystallizes in a cubic motif called the fluorite structure. The same crystal structure is found in numerous ionic compounds with formula AB₂, such as ceria (CeO₂), zirconia (cubic ZrO₂), uranium dioxide (UO₂). In the corresponding anti-structure, called the antifluorite structure, anions and cations are swapped, such as beryllium carbide (Be₂C) or lithium oxide (Li₂O), potassium sulfate (K₂SO₄).

Other anti-structures include:[2]

• anti-SnO₂: Ti₂N
• anti-PbCl₂: Co₂P
• anti-CdCl₂: Co₂N
• anti-SdI₂: Cs₂O
• anti-NbS₂: Hf₂S
• anti-ReO₃: Cu₃N
• anti-LaF₃:[3] Cu₃P, Cu₃As