In crystallography, an anti-structure is obtained from a salt structure by exchanging anion and cation positions.
For instance, calcium fluoride, CaF<sub>2</sub>, crystallizes in a cubic motif called the fluorite structure. The same crystal structure is found in numerous ionic compounds with formula AB<sub>2</sub>, such as ceria (CeO<sub>2</sub>), zirconia (cubic ZrO<sub>2</sub>), uranium dioxide (UO<sub>2</sub>). In the corresponding anti-structure, called the antifluorite structure, anions and cations are swapped, such as beryllium carbide (Be<sub>2</sub>C) or lithium oxide (Li<sub>2</sub>O), potassium sulfate (K<sub>2</sub>SO<sub>4</sub>).
Other anti-structures include: