Compounds with a `stuffed' anti‐bixbyite‐type structure, analysed in terms of the Zintl–Klemm and coordination‐defect concepts

The bixbyite structure (Mn 2 O 3 ) () is often described as a distorted face‐centered cubic (f.c.c.) array of Mn atoms, with O atoms occupying 3/4 of the tetrahedral holes. The empty M 4 tetrahedra are centred at 16 c . In anti‐bixbyite structures (Mg 3 N 2 ), cation vacancies are centred in empty N 4 tetrahedra. If 16 hypothetical atoms were located at this site they would form the structure of γ‐Si. This means that anti‐bixbyite structures are ideally prepared to accommodate Si(Ge) atoms at these holes. Several compounds (Li 3 AlN 2 and Li 3 ScN 2 ) fully satisfy this expectation. They are really anti‐bixbyites `stuffed' with Al(Sc). The presence of these atoms in 16 c is illuminated in the light of the extended Zintl–Klemm concept (EZKC) [Vegas & García‐Baonza (2007). Acta Cryst. B 63 , 339–345], from which a compound would be the result of `multiple resonance' pseudo‐structures, emerging from electron transfers between any species pair (like or unlike atoms, cations or anions). The coordination‐defect (CD) concept [Bevan & Martin (2008). J. Solid State Chem. 181 , 2250–2259] is also consistent with the EZKC description of the pseudo‐structures. A more profound insight into crystal structures is gained if one is not restricted to the contemplation of classical anions and cations in their conventional oxidation states.