E21, perovskite (cubic)

Structural typeE21
Common nameperovskite (cubic)
Pearson symbolcP5
Space group221 Pm-3m Oh1
Chemical formulaABX3
Bondingionic or ionic-covalent
Atomic positions A (1a)   0    0    0   (marked by C)
 B (1b)  1/2  1/2  1/2  (marked by N)
 X (3c)  1/2  1/2   0   (marked by S)
PDB filesE2_1
SubstancesI+V+O (KNbO3, NaNbO3), II+IV+O (CaTiO3, MgSiO3, BaTiO3), III+III+O, PbTiO3, PbZrO3,
Links Schinzer C, Mixed metal perovskites (2000)

In many cases cubic structure is not the ground state in normal conditions, it is high temperature phase. Therefore deformations (4mm, mmm, 3m, mm) are also referred to as perovskite structural type. Here are examples of such distortions: CaTiO3 (Pnma), BaTiO3 (Pm-3m→P4mm→Amm2→R3m). Some perovskites are ferroelectrics (KNbO3, NaTaO3, BaTiO3, PbTiO3) or antiferroelectrics (NaNbO3, PbZrO3).

There are also more complicated deformations like LiNbO3 with paraelectric R-3c to ferroelectric R3c transition.

Note that A and X atoms together form fcc lattice and B atoms being smaller cations fill ¼ of octahedral voids.