Abstract

Ideal AB₆ octahedra are known to form, by sharing all corners (but no edges), three topologically different kinds of three-dimensional periodic nets: perovskite type, hexagonal tungsten bronze type, and pyrochlore type. The geometries of the arrangements with the highest possible symmetry as well as some of the distortions are discussed and electrostatic lattice energies are computed. The electrostatic lattice energies are in part compatible with experience in stereochemistry. Notable exceptions are several trifluorides that do not adopt the perovskite arrangement with the highest possible symmetry but rather rhombohedrically distorted structures. This behaviour is tentatively interpreted to be a consequence of the polarizability of the F⁻and/or the admixture of covalent bonds.