Читать книгу Solid State Chemistry and its Applications - Anthony R. West - Страница 39

A general guideline is that tetrahedra are smaller than octahedra which, in turn, are smaller than polyhedra of higher coordination number; for a given anion array, the relative sizes of sites may be obtained quantitatively by geometric calculations (see the next section). In ionic structures, cations M occupy those sites or polyhedra of most suitable size. Cations may occupy sites which, at first sight, appear too small, by pushing apart anions and expanding the structure (as in eutactic structures). By contrast, cations tend not to occupy sites that are too large, unless the structure can adjust itself, by twisting or distorting the anion array so as to reduce the size of the sites; this happens in many distorted perovskite structures in which cations are too small to occupy large 12‐coordinate sites and a partial structural collapse occurs (see Section 1.17.7). Thus, cations do not occupy large sites in which they can ‘rattle’. An interesting intermediate situation occurs in some structures in which a cation site is marginally too large. The cation can then undergo small off‐centre displacements giving rise to high polarisability, high permittivity and the phenomenon of ferroelectricity (Section 8.7).