Читать книгу Solid State Chemistry and its Applications - Anthony R. West - Страница 59

BaTiO₃ is tetragonal at room temperature, a = 3.995, c = 4.034 Å, with the structure shown in projection on the ac plane in Fig. 1.41(h). Since Ti is slightly too small for its octahedral site, it displaces by about 6% of the Ti–O distance towards one of the corner oxygens; Ba²⁺ ions also undergo a smaller displacement in the same direction. This reduces the coordination of Ti to five (square pyramidal) and, to have reasonable Ti–O bond lengths, the structure also contracts slightly in the ab plane [not shown in (h)].

Ti atoms in adjacent unit cells undergo a similar displacement in the same direction and the resulting structure has a large dipole moment due to the separation of positive and negative charge centres. It is possible to flip the orientation of the dipoles: under the action of an applied electric field, the Ti atoms move through the centre of the octahedral site towards one of the other corner oxygens. This ready reversibility gives the structure high polarisability and a high permittivity (or dielectric constant) and is responsible for the property of ferroelectricity (see Section 8.7).