Origin of ferroelectricity in perovskite oxides

FERROELECTRIC materials are characterized by a switchable macroscopic polarization. Most technologically important ferroelectrics are oxides with a perovskite structure. The origin of their ferroelectric behaviour is unclear, however, and there is incomplete understanding of why similar, but chemically different, perovskites should display very different ferroelectric behaviour. The great sensitivity of ferroelectrics to chemistry, defects, electrical boundary conditions and pressure arises from a delicate balance between long-range Coulomb forces (which favour the ferroelectric state) and short-range repulsions (which favour the nonpolar cubic structure). To model the transition accurately, total-energy techniques are required which incorporate the effects of charge distortion and covalency. Here I report results of electronic-structure calculations on two classic examples of ferroelectric perovskites, BaTiO3 and PbTiO3, and demonstrate that hybridization between the titanium 3d states and the oxygen 2p states is essential for ferroelectricity. The different ferroelectric phase behaviour of the two materials is also clear: in PbTiO3, the lead and oxygen states hybridize, leading to a large strain that stabilizes the tetragonal phase, whereas in BaTiO3 the interaction between barium and oxygen is completely ionic, favouring a rhombohedral structure.