Ferroelectricity

When the polarity of a non-centrosymmetric polar structure can be reversed one normally finds a set of additional characteristic properties, altogether designated as ferroelectricity. We outline these properties and the systems in which they are materialized with some representative examples. In particular, we delineate the standard case in which ferroelectricity is associated with a phase transition—that is, to some sort of underlying instability—and emphasize another class of ferroelectricity that is beyond this paradigm. The former can be described within the Landau theory of phase transitions and its extensions and takes place in proper ferroelectrics (e.g., BaTiO3, PbTiO3, BiFeO3 and HfO2), improper ferroelectrics (e.g., hexagonal manganites such as YMnO3) and multiferroics (e.g., orthorhombic manganites such as HoMnO3 and TbMnO3). The latter, however, is beyond the Landau framework and is found in 2D van der Waals materials for example.