Thermodynamic Theory

The classic work of Devonshire [1, 2] in developing the phenomenological theory of barium titanate represented a significant advance in the study of the physical properties of piezoelectric crystals and ceramics based on the perovskite-structured ferroelectric oxides. Although various theories of ferroelectricity in BaTiO₃ had been put forward by other workers during the period, the phenomenological theory as formulated by Devonshire provided the first unified description of its changes in crystal form and of the variations of the thermal, dielectric, and piezoelectric properties observed on cooling through the sequence of cubic → tetragonal → orthorhombic → rhombohedral ferroelectric phase transitions. By taking account of the multicomponent nature of the ferroelectric polarization in the perovskite oxides, the Devonshire theory allowed the full matrix of tensor components describing the properties of the monodomain single-crystalline state to be assembled, even within the constraints of a comparatively limited set of available experimental data. The influence of the Devonshire theory on contemporary workers in the field is evident in the early investigations of Slater [3] into the origins of ferroelectricity in BaTiO₃, in that of Kittel [4] who predicted the possibility of antiferroelectricity in crystals of the perovskite type before the effect had been experimentally verified, and in that of Cross [5] in his treatment of ferroelectricity and antiferroelectricity in NaNbO₃-KNbO₃ and related perovskite systems.