Quantitative study of low-frequency hysteresis loops of polarized polycrystalline barium titanate

Ferroelectrics belong to that group of dielectrics which are said to be nonlinear, but a nonlinear dielectric is not necessarily ferroelectric. The statement that a dielectric is nonlinear requires rather careful consideration.1 If the temperature of a ferroelectric ceramic and also the external pressure on it are maintained constant, and if an electric field is applied which is below the saturation level, then the electric displacement D, or equivalently the polarization P, depends not only on the magnitude of the applied field but also on the past history and state of strain of the material. Linear dielectrics are characterized by the unique function D(¿) = ¿(¿)E(¿), which does not hold true for nonlinear dielectrics. This implies that when one is dealing with a nonlinear dielectric: (1) frequencies appear in P(t), or in D(t), which are not present in E(t); and/or (2) if E1(t) produces D1(t) and E2 produces D2(t), then A1E1(t) + A2E2(t) does not produce A1D1(t) + A2D2(t).