Mechanisms of extrinsic and intrinsic switching phenomena in ferroelectrics are explained and existing models are summarized. Then, criteria for an experimental distinction between both models are elaborated. Samples with thicknesses ranging from 2.7 to 63.8 nm prepared by a Langmuir–Blodgett technique were investigated with respect to these criteria. Measurements of their polarization switching behaviour, their polarization hysteresis loops, and their coercive fields were carried out. It is found that the coercive fields increase with decreasing sample thickness. Also, the switching time increases with decreasing sample thickness and it increases with decreasing field strength. The switching process turns out to be thermally activated. We find that neither intrinsic nor extrinsic models are sufficient to describe the experimental situation.

Thin films of ion-conducting polyethylene oxide ͑PEO͒ are prepared as a model system for solid polymer electrolytes. Plane-parallel capacitance structures of the type Al-PEO-Al ͑Metal-Insulator-Metal, MIM͒ with blocking electrodes are produced. In the short time range t ഛ 1 s after the application of an external field, an ionic space-charge polarization with an exponential time dependence prevails. In the long time range the samples exhibit a Kohlrausch behavior of the polarization current, i.e., j ϰ t −␣ . This time dependence can be explained by image charges in the electrodes which attract the mobile ions. This attraction of ions is responsible for the static and the dynamic behavior of the system. The remanent polarization appearing in hysteresis measurements is an affirmation of the presence of these image charges. Computer simulations of a three-dimensional hopping model yield qualitatively the same results. Here the attraction of ions at the electrodes due to the image charges can be observed, as well as the experimentally determined t −␣ currents and the hysteresis of the polarization. This behavior cannot be simulated with a standard continuum model.

This paper reports first principle calculations and analysis of the molecular mechanism of the polarization switching in polyvinylidene fluoride and its copolymer with trifluoroethylene (P(VDF–TrFE)) using semi-empirical and ab initio quantum chemical methods based on the HyperChem 7.5 and Gaussian98 programs. The simulations were performed for different copolymer contents in P(VDF–TrFE)—(70:30), (60:40) and pure PVDF. The calculated values of the dipole moment and average polarization of the molecular chains show a clear hysteresis under varying electric field with polarization saturated at ∼0.1–0.14 C m−2. The calculated coercive fields (corresponding to the rotation of molecular chains to opposite orientation) are consistent (within an order of magnitude) with experimental data obtained for thin films (Ec = 5–18 MV cm−1). In the absence of external electric fields, the interactions between several molecular chains lead to the orientation of all dipole moments along one direction parallel to the chain plane. This model corresponds to the PVDF layer on the dielectric surface. For the electric field in the perpendicular direction, all chains are rotated along this direction corresponding to the model of conductive substrate.

In addition to our previous work, we report on further experimental results concerning Langmuir–Blodgett polyvinylidenefluoride-copolymer thin films. Polarization hysteresis loops and polarization switching behaviour of samples with thicknesses below 3 nm were measured. Also, the time and temperature comportments of the remanent polarization were investigated for various film thicknesses, ranging from 2.7 up to 63.8 nm. The remanent polarization shows a loss of about 10% over 3 decades of time, ranging from 100 to 2 · 105 s. For thinner films, the remanent polarization exhibits the typical behaviour of a first-order transition, whereas for thicker samples a diffuse transition is found. Initial polarization curves obtained from the unpolarized state were finally measured. They are temperature and thickness dependent.

Dynamic Monte-Carlo simulations of ionic space charge relaxations are carried out using a three-dimensional model for thermally activated ion hopping in a multiwell energy structure. In this model a solid polymer electrolyte is embedded between two ideal blocking electrodes. The polymer is subdivided into 100ϫ100ϫ100 lattice cells. Positive ions ͑typically 1000͒ are distributed on the cells. To provide charge neutrality a negative background charge, constant in space and time, is introduced. The positive ions are able to hop between neighboring cells, surmounting energy barriers of distributed heights. The barrier heights consist of an intrinsic part due to the polymer structure, a part due to the Coulomb interaction of the ions, and a part due to the externally applied field. To calculate the interaction between the ions and the electrodes a method of images is used. Periodical boundary conditions are used for those lattice surfaces which are not in contact with the electrodes. The ionic space charge polarization process is simulated as dependent on the time, the sample thickness, the ion concentration, and the externally applied voltage. The polarization current after a step of the electric field shows dispersion due to distributed energy barrier heights in the short time range and a Kohlrausch behavior due to image charges in the long time range.