A method to study ageing of polydomain ferroelectrics using measurements   of nonlinear permittivity
  It is known that the permittivity of the ferroelectric films is affected by several phenomena, which deteriorate the material quality (e.g. the redistribution of the crystal lattice defects, appearance of the electrode-adjacent non-ferroelectric layers or the spontaneous polarization screening due to a free charge injection across the electrode-adjacent layer, etc.). It is also known that the permittivity of ferroelectric polydomain films is controlled by the sum of two contributions: the crystal lattice (intrinsic) contribution and the domain wall movement (extrinsic) contribution. It is the latter one, which is very sensitive to the aforementioned phenomena and which plays a key role in the deterioration of the dielectric response of the ferroelectric polydomain films. In this Article, there is presented a method for the identification of the process, which is responsible for the ferroelectric ageing. The method is based on the analysis of the evolution of both the linear and nonlinear permittivity during ageing. Applicability of the method is theoretically demonstrated on four ageing scenarios in two qualitatively different systems where the evolution of the nonlinear permittivity is controlled, first, by a redistribution of the pinning centers on the domain wall and, second, by microstructural changes at the interface between the ferroelectric layer and the electrode. It is shown that each ageing scenario is characterized by unique trend in the evolution between the linear and nonlinear part of the permittivity, which can be verified experimentally. 
Pavel Mokry