  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.
