  Using Landau-Ginzburg-Devonshire theory, we considered the impact of the
flexoelectro-chemical coupling on the size effects inpolar properties and phase
transitions of thin ferroelectric films with a layer of elastic defects. We
investigated a typical case, when defects fill a thin layer below the top film
surface with a constant concentration creating an additional gradient of
elastic fields. The defective surface of the film is not covered with an
electrode, but instead with an ultra-thin layer of ambient screening charges,
characterized by a surface screening length. This geometry is typical for the
scanning probe piezoelectric force microscopy. Obtained results revealed an
unexpectedly strong effect of the joint action of Vegard stresses and
flexoelectric effect (shortly flexo-chemical coupling) on the ferroelectric
transition temperature, distribution of the spontaneous polarization and
elastic fields, domain wall structure and period in thin PbTiO3 films
containing a layer of elastic defects. A nontrivial result is the
ferroelectricity persisting at film thicknesses below 4 nm, temperatures lower
than 350 K and relatively high surface screening length (~0.1 nm). The origin
of this phenomenon is the re-building of the domain structure in the film
(namely the cross-over from c-domain stripes to a-type closure domains) when
its thickness decreases below 4 nm, conditioned by the flexoelectric coupling
and facilitated by negative Vegard effect. For positive Vegard effect, thicker
films exhibit the appearance of pronounced maxima on the thickness dependence
of the transition temperature, whose position and height can be controlled by
the defect type and concentration. The revealed features may have important
implications for miniaturization of ferroelectric-based devices.
