Abstract
We explore the role of flexoelectric effect in functional properties of nanoscale ferroelectric films with mixed electronic-ionic conductivity. Using a coupled Ginzburg-Landau model, we calculate spontaneous polarization, effective piezoresponse, elastic strain and compliance, carrier concentration, and piezoconductance as a function of thickness and applied pressure. In the absence of flexoelectric coupling, the studied physical quantities manifest well-explored size-induced phase transitions, including transition to paraelectric phase below critical thickness. Similarly, in the absence of external pressure flexoelectric coupling affects properties of these films only weakly. However, the combined effect of flexoelectric coupling and external pressure induces polarizations at the film surfaces, which cause the electric built-in field that destroys the thickness-induced phase transition to paraelectric phase and induces the electretlike state with irreversible spontaneous polarization below critical thickness. Interestingly, the built-in field leads to noticeable increase of the average strain and elastic compliance in this thickness range. We further illustrate that the changes of the electron concentration by several orders of magnitude under positive or negative pressures can lead to the occurrence of high- or low-conductivity states, i.e., the nonvolatile piezoresistive switching, in which the swing can be controlled by the film thickness and flexoelectric coupling. The obtained theoretical results can be of fundamental interest for ferroic systems, and can provide a theoretical model for explanation of a set of recent experimental results on resistive switching and transient polar states in these systems.
