Appearance of ferroelectricity in thin films of incipient ferroelectric

TL;DR

This paper uses phenomenological theory to show that thin films of incipient ferroelectrics like KTaO3 can exhibit ferroelectricity due to size effects, surface stress, and strain, despite being non-ferroelectric in bulk.

Contribution

It introduces a generalized surface free energy model including surface stress and piezoelectric effects, deriving expressions for thickness-dependent transition temperatures in incipient ferroelectric thin films.

Findings

Ferroelectricity appears in thin films of incipient ferroelectrics due to surface effects.

Thickness-induced ferroelectricity is predicted for KTaO3 at room temperature.

Surface effects significantly influence phase transitions in thin films of incipient ferroelectrics.

Abstract

The consideration of size-induced ferroelectric-paraelectric phase transition for conventional and incipient ferroelectrics thin films with perovskite structure was carried out in phenomenological theory framework of Ginsburg-Landau-Devonshire. The more general form of surface free energy expansion that includes intrinsic surface stress tensor, surface piezoelectric effect and electrostriction as well as quadratic and quartic powers of surface polarization has been considered. The analytical expressions for thickness dependence of transition temperature was derived both for the conventional and incipient ferroelectrics. It was shown that although there is no ferroelectricity in the bulk incipient ferroelectrics it appears in thin film for the negative extrapolation length that is realized e.g. at positive surface stress coefficient and negative or zero misfit strain. In our consideration we came to the conclusion about thickness induced ferroelectricity in incipient ferroelectrics KTaO3 at room temperature for the thin enough films. The similar surface effects can influence strongly on the phase transitions in the thin films of another incipient ferroelectrics, SrTiO3.