Phase diagram of ferroelectric BaTiO3 ultrathin films under open-circuit conditions

Abstract

Using a first-principles-based Monte Carlo approach, we investigate the strain–temperature phase diagramof BaTiO3 ultrathin film without any surface charge screening. Although the depolarizing field haslittle effect on the phase types, a unique stripe domain structure with periodic out-of-planepolarization forms in the ultrathin films to minimize the energy of the depolarizing fieldunder the constraint of the boundary condition, which leads to an unusual characteristic ofthe phase diagram. As a result, the total out-of-plane polarization remains zero at anystrain and temperature. The film thickness has obvious effects on the phase diagram,e.g. the Curie temperature decreases and the strain moves to more negative valueswhen the film thickness decreases. It is found that the compressive strain plays adecisive role in the characterization of ferroelectric polarization in such films, whichsuppresses the in-plane polarization while supporting the out-of-plane polarization, andthus drives the polarization to rotate from the in-plane to the normal direction.