Phase structures and stability in barium titanate ferroelectric ultrathin films

Under taking into account both stress and surface effects, an extended Landau–Devonshire theory is developed to investigate the phase structures and ferroelectric stability in barium titanate ultrathin films. The phase diagrams of temperature-stress, film thickness-stress and polarization-film thickness are calculated. For the surface polarization smaller than the bulk, the paraelectric phase starts at temperature below the bulk Curie temperature under small stresses and the ferroelectric phases can be stabilized in the ultrathin films under large stresses. For the surface polarization larger than the bulk, the paraelectric phase appears at temperature above the bulk Curie temperature and the ferroelectricity can be sustained in the ultrathin films by the “superpolarized” surface layer.