Origin of the vanishing critical thickness for ferroelectricity in free-standing PbTiO3 ultrathin films from first principles

Theoretical investigations in earlier literatures have shown that there is no critical thickness for ferroelectricity existing in the free-standing PbTiO3 ultrathin films. In this work, we report the origin of the vanishing critical thickness by comparing the properties of PbTiO3 and BaTiO3 ultrathin films based on ab initio investigations. Attributing to the covalency of the Pb-O bond, spontaneous strain gradient appears in the PbTiO3 ultrathin films, which breaks the inversion symmetry and makes the ferroelectric polarizations be sustained. (C) 2013 AIP Publishing LLC.