Surface reconstruction and ferroelectricity in PbTiO3 thin films -: art. no. 014110

Surface and ferroelectric properties of PbTiO3 thin films are investigated using an interatomic potential approach with parameters determined from first-principles calculations. We show that a model developed for the bulk describes properly the surface properties of PbTiO3. In particular, the antiferrodistortive surface reconstruction, recently observed with x-ray scattering, is correctly reproduced as a result of the change in the balance of long-range Coulombic and short-range interactions at the surface. The effects of the surface reconstruction on the ferroelectric properties of ultrathin films are investigated. Under the imposed open-circuit electrical boundary conditions, the model gives a critical thickness for the ferroelectricity of four unit cells. The surface layer, which forms the antiferrodistortive reconstruction, participates in the ferroelectricity. A decrease in the tetragonality of the films leads to the stabilization of a phase with nonvanishing in-plane polarization. A peculiar effect of the surface reconstruction on the in-plane polarization profile is found.