Ferroelectricity and tetragonality in ultrathin PbTiO3 films

The evolution of tetragonality with thickness has been probed in epitaxial c-axis oriented PbTiO3 films with thicknesses ranging from 500 down to 24 Angstroms. High resolution x-ray pointed out a systematic decrease of the c-axis lattice parameter with decreasing film thickness below 200 Angstroms. Using a first-principles model Hamiltonian approach, the decrease in tetragonality is related to a reduction of the polarization attributed to the presence of a residual unscreened depolarizing field. It is shown that films below 50 Angstroms display a significantly reduced polarization but still remain ferroelectric.