Ab initiostudy ofBaTiO3andPbTiO3surfaces in …

For the ferroelectric perovskite compounds ${\mathrm{BaTiO}}_{3}$ and ${\mathrm{PbTiO}}_{3},$ we have studied the effects of external electric fields on the structural properties of the (001) surfaces. The field-induced changes in the surface interlayer spacings and bucklings have been calculated using a first-principles ultrasoft-pseudopotential approach, and the change of the polarization and the ferroelectric distortions in the surface layers have been obtained. The surfaces are represented by periodically repeated slabs, and an external dipole layer is included in the vacuum region of the supercells to control the electric field normal to the surfaces. The influence of the electrical boundary conditions on the ferroelectric properties of the slabs is discussed. In the case of a vanishing internal electric field, our study indicates that even very thin slabs can show a ferroelectric instability.