Ab initiostudy ofBaTiO3andPbTiO3surfaces in external electric fields

For the ferroelectric perovskite compounds BaTiO3 and PbTiO3, we have studied\nthe effects of external electric fields on the structural properties of the\n(001) surfaces. The field-induced changes in the surface interlayer spacings\nand bucklings have been calculated using a first-principles\nultrasoft-pseudopotential approach, and the change of the polarization and the\nferroelectric distortions in the surface layers have been obtained. The\nsurfaces are represented by periodically repeated slabs, and an external dipole\nlayer is included in the vacuum region of the supercells to control the\nelectric field normal to the surfaces. The influence of the electrical boundary\nconditions on the ferroelectric properties of the slabs is discussed. In the\ncase of a vanishing internal electric field, our study indicates that even very\nthin slabs can show a ferroelectric instability.\n