The first-principles study of ferroelectric behaviours of PbTiO₃/SrTiO₃ and BaTiO₃/SrTiO₃ superlattices

We have performed the first-principles calculation to investigate the origins of ferroelectricities and different polarization behaviours of superlattices BaTiO_3/SrTiO_3 and PbTiO_3/SrTiO_3. The density of state (DOS) and electronic charge profiles show that there are strong hybridizations between atoms Ti and O and between atoms Pb and O which play very important roles in producing the ferroelectricities of superlattices BaTiO_3/SrTiO_3 and PbTiO_3/SrTiO_3. Owing to the decline of internal electric field in SrTiO_3 (ST) layer, the tetragonality and polarizations of superlattices decrease with increasing the fraction of SrTiO_3 in the superlattices. We find that the polarization of PbTiO_3/SrTiO_3 is larger_ than that of BaTiO_3/SrTiO_3 at the same ratio of components, because the polarization mismatch between PbTiO_3 and SrTiO_3 is larger than that between BaTiO_3 and SrTiO_3. The polarization and tetragonality are enhanced with respect to those of bulk tetragonal BaTiO_3 in the superlattices BaTiO_3/SrTiO_3, while the polarization and tetragonality are reduced with respect to those of bulk tetragonal PbTiO_3 in superlattices PbTiO_3/SrTiO_3.