First-principle calculations of the cohesive energy and the electronic properties of PbTiO3

The cohesive energy and electronic properties of PbTiO3 have been studied using the full-potential-linearized augmented-plane wave method (FP-LAPW). The calculations have been made within the framework of density functional theory (DFT), with local density approximation (LDA) and generalized gradient approximation (GGA). The cohesive energy of PbTiO3 with 19 core states for Pb, 4 for Ti and 1 for O atoms per unit cell for tetragonal phase lowers by 0.27 eV, since the Pb and O states hybridize, leading to a large strain that stabilizes this phase. In the tetragonal phase, it is found that the bond between Pb and O is ionic, while there is strong hybridization between Ti-3d and O-2p, which is necessary for ferroelectricity in PbTiO3. It is also indicated that shallow Pb-6s semicore hybridize with the O-2p state. The results also show an indirect band gap of 1.7 eV at X–Γ points in the Brillouin zone for the cubic phase and an indirect band gap of 2.0 eV at X–Γ point for the tetragonal phase, in the absence of the scissors operation. These results are in better agreement with the published theoretical and experimental data.