Theoretical Study Of Lead Titanate (110) And (001) Surface Structure And Stability

The cubic lead titanate surfaces were systematically investigated by means of the density functional theory (DFT) plane-wave pseudopotentials method using a periodic slab model. We used the CASTEP computational code to perform the atomic, electronic structures and thermodynamic stability calculations for the bulk crystal as well as the considered two (001) non-polar and five (110) polar surfaces, and thus explore the connections between structures and properties of materials.The results of detailed calculations for the bulk electronic structure of the cubic phase of PbTiO3 showed the partially covalent characteristics o Ti-O and Pb-O pairs, confirmed by the mixtures of Ti3d, Pb6s as well as Pb6p with O2p orbitals. The calculated results indicated the atomic and electronic structures of the (001) non-polar surfaces have little change in comparison with the bulk crystal with obscure relaxation effects and charge redistributions. In contrast, the two stoichiometric terminations of the (110) polar surface underwent significant changes with respect to bulk materials. On the PbTiO termination, an anomalous filling of conduction band was observed, and this surface possessed metallic characteristic. While on the O2 termination, two surface oxygen atoms closed to each other and formed a peroxo group. However, for the three nonstoichiometric TiO-, Pb- and O-terminated surfaces, their electronic structures were very similar to the bulk crystal, and kept insulating property. Charge redistribution results for the five terminations confirmed that electronic structure and surface composition changes are responsible for their polarity compensation. However, which mechanism actually dominates the stabilization process depends upon energetic considerations. A thermodynamic stability diagram suggested that the two stoichiometric terminations were unstable; however, the three nonstoichiometric terminations can be stabilized in some given regions. Furthermore, the present study indicates that the very different stabilities and surface states filling behaviors of the PbTiO3 (110) polar surface with respect to SrTiO3 and BaTiO3 ones seems to originate from the partially covalent characteristics of Pb-O pairs.