First-principles study on ferroelectricity at PbTiO3 surface steps.

We performed ab initio density functional theory calculations to investigate ferroelectricity at PbTiO(3) surface steps consisting of (100) and (001) surfaces with the polar axis in the [010] direction. Ferroelectricity was enhanced at PbO-terminated surface steps due to enhanced covalent Pb-O bonding because of the low coordination number of Pb atoms at the step edge. In contrast, ferroelectric distortions were suppressed at TiO(2)-terminations, because of electron transfer from Pb-O sites to Ti-O sites. Spontaneous polarization at the surface step increased when tensile strain was applied in the [010] direction and decreased when compressive strain was applied. At a critical compressive strain, the polarization direction changed and a polydomain structure was formed that consisted of 90° and 180° domain walls aligned with the surface step edge. This polydomain structure compensates surface charges that would generate a depolarizing field, thereby stabilizing ferroelectric distortions at the surface step. The polydomain structure also explains the formation mechanism of the experimentally observed 180° domain wall pinned at the surface step edge.