Self-Polarization in PbTiO3 Crystals Induced by Chemical Inhomogeneity in the Surface Layer

We study the so-called self-polarization phenomenon in single, electroded PbTiO3 crystals. In this case, near the electrodes, surface layers are formed with a chemically modified perovskite structure. This generates a built-in electric polarization, which cannot be switched permanently by an external electric field. While the initial samples, having two such surface layers with opposite directions of built-in polarization, exhibit ordinary symmetric hysteresis loops, the “asymmetric” samples, with one of these surface layers removed, show asymmetric hysteresis loops. To describe our experimental findings, we combine two kinds of models: one is phenomenological, utilizing the above general features; and the other is ab initio, taking into account the actual atomic structure at the bulk ferroelectric–surface layer–electrode interface. Namely, the ab initio calculations show that the electric polarization within the surface layer occurs due to the shifts of the relaxed Ti ions with respect to the oxygen ion octahedra on the PbO-terminated surface. We ascribe the self-polarization effect to the occurrence of the built-in electric field resulting from the formation of Pb-O planes within the surface layer.