Equilibrium Polarization under Ionic Surface Compensation in Ultrathin PbTiO$_{3}$

The polarization structure of a ferroelectric film depends strongly on the charge compensation at its interfaces. When there is no electrode, ions from the environment can compensate a surface. We have previously shown that changing the chemistry of the environment can drive polarization switching in a ferroelectric film, and for the thinnest films the switching occurs continuously, without domain nucleation. In this talk we present results on the equilibrium polarization of a film as a function of the ionic compensation of its surface. Synchrotron x-ray scattering is used to determine the polarization structure of epitaxial PbTiO$_{3}$ films on conductive SrRuO$_{3}$ layers coherently strained to SrTiO$_{3}$ (001) substrates as a function of temperature, film thickness, and external oxygen partial pressure (pO$_{2})$. We observe a suppression of the Curie temperature (T$_{C})$ at intermediate values of pO$_{2}$, which becomes very large for films thinner than $\sim $5 nm. This suppression of T$_{C}$ is explained by a model for the equilibrium between the chemical environment and the surface of the ferroelectric. Work supported by the U. S. Department of Energy under Contract No. DE-AC02-06CH11357.