Density-functional calculations have been performed on ${\text{BaTiO}}_3\left(001\right)$ bare, with adsorbed molecular water and hydroxylated surfaces in the case of surfaces having in-plane polarization. We show that in-plane-polarized ${\text{BaTiO}}_3\left(001\right)$ flat surfaces have very strong interaction energies with water and that ${\text{H}}_2\text{O}$ is chemisorbed on both BaO and ${\text{TiO}}_2$ terminations in that case. The adsorption results into surface hydroxyl groups OH with electric dipoles that couple to ferroelectricity in the materials (imprint effect) and point preferentially in the same direction as the polarization in the slab or into molecular water strongly bound to the surface. Moreover we find that the in-plane polarization in the last ${\text{TiO}}_2$ layer is pinned by the dipoles of the OH groups adsorbed for both terminations (especially in the case of the ${\text{TiO}}_2$ one). The influence of strain on hydroxylation is also examined in the case of ${\text{TiO}}_2$ terminations, and tensile strain is found to increase the reactivity of ${\text{TiO}}_2$-terminated ${\text{BaTiO}}_3\left(001\right)$ surfaces with respect to water dissociation. The evidence of spontaneous water chemisorption on in-plane-polarized ${\text{BaTiO}}_3$ surfaces in ambient moisture conditions might partly explain screening mechanisms on ferroelectric films.

• Received 5 January 2009

DOI:https://doi.org/10.1103/PhysRevB.79.235420