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Ionic polarizability of conductive metal oxides and critical thickness for ferroelectricity in BaTiO3.

We report a first-principles investigation of ultrathin BaTiO(3) films with SrRuO(3) electrodes. We find that the ionic relaxations in the metal-oxide electrode play a crucial role in stabilizing the ferroelectric phase. Comparison with frozen-phonon calculations shows that the degree of softness of the SrRuO(3) lattice has an essential impact on the screening of ferroelectric polarization in BaTiO(3). The critical thickness for ferroelectricity in BaTiO(3) is found to be 1.2 nm. The results of our calculations provide a possible explanation for the beneficial impact of oxide electrodes on the switching and dielectric properties of ferroelectric capacitors.