Structural and electronic properties of BaTiO₃ slabs: Mechanism for surface conduction

The structural and electronic properties of BaTiO₃ (001) slabs in paraelectric (PE) phase and ferroelectric (FE) phases with a [001] FE distortion are investigated by first-principles calculations to explore a possible scenario for the surface conduction recently observed in the FE phase. Both TiO₂(S1)-TiO₂(S2) and TiO₂(S1)-BaO(S2) terminated surfaces are considered, where S1 (S2) denotes a top (bottom) slab surface. The PE phase electronic structure possesses fully occupied surface oxygen p states in the bulk band gap, and the empty surface titanium d states at the edge of the bulk conduction band. In the FE phase, both the top surface titanium d state and the bottom surface oxygen p state move into the bulk midgap; the states exchange charge, becoming partially occupied and conducting. The intrinsic carrier layers are formed by a long-range charge transfer from O(S2) to Ti(S1) states.