Subsurface Oxygen Vacancy Mediated Surface Reconstruction and Depolarization of Ferroelectric BaTiO (001) Surface.

The interplay between surface reconstruction and depolarization of ferroelectric oxide surfaces is strongly influenced by oxygen vacancies (V). Using in-situ atomic-resolution electron microscopy imaging and spectroscopy techniques, it is directly observed that a clean BaTiO (001) surface stabilizes into (2 × 1) BaO-terminated reconstruction during vacuum annealing. This surface reconstruction is achieved with accommodating BaO deficiency and incorporates TiO adunits. The cooperative atomic rumpling in both the surface and subsurface layers, arranged in a tail-to-tail configuration, is stabilized by planar accumulation of V in the subsurface TiO layer. This reduces the net polarization of surface unit cells, contributing to overall depolarization. Under this atomic rumpling, the polarization-down (P↓) state is energetically favored over the polarization-up (P↑) state, as the P↓ state requires less atomic relaxation in the bulk layers to achieve dipole inversion at the subsurface. The energetic preference for V in the subsurface TiO layer of the P↓ state is confirmed through calculations of V formation energy and the energy barrier for surface-to-subsurface migration. These findings reveal that the presence of V in the subsurface layer lifts the degeneracy in the double-well potential between the P↓ and P↑ states in BaTiO (001).