Oxygen vacancies are often present in complex oxides as point defects, and their effect on the electronic properties is typically uniform and isotropic. Exploiting oxygen deficiency in order to generate controllably novel structures and functional properties remains a challenging goal. Here we show that epitaxial strontium chromite films can be transformed, reversibly and at low temperature, from rhombohedral, semiconducting SrCrO 2.8 to cubic, metallic perovskite SrCrO 3-d . Oxygen vacancies in SrCrO 2.8 aggregate and give rise to ordered arrays of {111}-oriented SrO 2 planes interleaved between layers of tetrahedrally coordinated Cr 4 þ and separated by B1 nm. First-principle calculations provide insight into the origin of the stability of such nanostructures and, consistent with the experimental data, predict that the barrier for O 2 À diffusion along these quasi-two-dimensional nanostructures is significantly lower than that in cubic SrCrO 3-d . This property is of considerable relevance to solid oxide fuel cells in which fast O 2 À diffusion reduces the required operating temperature.

We have investigated the evolution of the electronic properties of La 1-x Sr x CrO 3 (0 ≤ x ≤ 1) epitaxial films deposited by molecular beam epitaxy (MBE) using x-ray diffraction, x-ray photoemission spectroscopy, Rutherford backscattering spectrometry, x-ray absorption spectroscopy, electrical transport, and ab initio modeling. LaCrO 3 is an antiferromagnetic insulator whereas SrCrO 3 is a metal. Substituting Sr