Direct observation of apical oxygen vacancies in the high-temperature superconductor YB a 2 C u 3 O 7 − x

The properties of the high-temperature superconductor $\mathrm{YB}{\mathrm{a}}_{2}\mathrm{C}{\mathrm{u}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ (YBCO) depend on the concentration of oxygen vacancies (${V}_{\mathrm{O}}$). It is generally agreed upon that ${V}_{\mathrm{O}}$ form in the CuO chains, even at low concentrations where the critical temperature for superconductivity peaks ($x\ensuremath{\approx}0.07$), with only a handful of reports suggesting the presence of ${V}_{\mathrm{O}}$ at the apical sites. In this paper, we show direct evidence of apical ${V}_{\mathrm{O}}$ in optimally doped YBCO samples. Using density-functional-theory calculations, we predict that isolated ${V}_{\mathrm{O}}$ are equally favorable to form in either the CuO chains or the apical sites, which we confirm using atomic-resolution scanning transmission electron microscope imaging and spectroscopy. We further show that apical ${V}_{\mathrm{O}}$ lead to significant lattice distortions and changes in the electronic structure of YBCO, indicating they should be considered on an equal footing with chain ${V}_{\mathrm{O}}$ to understand the superconducting properties of YBCO in the optimal doping region.