The structure of the oxygen-annealed Fe$_{1.08}$Te$_{0.55}$Se$_{0.45}$O$_{x}$ superconductor

Effect of oxygen annealing on the iron chalcogenide superconductor with excess Fe is studied and structure change is investigated by using electron microscopy. The as-grown single crystal Fe$_{1.08}$Te$_{0.55}$Se$_{0.45}$ with the tetragonal PbO-type structure is non-superconducting owing to the excess Fe. Superconductivity is induced after oxygen annealing with an onset and zero resistance transition temperature around 14.5 K and 11.5 K, respectively. The oxygen doping is evidenced by electron energy loss spectroscopy, and accompanied by improved homogeneity in the remaining PbO-type phase as well as an increase in the L$_{3}$/L$_{2}$ intensity ratio of the Fe-L$_{2,3}$ edge, indicating an increase in Fe valence. Local phase transformation from the tetragonal PbO-type phase to the hexagonal NiAs-type phase is also observed after oxygen annealing.