Structure of the oxygen-annealed chalcogenide superconductor Fe1.08Te0.55Se

The as-grown single crystal Fe${}_{1.08}$Te${}_{0.55}$Se${}_{0.45}$ with the tetragonal PbO-type structure is nonsuperconducting 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.