Microstructural evolution and its influence on the superconductivity induced by oxygen vacancies in Sr2VO3-δFeAs (δ=0.1, 0.5)

Superconducting transition temperatures of Sr2VO3-delta FeAs (delta = 0.1, 0.5) can be tuned by adjusting the dopant concentration of oxygen vacancy. The association between microstructure and superconductivity is not clear and urgently needs to be clarified. Directly via the in situ transmission electron microscopy (TEM) cooling experiment, we demonstrate novel atomic-level microstructural features, which are induced by oxygen deficiency and play key roles in determining the superconducting property. The poor superconducting sample Sr2VO2.5FeAs exhibits high density of stacking faults distributed along the [001] orientation with periodic weak image contrast and extra streaking diffraction spots, due to the periodic extraction of partial SrO atom stripes from the Sr2VO2.5 blocking unit. The superconducting sample Sr2VO2.9FeAs shows a well-formed structure with only a limited amount of point vacancies. When delta is changed from 0.1 to 0.5, the valence state of the Fe ion inside the FeAs blocking unit is slightly reduced from +2.0 to +(2 - delta). Surprisingly, when the sample Sr2VO2.5FeAs is cooled down to the superconducting state, a structural reconstruction process occurs as is inferred from the in situ TEM cooling experiments. The evolution of superconductivity on oxygen stoichiometry can further be supported by the dependence of the Hall coefficient and resistivity. Our findings might shed new light on understanding the superconductivity of the Sr2VO3-delta FeAs system.