Superconductivity in anti-PbO-type iron chalcogenides Fe1+xTe1−ySey (x = 0, 0.1, y = 0.1–0.4) depends on the amount (x) of interstitial iron atoms located between the FeTe1−ySey layers. Non-superconducting samples of nominal Fe1.1Te1−ySey convert to superconductors with critical temperatures up to 14 K after annealing at 300 °C in an oxygen atmosphere. The process is irreversible upon subsequent hydrogen annealing. Magnetic measurements are consistent with the formation of iron oxides, suggesting that oxygen annealing preferably extracts interstitial iron from Fe1+xTe1−ySey, which interferes with superconductivity.

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