Structural disorder in Lix(C5H5N)yFe2−zSe2 and CsxFe2−zSe2 superconductors studied by Mössbauer spectroscopy

• Secondary phases dispersed in single crystal of CsxFe2−zSe2.
• α-Fe precipitates between Fe–Se layers in Lix(C5H5N)yFe2−zSe2.
• Superconductivity is obtained by removing of the iron vacancies within Fe–Se sheets.

Two iron-chalcogenide superconductors Lix(C5H5N)yFe2−zSe2 and CsxFe2–zSe2 in the as-prepared and annealed state have been investigated by means of the Mössbauer spectroscopy versus temperature. Multi-component spectra are obtained. One can see a non-magnetic component due to iron located in the unperturbed Fe–Se sheets responsible for superconductivity. Remaining components are magnetically ordered even at room temperature. There is some magnetically ordered iron in Fe–Se sheets perturbed by presence of the iron vacancies. Additionally, one can see iron dispersed between sheets in the form of magnetically ordered high spin trivalent ions, some clusters of above ions, and in the case of pyridine intercalated compound in the form of α-Fe precipitates. Pyridine intercalated sample shows traces of superconductivity in the as-prepared state, while cesium intercalated sample in the as-prepared state does not show any superconductivity. Superconductors with transition temperatures being 40 K and 25 K, respectively, are obtained upon annealing. Annealing leads to removal/ordering of the iron vacancies within Fe–Se sheets, while clusters of α-Fe grow in the pyridine intercalated sample.

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