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The Fe-based superconducting KxFe2-ySe2 (0.6 {\leg} x {\leg} 1, 0.2{\leg} y{\leg} 0.4) compounds, unlike the well-known RFe2As2 (R = Ba, Sr, Ca)superconductors, contain complex structural features and notable physicalproperties, such as the Fe-vacancy ordering, multi-superconducting transitionsand the phase separation[1]. Recent experimental studies also suggested thepresence of remarkable interplay among the Fe-vacancy order and theantiferromagnetic structure[2]. Here we demonstrate that the well-characterizedsuperconducting KxFe2-ySe2 (0.6 {\leg} x {\leg} 0.8, 0.2 {\leg} x {\leg} 0.3)samples contain complex microstructure features and undergo successive phasetransitions at low temperatures. In-situ TEM observations on a number of theKxFe2-ySe2 superconductors demonstrated the presence of a remarkable collapseof the Fe-vacancy order above the superconducting transition, as a result, thesuperconducting phase actually adopts a tetragonal structure without theFe-vacancy ordering. Moreover, our analysis at the low temperatures suggeststhat the superconductors likely adopt a Fe-deficient structure with compositionof K0.75Fe2-ySe2. These results are important not only for the furtheroptimization of superconducting phase in present system but also forunderstanding the mechanism of the Fe-based superconductivity.

Collapse of the Fe-vacancy order and successive phase transitions in superconducting KxFe2-ySe2 (0.7 \leg x \leg 0.8, 0.2 \leg y \leg 0.3)