Origin of the Higher-Tc Phase in the KxFe2-ySe2 System

Potassium intercalated FeSe has been reported as a potential superconductor with a maximum superconducting transition temperature (Tc) of up to 44-48 K. There is no clear answer to identify the relationship between the composition and crystal structure. This report shows that only a specific heating treatment process can realize a high Tc around 44 K for single crystals. In-situ high-temperature single crystal X-ray diffraction measurements provides us with a hint as to how to fabricate said high Tc superconducting single crystals, and solve the puzzle behind superconductivity in the KxFe2-ySe2 system.