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

Single crystals of KxFe2−ySe2 are prepared by quenching at various temperatures. The crystals obtained at higher quenching temperatures have a surface morphology with mesh-like texture. They show a sharp superconducting transition at Tc ∼ 32 K with a large shielding volume fraction. On the other hand, the crystals prepared without quenching show an onset superconducting transition at ∼44 K and a zero resistivity at approximately ∼33 K, and they possess island-like regions on the surface with a larger amount of Fe incorporation. In situ high-temperature single-crystal X-ray diffraction measurements tell us the Fe-vacancy-ordered phase is generated at a temperature region of around 270 °C via iron diffusion. The generation of this Fe-vacancy-ordered phase may become a driving force for the growth of the higher-Tc phase. The superconductivity at ∼44 K is attributed to a metallic phase with no Fe vacancy.