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 T-c similar to 32 K with a large shielding volume fraction. On the other hand, the crystals prepared without quenching show an onset superconducting transition at similar to 44 K and a zero resistivity at approximately similar to 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 degrees C via iron diffusion. The generation of this Fe-vacancy-ordered phase may become a driving force for the growth of the higher-T-c phase. The superconductivity at similar to 44 K is attributed to a metallic phase with no Fe vacancy.