Effect of the FeSe Precursor on the Superconductivity of K x Fe 2− y Se 2

Abstract

Three single crystals of K_xFe_2−ySe₂ with nominal compositions of K_0.8Fe₂Se₂ were prepared by self-flux method in one step or two steps, employing different FeSe precursors as raw materials. Depending on the heat treatment times of FeSe precursors, these three K_xFe_2−ySe₂ crystals are symbolized by KFeSe-0, KFeSe-1, and KFeSe-2, respectively. The onset superconducting transition temperature for KFeSe-1 and KFeSe-2 is around 44 K which is much higher than that of 34.8 K for KFeSe-0. According to the structural and transport properties, it is inferred that the higher-onset transition temperature at 44 K might be derived from another iron-rich K_xFe_2−ySe₂ phase. The employment of FeSe precursors makes it more difficult for K ions to intercalate into FeSe layers, inducing a phase separation and the formation of a second iron-rich phase.