Synthesis and Characterizations of Arsenic Doped FeSe Bulks

FeSe(11) family has a simple crystal structure belonging to iron-based superconductors (FBS) and has many stable phases including hexagonal and tetragonal structures, but only the tetragonal phase exhibits the superconductivity. In this study, we have investigated the effects of chemical pressure induced by As-doping at Se sites in the FeSe system by preparing a series of FeSe_1-xAs_x (x = 0.005, 0.01, 0.02, 0.05, 0.1 and 0.2) bulks. A broad characterization has been performed on these samples using structural, microstructural, transport and magnetic measurements. The obtained lattice parameters are increased by As-doping, which suggests the successful insertion of As at Se sites into the tetragonal lattice for low doping contents up to 5%, whereas the higher As substitution appears in the form of the FeAs impurity phase. The temperature dependence of the resistivity of all samples has similar behaviour and depicts the highest onset transition temperature of around 11.5 K, but the zero resistivity is not reached until the measured temperature of 7 K, which could be due to the presence of the impurity phases. Our study suggests that a dopant with a large ionic radius, i.e. arsenic, promotes the formation of the hexagonal phase of the 11 family and is effective for a small amount of doping level for the superconducting properties, whereas higher As-doping levels reduce the superconducting properties.