Role of the extra Fe in K2-xFe4+ySe5 superconductors

This paper presents the results of a detailed structural study using synchrotron X-ray diffraction at various temperatures up to 850 °C on K_2xFe_4+ySe₅ superconductors. The results confirm that extra Fe atoms begin to fill the empty 4d site and stabilize the structural-phase I4/m symmetry so that the lattice size remains the same even above the order-disorder temperature. The results demonstrate that the addition of extra Fe suppresses the Fe vacancy long-range order and the accompanied magnetic order so that superconductivity emerges. The significance of this research is that it provides unambiguously the structural origin for superconductivity in the K_2-xFe_4+ySe₅ superconducting system.