The crystal structure of

The crystal structure of the superconductor FeSe 0.44 Te 0.56  was redetermined by high-resolution X-ray single crystal diffraction at 173 K (anti-PbO-type, P 4 / n m m , a = 3.7996 ( 2 ) , c = 5.9895 ( 6 ) Å, R 1 = 0.022 , w R 2 = 0.041 , 173 F 2 ). Significantly different z -coordinates of tellurium and selenium at the 2 c site are clearly discernible and were refined to z Te = 0.2868 ( 3 ) and z Se = 0.2468 ( 7 ) . Thus the chalcogen heights differ by 0.24 Å and the Fe–Se bonds are by 0.154 Å shorter than the Fe–Te bonds, while three independent (Te, Se)–Fe–(Te, Se) bond angles occur. An elevated U 33 displacement parameter of the iron atom is suggestive of a slightly puckered Fe layer resulting from different combinations of Se or Te neighbors. Such strong disorder underlines the robustness of superconductivity against structural randomness and has not yet been considered in theoretical studies of this system.