The crystal structure of FeSe0.44Te0.56

The crystal structure of the superconductor FeSe0.44Te0.56 was redetermined by high-resolution X-ray single crystal diffraction at 173 K (anti-PbO-type, P4/nmm, a=3.7996(2), c=5.9895(6) A, R1=0.022, wR2=0.041, 173 F^2). Significantly different z-coordinates of tellurium and selenium at the 2c 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 A and the Fe-Se bonds are by 0.154 A shorter than the Fe-Te bonds, while three independent (Te,Se)-Fe-(Te,Se) bond angles occur. An elevated U33 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.