  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.
