Local atomic structure of superconducting FeSe_1-x Te_x

The isovalent substitution of Te for Se in the superconducting α-FeSe raises T_C where the average chalcogen-Fe bond angle decreases and the chalcogen-Fe distance increases. Locally, however, the Se and Te ions do not share the same site and have two distinct z coordinates, in contrast to what is presumed in the P4/nmm symmetry. The local bond angle between the chalcogens and Fe increases with the substitution, consistent with the rise in T _C, the Fe-Te bonds become shorter than in the binary FeTe, while the Fe-Se bonds stay the same as in the binary. Ab initio calculations based on spin density functional theory yielded an optimized structure with distinct z coordinates for Se and Te, in addition to a stronger hybridization of Te with Fe.