Abstract
The isovalent substitution of Te for Se in the superconducting $\ensuremath{\alpha}\text{-FeSe}$ 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.
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