The local atomic structure of superconducting Fe-Se-Te

TL;DR

This study reveals that local atomic structure variations in FeSe1-xTex superconductors, especially bond angles and site occupancy, are directly linked to their superconducting transition temperature, highlighting the importance of local symmetry and structure.

Contribution

First local atomic structure analysis of FeSe1-xTex using pair density function, showing local symmetry lower than average and its correlation with Tc.

Findings

Local symmetry is lower than average crystal symmetry.

Te and Se occupy distinct sites with different z-coordinates.

Bond angles increase with Te substitution, correlating with higher Tc.

Abstract

We report on the first local atomic structure study via the pair density function (PDF) analysis of neutron diffraction data and show a direct correlation of local coordinates to TC in the newly discovered superconducting FeSe1-xTex. The isovalent substitution of Te for Se such as in FeSe0.5Te0.5 increases Tc by twofold in comparison to a-FeSe without changing the carrier concentration but, on average, decreases the chalcogen-Fe bond angle. However, we find that the local symmetry is lower than the average P4/nmm crystal symmetry, because the Se and Te ions do not share the same site, leading to two distinct z-coordinates that exhibit two types of bond angles with Fe. The angle indeed increases from ~ 104.02o in FeSe to ~105.20o in FeSe0.5Te0.5 between Fe and Se. Simultaneously, ab-initio calculations based on spin density function theory yielded an optimized structure with distinct z-coordinates for Se and Te, in agreement with the experiment. The valence charge distribution in the Fe-Se bonds was found to be different from that in the Fe-Te bonds. Thus, superconductivity in this chalcogenide is closely related to the local structural environment, with direct implications on the multiband magnetism where modulations of the ionic lattice can change the distribution of valence electrons.