Suppression of Superconductivity and Nematic Order in Fe 1- y Se 1- x S x (0 ≤ x ≤ 1; y ≤ 0.1) Crystals by Anion Height Disorder.

Connections between crystal chemistry and critical temperature T c have been in the focus of superconductivity, one of the most widely studied phenomena in physics, chemistry, and materials science alike. In most Fe-based superconductors, materials chemistry and physics conspire so that T c correlates with the average anion height above the Fe plane, i.e., with the geometry of the FeAs 4 or FeCh 4 (Ch = Te, Se, or S) tetrahedron. By synthesizing Fe 1- y Se 1 -x S x (0 ≤ x ≤ 1; y ≤ 0.1), we find that in alloyed crystals T c is not correlated with the anion height like it is for most other Fe superconductors. Instead, changes in T c ( x ) and tetragonal-to-orthorhombic (nematic) transition T s ( x ) upon cooling are correlated with disorder in Fe vibrations in the direction orthogonal to Fe planes, along the crystallographic c -axis. The disorder stems from the random nature of S substitution, causing deformed Fe(Se,S) 4 tetrahedra with different Fe-Se and Fe-S bond distances. Our results provide evidence of T c and T s suppression by disorder in anion height. The connection to local crystal chemistry may be exploited in computational prediction of new superconducting materials with FeSe/S building blocks.