Phase Diagram of KxFe2-ySe2-zSz and the Suppression of its   Superconducting State by an Fe2-Se/S Tetrahedron Distortion
  We report structurally tuned superconductivity in KxFe2-ySe2-zSz (0<=z<=2) phase diagram. Superconducting Tc is suppressed as S is incorporated into the lattice, eventually vanishing at 80% of S. The magnetic and conductivity properties can be related to stoichiometry on poorly occupied Fe1 site and the local environment of nearly fully occupied Fe2 site. The decreasing Tc coincides with the increasing Fe1 occupancy and the overall increase in Fe stoichiometry from z = 0 to z = 2. Our results indicate that the irregularity of Fe2-Se/S tetrahedron is an important controlling parameter that can be used to tune the ground state in the new superconductor family. 
Hechang Lei, Milinda Abeykoon, Emil S. Bozin, Kefeng Wang, J. B.   Warren and C. Petrovic
cond-mat.supr-con cond-mat.mtrl-sci cond-mat.str-el