Suppression of superconductivity in Fe chalcogenides by annealing: A reverse effect to pressure

Superconductivity in FeTe_1-xSe_x can be controlled by annealing in the absence of extrinsic influences. Using neutron diffraction, we show that the superconducting transition temperature (T_C) sensitively depends on the atomic configurations of the Te and Se ions. Low-temperature annealing not only homogenizes the Te and Se ion distribution, it suppresses T_C because of changes in the chalcogen ion's z parameter. In particular, the height of Te from the Fe basal plane is much reduced while that for Se shows a modest increase. These trends are the reverse of the effects induced by pressure.