Phase separation and superconductivity in Fe1+xTe0.5Se0.5

Fe(1+x)Te(0.5)Se(0.5) is the archetypical iron-based superconductor. Here we show that the superconducting state is controlled by the stacking of its anti-PbO layers, such that homogeneous ordering hinders superconductivity and the highest volume fractions are observed in phase separated structures as evidenced by either a distribution of lattice parameters or microstrain.