  Fe1+xTe0.5Se0.5 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.
