Phase Separation and Superconductivity in Fe1+xTe0.5Se0.5
  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. 
Vikas Bhatia, Efrain E. Rodriguez, Nicholas P. Butch, Johnpierre   Paglione and Mark A. Green
cond-mat.supr-con