Superconductivity tuned by the iron vacancy order in K$_{\bf x}$Fe$_{\bf   2-y}$Se$_2$
  Combining in-depth neutron diffraction and systematic bulk studies, we discover that the $\sqrt{5}\times\sqrt{5}$ Fe vacancy order with its associated block antiferromagnetic order is the ground state, with varying occupancy ratio of the iron 16i and vacancy 4d sites, across the phase-diagram of K$_{\bf x}$Fe$_{\bf 2-y}$Se$_2$. The orthorhombic order with one of the four Fe sites vacant appears only at intermediate temperature as a competing phase. The material experiences an insulator to metal crossover when the $\sqrt{5}\times\sqrt{5}$ order has highly developed. Superconductivity occurs in such a metallic phase. 
Wei Bao, G. N. Li, Q. Huang, G. F. Chen, J. B. He, M. A. Green, Y.   Qiu, D. M. Wang, J. L. Luo
cond-mat.str-el cond-mat.supr-con