Superconductivity tuned by the iron vacancy order in K_xFe_2-ySe₂

Combining in-depth neutron diffraction and systematic bulk studies, we discover that the 5 × 5 Fe vacancy order, with its associated block antiferromagnetic order, is the ground state with varying occupancy ratios of the iron 16i and vacancy 4d sites across the phase-diagram of K_xFe_2-ySe₂ . The orthorhombic order, with one of the four Fe sites vacant, appears only at intermediate temperatures as a competing phase. The material experiences an insulator to metal crossover when the  √5 × √5 order is highly developed. Superconductivity occurs in such a metallic phase.