Elucidating the magnetic and superconducting phases in the alkali metal intercalated iron chalcogenides

The complex interdigitated phases have greatly frustrated attempts to document the basic features of the superconductivity in the alkali metal intercalated iron chalcogenides. Here, using elastic neutron scattering, energy-dispersive x-ray spectroscopy, and resistivity measurements, we elucidate the relations of these phases in RbxFeySe2-zSz. We find (i) the iron content is crucial in stabilizing the stripe antiferromagnetic (AF) phase with rhombic iron vacancy order (y≈1.5), the block AF phase with 5×5 iron vacancy order (y≈1.6), and the iron vacancy-free phase (y≈2); and (ii) the iron vacancy-free superconducting phase (z=0) evolves into an iron vacancy-free metallic phase with sulfur substitution (z>1.5) due to the progressive decrease of the electronic correlation strength. Both the stripe AF phase and the block AF phase are Mott insulators. The iron-rich compounds (y>1.6) undergo a first order transition from an iron vacancy disordered phase at high temperatures into the 5×5 iron vacancy ordered phase and the iron vacancy-free phase below Ts. Our data demonstrate that there are miscibility gaps between these three phases. The existence of the miscibility gaps in the iron content is a key to understanding the relationship between these complicated phases.