UC Berkeley

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.