Antiferromagnetic order and superlattice structure in nonsuperconducting and superconducting RbyFe1.6+xSe

Neutron diffraction has been used to study the lattice and magnetic structures of the insulating and superconducting Rb${}_{y}$Fe${}_{1.6+x}$Se${}_{2}$. For the insulating Rb${}_{y}$Fe${}_{1.6+x}$Se${}_{2}$, neutron polarization analysis and single-crystal neutron diffraction unambiguously confirm the earlier proposed $\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5}$ block antiferromagnetic structure. For superconducting samples (${T}_{c}=30$ K), we find that in addition to the tetragonal $\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5}$ superlattice structure transition at 513 K, the material develops a separate $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$ superlattice structure at a lower temperature of 480 K. These results suggest that superconducting Rb${}_{y}$Fe${}_{1.6+x}$Se${}_{2}$ is phase separated with coexisting $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$ and $\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5}$ superlattice structures.