Antiferromagnetic order and superlattice structure in nonsuperconducting and superconducting RbyFe1.6+xSe2

Neutron diffraction has been used to study the lattice and magnetic structures of the insulating and superconducting RbyFe1.6+ xSe2. For the insulating RbyFe1.6+ xSe2, neutron polarization analysis and single-crystal neutron diffraction unambiguously confirm the earlier proposed root 5 x root 5 block antiferromagnetic structure. For superconducting samples (T-c = 30 K), we find that in addition to the tetragonal root 5 x root 5 superlattice structure transition at 513 K, the material develops a separate root 2 x root 2 superlattice structure at a lower temperature of 480 K. These results suggest that superconducting RbyFe1.6+xSe2 is phase separated with coexisting root 2 x root 2 and root 5 x root 5 superlattice structures.