Antiferromagnetic order and superlattice structure in nonsuperconducting and superconducting Rb_yFe_1.6+xSe₂

Neutron diffraction has been used to study the lattice and magnetic structures of the insulating and superconducting Rb_yFe _1.6+xSe₂. For the insulating Rb_yFe _1.6+xSe₂, neutron polarization analysis and single-crystal neutron diffraction unambiguously confirm the earlier proposed √5×√5 block antiferromagnetic structure. For superconducting samples (T_c=30 K), we find that in addition to the tetragonal √5×√5 superlattice structure transition at 513 K, the material develops a separate √2×√2 superlattice structure at a lower temperature of 480 K. These results suggest that superconducting Rb _yFe_1.6+xSe₂ is phase separated with coexisting √2×√2 and √5×√5 superlattice structures.