Room temperature antiferromagnetic order in superconducting X(y)Fe(2-x)Se₂ (X = Rb, K): a neutron powder diffraction study.

Magnetic and crystal structures of superconducting X(y)Fe(2-x)Se₂ (X = Rb and K with T(c) = 31.5 and 29.5 K) have been studied by neutron powder diffraction at room temperature. Both crystals show an ordered iron vacancy pattern and the crystal structure is well described by the I4/m space group with the lattice constants a = 8.799, c = 14.576 and a = 8.730, c = 14.115 Å and the refined stoichiometry x = 0.30(1), y = 0.83(2) and x = 0.34(1), y = 0.83(1) for Rb and K crystals, respectively. The structure contains one fully occupied iron position and one almost empty vacancy position. Assuming that the iron moment is ordered only on the fully occupied site we have sorted out all eight irreducible representations (irreps) for the propagation vector k = 0 and have found that irreps τ₂ and τ₇ fit the experimental data well with the moments along the c axis. The moment amplitudes amounted to 2.15(3) µ(B), 2.55(3) μ(B) for τ₂ and 2.08(6) μ(B), 2.57(3) μ(B) for τ₇ for Rb and K crystals, respectively. Irrep τ₂ corresponds to the Shubnikov group I4/m' and gives a constant moment antiferromagnetic configuration, whereas τ₇ does not have a Shubnikov counterpart and allows two different magnetic moments in the structure.