Room temperature antiferromagnetic order in superconducting XyFe2-xSe2 (X = Rb, K): a neutron powder diffraction study

Magnetic and crystal structures of superconducting XyFe2-xSe2 (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 I 4/m space group with the lattice constants a = 8.799, c = 14.576 and a = 8.730, c = 14.115 angstrom 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 tau(2) and tau(7) fit the experimental data well with the moments along the c axis. The moment amplitudes amounted to 2.15(3) mu(B), 2.55(3) mu(B) for tau(2) and 2.08(6) mu(B), 2.57(3) mu(B) for tau(7) for Rb and K crystals, respectively. Irrep tau(2) corresponds to the Shubnikov group I 4/m' and gives a constant moment antiferromagnetic configuration, whereas tau(7) does not have a Shubnikov counterpart and allows two different magnetic moments in the structure.