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Magnetic and crystal structures of superconducting X yFe 2-xSe 2 (X= Rb and Kwith Tc=31.5 K and 29.5 K) have been studied by neutron powder diffraction atroom temperature. Both crystals show ordered iron vacancy pattern and thecrystal structure is well described in the I4/m space group with the latticeconstants a=8.799, c=14.576 and a=8.730, c=14.115 A, and the refinedstoichiometry x=0.30(1), y=0.83(2) and x=0.34(1), y=0.83(1) for Rb- andK-crystals respectively. The structure contains one fully occupied ironposition and one almost empty vacancy position. Assuming that the iron momentis ordered only on the fully occupied site we have sorted out all eightirreducible representations (irreps) for the propagation vector k=0 and havefound that irreps tau_2 and tau_7 well fit the experimental data with themoments along 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-crystalsrespectively. Irrep tau_2 corresponds to the Shubnikov group I4/m' and gives aconstant moment antiferromagnetic configuration, whereas tau_7 does not haveShubnikov counterpart and allows two different magnetic moments in thestructure.

Room temperature antiferromagnetic order in superconducting X_yFe_{2-x}Se_ 2, (X= Rb, K): a powder neutron diffraction study