  Magnetic and crystal structures of superconducting X yFe 2-xSe 2 (X= Rb and K
with Tc=31.5 K and 29.5 K) have been studied by neutron powder diffraction at
room temperature. Both crystals show ordered iron vacancy pattern and the
crystal structure is well described in the I4/m space group with the lattice
constants a=8.799, c=14.576 and a=8.730, c=14.115 A, 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 well fit the experimental data with the
moments 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-crystals
respectively. Irrep tau_2 corresponds to the Shubnikov group I4/m' and gives a
constant moment antiferromagnetic configuration, whereas tau_7 does not have
Shubnikov counterpart and allows two different magnetic moments in the
structure.
