  Neutron and x-ray powder and single crystal synchrotron diffraction of
CsyFe2-xSe2 show the presence of superstructure reflections with propagation
vector k=[2/5, 1/5, 1] with respect to the average crystal structure I4/mmm (a
= 4, c = 15A). The propagation vector star corresponds to the 5 times bigger
unit cell given by transformation A=2a+b, B= -a+2b, C= c. A solution for the
atomic structure is found in the space groups P42/n and I4/m with an ordered
pattern of iron vacancies corresponding to the iron deficiency x = 0.29 and Cs
stoichiometry y = 0.83. The superstructure satellites are more pronounced in
the neutron diffraction patterns suggesting that they can have some magnetic
contribution. We have sorted out possible symmetry adapted magnetic
configurations and found that the presence of AFM ordering with the ordered
magnetic moment of Fe with 2{\mu}B does not contradict to the experimental
data. However, the solutions space is highly degenerate and we cannot choose a
specific solution. Instead we propose possible magnetic configurations with the
Fe magnetic moments in (ab)-plane or along c-axis. The superstructure is
destroyed above Ts ~ 500 K by a first-order-like transition.
