  The structural features and physical properties of the antiferromagnetic
K0.8Fe1.6Se2 (so called K2Fe4Se5 phase) have been studied in the temperature
range from 300K up to 600K. Resistivity measurements on both single crystal and
polycrystalline samples reveal a semiconducting behavior. Structural
investigations of K2Fe4Se5 by means of transmission electron microscopy and
powder x-ray diffraction demonstrate the presence of a well-defined
superstructure within the a-b plane originating from a Fe-vacancy order along
the [130] direction. Moreover, in-situ heating structural analysis shows that
K0.8Fe1.6Se2 undergoes a transition of the Fe-vacancy order to disorder at
about 600K. The phase separation and the Fe-vacancy ordering in the
superconducting materials of KxFe2-ySe2 (0.2 \leq y \leq 0.3) has been briefly
discussed.
