Structural Study of Iron Selenides FeSex. I Ordered Arrangement of Defects of Fe Atoms

The ordered arrangements of the defects of iron atoms in FeSe x have been studied for several compositions by X-ray analysis. These defects tend to assume an ordered arrangement as in the case of pyrrhotite FeS 1.14 . New phases are found in the region ( x =8/7∼4/3) which have been considered as the single phase region. If we choose the orthohexagonal and pseudo-orthohexagonal unit cells for x =8/7 and 4/3, respectively, the unit cell dimensions of the superstructure resulting from this ordering are twice as large as that of the fundamental structure along the a - and b -axes, and three times along the c -axis for x =8/7, and on the other hand, twice only along the c -axis for x =4/3.