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

Abstract

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.