Crystal-structure units of the R (Fe3 Mo2) phase in the Fe-Mo alloy system

Around 37 at. % Mo in the Fe-Mo alloy system, in the temperature range between 1473 and 1761 K, there exists the intermetallic-compound R phase, whose crystal structure involves 12-, 14-, 15-, and 16-coordination polyhedra. To determine the crystal-structure units of the R phase, the crystallographic features of the bcc-to- R structural change in the (bcc→bcc+R) reaction of the Fe-Mo system have been investigated by transmission electron microscopy. It was found that atomic columns, which were identified as the secondary structural unit, appeared first in the initial stage of the reaction. Based on this fact, the R phase is understood to have a crystal structure consisting of atomic columns, just as in the case of the μ structure. Although only 12-coordination polyhedra form an atomic column in the μ structure, the column in the R structure consists of two kinds of polyhedra: that is, 16- and 12-coordination ones. Of these two polyhedra, in particular, the appearance of a 16-coordination polyhedron as the primary structural unit of the R structure is associated with the formation of covalent bonds in alloys.