Atomic channel occupation in disordered η-Al5Fe2 and in two of its low-temperatures phases, η″ and η‴

High-temperature, disordered η-Al5Fe2 intermetallic and its low-temperature long-range ordered structures have been investigated in the whole homogeneity range from 70.6 to 73.0 at.% Al based on selected area electron diffraction data and powder X-ray diffraction data. Using these data, the channel site occupation in the disordered η phase has been revised and a composition-dependent model with mixed occupation of the channels by both Al and Fe atoms was derived. It leads to an average number of atoms per channel and unit cell of approx. 1.5 in the whole homogeneity range. Below about 350 °C, four different phases exhibiting long-range order of the channel atoms appear to exist. Here the atomic ordering in the Al-poorest and Al-richest of these low-temperature phases, η″ and η‴ is reported. Both these phases can be characterized as incommensurately modulated composite crystal structures. Thereby, in both phases, the first composite subsystem contains atoms of the framework structure generating channels which are filled by atoms described by the second composite subsystem. The subsystems of the η″ phase have orthorhombic superspace groups Xmcm(00g)0s0 and Immm(00g)0s0 keeping the basic metric of the parent η phase. The subsystems of the η‴ can be described by the monoclinic superspace groups P21/c(0b0)00 and P21/c(0b0)s0 providing a lattice correspondence am = 1/2(a+b), bm = c and cm = a+b to the orthorhombic parent η phase. Although the number of atoms per channel is approx. 1.5 in η″ and η‴ phases, the channels are differently correlated with each other which is attributed to different Fe content in the channels inherited from the channel occupation in the disordered η phase.