Crystal structure of η'-Fe₃Al₈; low-temperature phase of η-Fe₂Al₅ accompanied by an ordered arrangement of Al atoms of full occupancy in the c-axis chain sites

The low-temperature phase of η-Fe₂Al₅ has been identified experimentally for the first time and the crystal structure of the low-temperature phase (designated as η') has been refined by scanning transmission electron microscopy, combined with first-principles calculations. The low-temperature η' phase has a tripled superlattice structure based on the orthorhombic η-Fe₂Al₅ (space group Cmcm), in which Al atoms occupy two ninths of the partially occupied Al sites (Al2 and Al3) in a chain along the c-axis of the orthorhombic parent lattice in an ordered manner, giving rise to the monoclinic space group C2/c, the stoichiometric composition Fe₃Al₈ (Al-27.3 at.%Fe), and the Pearson symbol mC44. The partially occupied Al sites along the c-axis of the orthorhombic parent lattice are converted to the Al site (Al3″ site) of full occupancy in the monoclinic superlattice of the η'-Fe₃Al₈. The η-η' phase transformation temperature decreases from between 350 and 400 °C at the composition of Al-26.7 at.%Fe to between 300 and 350 °C at compositions in the range of Al-27.3 to 28.6 at.%Fe, indicating that the η' phase exists primarily at the Fe-lean side of the solid-solubility region of the η-Fe₂Al₅ phase.