Powder-X-ray diffraction analysis of the crystal structure of the η′-Al8Fe3 (η′-Al2.67Fe) phase

Al2+δFe intermetallic for 0.4 < δ < 0.7, frequently also referred to as η-Al5Fe2, has been investigated in detail by powder-X-ray diffraction. The lattice parameters of the disordered orthorhombic η-Al2+δFe phase quenched from 750 °C were precisely determined. The lattice parameter cη increases non-linearly with increasing Al content, whereas aη increases and bη decreases linearly to a respectively lesser extent. Moreover, the recently reported long-range ordered η′-Al2+δFe low-temperature phase (C2/c symmetry) was confirmed to exist around an ideal value of δ = 2/3 after annealing at 250 °C or 350 °C. The monoclinic long-range ordering of some Al atoms in channels of a framework structure is accompanied by a characteristic ordering strain, which is revealed by the refined lattice parameters aη' = 11.35940(5) Å, bη' = 6.42493(2) Å and cη' = 8.68218(3) Å and βη' = 104.2519(3)° determined by X-ray diffraction. These values imply a change of the β angle pertaining to the original orthorhombic cell to β'η' = 90.51°. Experimentally determined atomic positions from Rietveld refinement of the η′ phase well agree with results from relaxed density functional energy calculations and can be related with earlier reported strongly anisotropic displacement parameters of certain atomic sites in the disordered η phase.