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

Al2+delta Fe intermetallic for 0.4 < delta < 0.7, frequently also referred to as eta-Al5Fe2, has been investigated in detail by powder-X-ray diffraction. The lattice parameters of the disordered orthorhombic eta-Al2+delta Fe phase quenched from 750 degrees C were precisely determined. The lattice parameter c(eta) increases non-linearly with increasing Al content, whereas a(eta) increases and b(eta), decreases linearly to a respectively lesser extent. Moreover, the recently reported long-range ordered eta-Al2+delta Fe low-temperature phase (C2/c symmetry) was confirmed to exist around an ideal value of = 2/3 after annealing at 250 degrees C or 350 degrees 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 a(eta') = 11.35940(5) angstrom = 6.42493(2) A and c(eta') = 8.68218(3) angstrom and beta'(eta') = 104.2519(3)degrees determined by X-ray diffraction. These values imply a change of the beta angle pertaining to the original orthorhombic cell to beta'(eta') = 90.51 degrees. Experimentally determined atomic positions from Rietveld refinement of the eta' 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 eta phase. (C) 2017 Elsevier B.V. All rights reserved.