Polymorphic phase transformation of Fe4Al13 causing multiple twinning with decagonal pseudo-symmetry

The polymorphic phase transformation of alloys Fe 25 Al 75 −Fe 23 Al 77 found recently by metallographical investigation was confirmed by X-ray diffraction. The high-temperature phase Fe 4 Al 13 (h) has an oB ∼ 50 = q unit cell, a = 7.7510 (9), b = 4.0336 (5) and c = 23.771 (3) A, space group Bmmm, c/a ≃ tan 2π/5. The low-temperature modification Fe 4 Al 13 (1) shows the structure of the Fe 4 Al 13 type [Black (1955a). Acta Cryst. 8, 43-48]. A twinning model is discussed in reciprocal space for structures with the orthorhombic face-centred Bravais lattice having the axial ratio c/a = tan 2π/5, i.e. the ratio d 002 /d 101 = ((5) 1/2 + 1]/2 = τ. Multiple-twinned Fe 4 Al 13 alloys yield diffraction patterns (hOl) q with decagonal pseudo-symmetry