Zirconium–aluminium ordering in Zr5Ni4Al—a new superstructure in the family of U3Si2 related intermetallics

Zr5Ni4Al was synthesized by arc-melting of the elements and subsequent annealing in an induction furnace at 1360–1440 K. This aluminide crystallizes according to X-ray diffraction data of a twinned crystal with an ordered version of the Zr3Al2 type: P42/m, a=716.7(3), c=659.8(4) pm, wR2=0.1174, 824 F 0 2 values, 28 variables. Due to the ordering of the zirconium and aluminium atoms, the space group symmetry is reduced from P42/mnm (Zr3Al2) to P42/m (Zr5Ni4Al), leading to merohedric twinning with a twin ratio of 52.5: 47.5 ( twin matrix 010 100 00 1 ¯ ) . The Zr5Ni4Al structure is an intergrowth of slightly distorted AlB2 and CsCl/W related slabs of compositions ZrNi2, ZrAl, and Zr2 with an Ni–Ni distance of 257 pm in the distorted AlB2 slab. A metastable Zr3Al2 type (P42/mnm) form of Zr5Ni4Al showing a disordered distribution of ZrAl and Zr2 slabs was obtained by crystallizing a metallic glass of the composition Zr50Ni40Al10 at 770 K. Further annealing at higher temperature led to the formation of ordered Zr5Ni4Al. The crystal chemical peculiarities of the different forms of Zr5Ni4Al are discussed on the basis of a group-subgroup scheme.