Crystal structure of τ5–TiNi2−xAl5 (x = 0.48) and isotypic {Zr,Hf}Ni2−xAl5−y

The crystal structure of the compound in the Al-rich region of the Ti–Ni–Al system, τ5–TiNi2−xAl5, x = 0.48, has been derived from X-ray powder and single crystal, neutron powder and electron diffraction (space group I4/mmm, a = 0.3984(2) nm, c = 1.4073(3) nm, RF2 = 0.0133). Titanium atoms were unambiguously located from neutron powder data. τ5 is isotypic with the crystal structure of ZrNi2Al5. Detailed transmission electron microscopy (TEM) in several crystallographic directions confirmed the lattice parameters and crystal symmetry. Although occupancy of Ni in the 4e site revealed a defect (occ. = 0.76), no significant homogeneity region was observed for this phase at 1020°C. Rietveld analyses of X-ray powder diffraction data for the Zr- and Hf-homologues confirmed for both compounds isotypism and revealed defects in the Ni sites and to a lesser extent also in the Al sites: ZrNi2−xAl5−y, x = 0.4, y = 0.4 and HfNi2−xAl5−y, x = 0.5, y = 0.2. The crystallographic relations among the structure types of Cu, TiAl3, ZrNi2Al5 and Zr(Ni,Ga)7 have been defined in terms of a Bärnighausen scheme.