Crystal structures of the Al-Ti-Pt τ5 and τ6 phases solved by zonal precession electron diffraction

The earlier reported Al-Ti-Pt phases τ5 and τ6 were studied using Precession Electron Diffraction (PED) and Convergent Beam Electron Diffraction (CBED). Their atomic models were proposed applying Direct Methods to the PED zonal data. The τ5-phase (FeCr-type, P42/mnm, a = 9.702 and c = 5.023 Å) can be described by the formula Al2Ti5Pt3, positioning 18 Ti, eight Pt and four Al atoms at five unique atomic positions in the unit cell. The τ6-phase (P213, a = 6.8477 Å) can be described by the formula Al3Ti5Pt2. Its unit cell consists of 20 atoms at 12b and two 4a atomic positions occupied by Ti, Pt and Al atoms, respectively. Due to deviation from the stoichiometry, the τ6-phase can be related to a defect variant of the Au4Al-type structure. The structures of the studied phases are presented as the stacking of coordination polyhedra.