Crystal structures of the Al-Ti-Pt [tau] sub(5) and [tau] sub(6) phases solved by zonal precession electron diffraction

The earlier reported Al-Ti-Pt phases [tau] sub(5) and [tau] sub(6) 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 [tau] sub(5)-phase (FeCr-type, P4 sub(2)/mnm, a = 9.702 and c = 5.023 [Angstrom]) can be described by the formula Al sub(2)Ti sub(5)Pt sub(3), positioning 18 Ti, eight Pt and four Al atoms at five unique atomic positions in the unit cell. The [tau] sub(6)-phase (P2 sub(1)3, a = 6.8477 [Angstrom]) can be described by the formula Al sub(3)Ti sub(5)Pt sub(2). 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 [tau] sub(6)-phase can be related to a defect variant of the Au sub(4)Al-type structure. The structures of the studied phases are presented as the stacking of coordination polyhedra.