Formation and crystal chemistry of cubic ternary phases with filled Th6Mn23-type and AuCu3-type in the systems Ti–MVIII–Al

Ternary compounds formed by Ti and Al with transition metals of the eighth group have been investigated by X-ray powder/single crystal and neutron powder diffraction. Two series of compounds have been identified crystallizing either with a new filled variant of the Th6Mn23-type (G-phase, space group Fm3̄m) or with the AuCu3-type (space group Pm3̄m). Structure refinements of the G-phases reveal for the chemical formula TiMAl∼2 (Ti32M22Al46; M=Ru, Os, Rh, Ir; and Ti25M25Al50; M=Pd, Pt,) a rather high degree of atom order with a random distribution of Ti/Al in only the 4b(M2), 24e(M4) and 32f(M6) sites: M-atoms are located in the 4a(M1) and 24d(M3) sites, whereas the 32f(M5) sites accept only Al-atoms. Ti-atoms prefer the 24e(M4) site. General details of site preference have been determined and are discussed. No hydrogen was absorbed by Ti32Ru22Al46, Ti32Rh22Al46 and Ti32Ir22Al46 at room temperature under an H2 pressure of 5 MPa. It was demonstrated that refinement of neutron powder diffraction data allows precise evaluation of symmetry and atom site occupation for the AuCu3-type phases, Ti25.5Ni8.5Al66 and Ti26Pt6Al68: titanium atoms were found to be located only in the 1a site whilst Ni- and Pt-atoms substitute for aluminium in the 3c sublattice. The Re-, Os-, and Pt-based compounds show an equal average number of electrons in both crystallographic sites, thus reflections with mixed indices are vanishing in the X-ray profiles masking the AuCu3-type by the appearance of simple Cu-type X-ray patterns. Phase equilibria have been derived at 950 °C for the Ti–Ir–Al partial system αTi3Al–TiAl3–G-phase.