Structural refinement of 1/1 bcc approximants to quasicrystals: Bergman-type W(TiZrNi) and Mackay-type M(TiZrFe)

We report the structural refinement of large-unit-cell bcc crystalline phases found in Ti-Zr-Ni and Ti-Zr-Fe alloys, which are 1/1 rational approximants of icosahedral quasicrystals in the same alloys. The structure of the stable 1/1 phase W(TiZrNi), lattice constant a(o) = 14.317 Angstrom, determined by a Rietveld analysis of x-ray and neutron powder diffraction data, is closely related to that of the 1/1 phases R(AlLiCu) and Bergman(AlMgZn), containing Bergman-type icosahedral clusters of atoms. Despite the similar chemistry of the 1/1 phases in Ti-Zr-Ni and Ti-Zr-Fe alloys, the 1/1 phase M(TiZrFe) contains double-shell Mackay icosahedra, Like those found in the 1/1 phase alpha(TiCrSiO). These results provide starting structures for six-dimensional refinements of the related quasicrystals. [S0163-1829(98)02729-5].