STRUCTURAL REFINEMENT OF 1 1 BCC APPROXIMANTS TO QUASI-CRYSTALS - 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 stru cture of the stable 1/1 phase W(TiZrNi), lattice constant a(o) = 14.31 7 Angstrom, determined by a Rietveld analysis of x-ray and neutron pow der diffraction data, is closely related to that of the 1/1 phases R(A lLiCu) and Bergman(AlMgZn), containing Bergman-type icosahedral cluste rs 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). T hese results provide starting structures for six-dimensional refinemen ts of the related quasicrystals. [S0163-1829(98)02729-5].