Structure of the icosahedral Ti-Zr-Ni quasicrystal

The atomic structure of the icosahedral Ti-Zr-Ni quasicrystal is determined by invoking similarities to periodic crystalline phases, diffraction data and the results from ab initio calculations. The structure is modeled by decorations of the canonical cell tiling geometry. The initial decoration model is based on the structure of the Frank-Kasper phase W-TiZrNi, the 1/1 approximant structure of the quasicrystal. The decoration model is optimized using a new method of structural analysis combining a least-squares refinement of diffraction data with results from ab initio calculations. The resulting structural model of icosahedral Ti-Zr-Ni is interpreted as a simple decoration rule and structural details are discussed.Comment: 12 pages, 8 figure

Icosahedral AlCuFe alloys : towards ideal quasicrystals

The topological and chemical long range orders in the icosahedral Al 65Cu20Fe15 alloy have been studied by X-ray powder diffraction. The spectacular enhancement of the quasicrystal-line state by annealing of rapidly quenched samples have been quantitatively measured showing that the final annealed product fits the theoretical predictions of the ideal quasicrystal model within the instrument resolution. Whereas, after annealing at 1 073 K, Al 65Cu2oFe15 is two-phased — that makes difficult any fine study of the structure and the properties of the ideal icosahedral phase — a close composition Al63Cu25Fe12 leads to a single-phased icosahedral structure suitable for single quasicrystal X-ray and neutron diffraction analysis.Le désordre topologique et l'ordre chimique à grande distance ont été étudiés par diffraction des rayons X sur poudre dans l'alliage i-Al65Cu 20Fe15 brut de trempe. Le perfectionnement spectaculaire de l'état quasicristallin, obtenu par recuit d'échantillons hypertrempés, a été mesuré quantitativement ; le matériau recuit final correspond aux prévisions théoriques du modèle quasicristallin idéal dans les limites de la résolution instrumentale. Après recuit à 1 073 K, l'alliage de composition nominale Al65Cu20Fe 15 est biphasé. La composition Al63Cu25Fe 12 conduit quant à elle à une structure icosaédrique monophasée qui convient à l'analyse par diffraction des rayons X ou des neutrons sur un mono quasi-cristal

Dynamical Diffraction and X-Ray Standing Waves from Atomic Planes Normal to a Twofold Symmetry Axis of the Quasicrystal AlPdMn

We have observed dynamical diffraction in the [[(02402̅ 4) and [[(04604̅ 6) reflections of the icosahedral quasicrystal AlPdMn in the back-reflection geometry ( θB = 90°). The x-ray fluorescence from the Al and Pd atoms exhibits strong standing wave behavior, similar to that observed in crystalline materials. The data indicate a long-range order of each species of atoms, with the coherent positions attributable to distributions of the Al and Pd, which we compare to a centrosymmetric model. We observe deviations from the model which imply small departures from inversion symmetry along the twofold symmetry axis and from the expected coherent fractions for Al.This article is from Physical Review Letters 82 (1999): 2904–2907, doi:10.1103/PhysRevLett.82.2904.</p