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

A molecular overlayer with the Fibonacci square grid structure

Quasicrystals differ from conventional crystals and amorphous materials in that they possess long-range order without periodicity. They exhibit orders of rotational symmetry which are forbidden in periodic crystals, such as five-, ten-, and twelve-fold, and their structures can be described with complex aperiodic tilings such as Penrose tilings and Stampfli-Gaehler tilings. Previous theoretical work explored the structure and properties of a hypothetical four-fold symmetric quasicrystal-the so-called Fibonacci square grid. Here, we show an experimental realisation of the Fibonacci square grid structure in a molecular overlayer. Scanning tunnelling microscopy reveals that fullerenes (C ) deposited on the two-fold surface of an icosahedral Al-Pd-Mn quasicrystal selectively adsorb atop Mn atoms, forming a Fibonacci square grid. The site-specific adsorption behaviour offers the potential to generate relatively simple quasicrystalline overlayer structures with tunable physical properties and demonstrates the use of molecules as a surface chemical probe to identify atomic species on similar metallic alloy surfaces

About the chemistry of the terminating 5f-planes in i-AlPdMn : A comparison between models and surface studies

Structural arguments are proposed that support the hypothesis that the equilibrium 5f-surfaces of i-AlPdMn icosahedral alloys are actual cuts of the bulk structure with almost pure Al termination planes [1, 2] corresponding to actual bulk properties with no need for possible Al segregation on the surface. Most of the 5f termination planes in i-AlPdMn are made of two close planes containing essentially Al at the upper one and Al and Pd at the second one (see for instance [3]). Here, perfect unrelaxed structural models that differ only in the chemical distribution of the atomic species are devised, close to the description proposed in [4]. Once compared to experimental X-rays data [5], the calculations show that although the chemical decoration has a rather limited in uence on the global R-factor, there is a clear tendency toward favoring models with almost pure Al on the atomic surface located on the node n, in excellent agreement with the surface studies.6th International Conference on Aperiodic Crystals (APERIODIC’09), 226 (2010): 012007, doi: 10.1088/1742-6596/226/1/012007. Posted with permission.</p

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