Co-rich decagonal Al-Co-Ni: predicting structure, orientational order, and puckering

We apply systematic methods previously used by Mihalkovic et al. to predict the structure of the `basic' Co-rich modification of the decagonal Al70 Co20 Ni10 layered quasicrystal, based on known lattice constants and previously calculated pair potentials. The modelling is based on Penrose tile decoration and uses Monte Carlo annealing to discover the dominant motifs, which are converted into rules for another level of description. The result is a network of edge-sharing large decagons on a binary tiling of edge 10.5 A. A detailed analysis is given of the instability of a four-layer structure towards $c$-doubling and puckering of the atoms out of the layers, which is applied to explain the (pentagonal) orientational order.Comment: IOP LaTex; 7 pp, 2 figures. In press, Phil. Mag. A (Proc. Intl. Conf. on Quasicrystals 9, Ames Iowa, May 2005

Twofold surface of the decagonal Al-Cu-Co quasicrystal

We have investigated the atomic structure of the twofold surface of the decagonal Al-Cu-Co quasicrystal using scanning tunneling microscopy and low-energy electron diffraction. We have found that most of the surface features can be interpreted using the bulk-structure model proposed by Deloudi and Steurer (S. Deloudi, Ph.D. thesis, ETH, Zürich, 2008). The surface consists of terraces separated by steps of various heights. Step heights and steps sequences match with the thickness and the stacking sequence of blocks of layers separated by gaps in the model. These blocks of layers define possible surface terminations consisting of periodic atomic rows which are aperiodically stacked. These surface terminations are dense (∼10 at./nm2) and are of three types. The first two types are pure or almost pure Al while the third one contains 30–40 at. % of transition-metal atoms. Experimentally, we observe three different types of fine structures on terraces, which can be interpreted using the three possible types of bulk terminations. Terraces containing transition metals exhibit a strong bias dependency and present a doubling of the basic 0.42 nm periodicity, in agreement with the 0.84 nm superstructure of the bulk. In addition, a high density of interlayer phason defects is observed on this surface that could contribute to the stabilization of this system through configurational entropy associated with phason disorder

High-resolution scanning tunneling microscopy investigation of the (12110) and (10000) two-fold symmetric d-Al-Ni-Co quasicrystalline surfaces

The d-Al-Ni-Co quasicrystal exhibits two crystallographically inequivalent twofold symmetric planes: the {10000} and {12110} surface planes. In particular the twofold symmetric surfaces are very interesting since they are spanned by perpendicular periodic and aperiodic axes. We investigated the (12110) surface of two different compositions d-Al70Ni15Co15 and d-Al72.9Ni10.4Co16.7 and the (10000) d-Al72.9Ni10.4Co16.7 surface by means of low-temperature (5 K) scanning tunneling microscopy (STM) in ultrahigh vacuum. All three surfaces are characterized with atomic resolution and display large and flat terraces with a columnar structure along the periodic axis. Both compositions of the (12110) d-Al-Ni-Co STM investigations revealed that the surface is faceted into {12110} and {10000} facets with a 1: 1 area ratio. The (12110) facets represent the bulk periodicity of 0.4 nm within the prominent columnar structure which is attributed to a stacking of Al dimers. The sequence of the columnar structure along the aperiodic axis as well as the step heights between the (12110) surfaces could be attributed to the pentagonal tiling edge length of the bulk model. Furthermore, the (12110) surface is identified as one of the densest planes in the bulk model possessing a slightly lower Al concentration as the nominal bulk composition. However, a difference in the fine structure within the columns is observed between the two investigated compositions. The (10000) facet presents an identical surface structure as the unfaceted (10000) surface of the quasicrystal. At the (10000) surface two terraces with different surface structures are identified and compared to the bulk model. One of them shows a bias voltage depending structure. In contrast to the bulk model the minimal observed periodicity on the (10000) surface is doubled to 0.8 nm. On the other hand the aperiodic step height sequence and the order of the columnar motifs along the aperiodic axis are in agreement with the bulk model. The height distribution analysis of the (10000) surfaces exhibits that the surface topmost layer consists of planes which are less dense than the (12110) surface planes

Colloidal quasicrystals with 12-fold and 18-fold diffraction symmetry

Micelles are the simplest example of self-assembly found in nature. As many other colloids, they can self-assemble in aqueous solution to form ordered periodic structures. These structures so far all exhibited classical crystallographic symmetries. Here we report that micelles in solution can self-assemble into quasicrystalline phases. We observe phases with 12-fold and 18-fold diffraction symmetry. Colloidal water-based quasicrystals are physically and chemically very simple systems. Macroscopic monodomain samples of centimeter dimension can be easily prepared. Phase transitions between the fcc phase and the two quasicrystalline phases can be easily followed in situ by time-resolved diffraction experiments. The discovery of quasicrystalline colloidal solutions advances the theoretical understanding of quasicrystals considerably, as for these systems the stability of quasicrystalline states has been theoretically predicted for the concentration and temperature range, where they are experimentally observed. Also for the use of quasicrystals in advanced materials this discovery is of particular importance, as it opens the route to quasicrystalline photonic band gap materials via established water-based colloidal self-assembly techniques

Twofold surface of the decagonal Al-Cu-Co quasicrystal

We have investigated the atomic structure of the twofold surface of the decagonal Al-Cu-Co quasicrystal using scanning tunneling microscopy and low-energy electron diffraction. We have found that most of the surface features can be interpreted using the bulk-structure model proposed by Deloudi and Steurer (S. Deloudi, Ph.D. thesis, ETH, Zürich, 2008). The surface consists of terraces separated by steps of various heights. Step heights and steps sequences match with the thickness and the stacking sequence of blocks of layers separated by gaps in the model. These blocks of layers define possible surface terminations consisting of periodic atomic rows which are aperiodically stacked. These surface terminations are dense (∼10 at./nm2) and are of three types. The first two types are pure or almost pure Al while the third one contains 30–40 at. % of transition-metal atoms. Experimentally, we observe three different types of fine structures on terraces, which can be interpreted using the three possible types of bulk terminations. Terraces containing transition metals exhibit a strong bias dependency and present a doubling of the basic 0.42 nm periodicity, in agreement with the 0.84 nm superstructure of the bulk. In addition, a high density of interlayer phason defects is observed on this surface that could contribute to the stabilization of this system through configurational entropy associated with phason disorder.This article is from Physical Review B 80, no. 2 (2009): 024201, doi:10.1103/PhysRevB.80.024201.</p