54 research outputs found
Energy-based Structure Prediction for d(Al70Co20Ni10)
We use energy minimization principles to predict the structure of a decagonal
quasicrystal - d(AlCoNi) - in the Cobalt-rich phase. Monte Carlo methods are
then used to explore configurations while relaxation and molecular dynamics are
used to obtain a more realistic structure once a low energy configuration has
been found. We find five-fold symmetric decagons 12.8 A in diameter as the
characteristic formation of this composition, along with smaller
pseudo-five-fold symmetric clusters filling the spaces between the decagons. We
use our method to make comparisons with a recent experimental approximant
structure model from Sugiyama et al (2002).Comment: 10pp, 2 figure
The Co-Ni distribution in decagonal Al69.7(4)Co10.0(4)Ni20.3(4)
The Co-Ni distribution in d-Al69.7(4)Co10.0(4)Ni20.3(4) was investigatedbased on X-ray and neutron diffraction data. The structure was modelledin higher dimensional space using the ‘charge-flipping’ and ‘low-densityelimination’ methods and it was quantitatively refined inthree-dimensional space employing a pseudo-approximant approach. Inhigher-dimensional description, the Co atoms are found at the centre ofone of the two symmetry independent occupation domains, enclosed byregions mainly occupied by Ni. The other occupation domain is mostlyoccupied by Al. In physical space Co atoms are located in the centres ofsmall Al pentagons and form pentagonal units, which are arranged indecagonal rings. On these sites Co is partly substituted by Ni, whileall other transition metal sites are occupied by Ni and to a minordegree by Al. The fraction of Co found on transition metal sitesdecreases with decreasing Co-Co distances, whereby Co is replaced by Ni
Quasiperiodic ordering in thick Sn layer on -Al-Pd-Mn: A possible quasicrystalline clathrate
Realization of an elemental solid-state quasicrystal has remained a distant
dream so far in spite of extensive work in this direction for almost two
decades. Here, we report the discovery of quasiperiodic ordering in a thick
layer of elemental Sn grown on icosahedral ()-Al-Pd-Mn. The STM images and
the LEED patterns of the Sn layer show specific structural signatures that
portray quasiperiodicity but are distinct from the substrate. Photoemission
spectroscopy reveals the existence of the pseudogap around the Fermi energy up
to the maximal Sn thickness. The structure of the Sn layer is modeled as a
novel form of quasicrystalline clathrate on the basis of the following:
Firstly, from ab-initio theory, the energy of bulk Sn clathrate quasicrystal is
lower than the high temperature metallic -Sn phase, but higher than the
low temperature -Sn phase. A comparative study of the free slab
energetics shows that surface energy favors clathrate over -Sn up to
about 4 nm layer thickness, and matches -Sn for narrow window of slab
thickness of 2-3 nm. Secondly, the bulk clathrate exhibits gap opening near
Fermi energy, while the free slab form exhibits a pronouced pseudogap, which
explains the pseudogap observed in photoemission. Thirdly, the STM images
exhibit good agreement with clathrate model. We establish the adlayer-substrate
compatibility based on very similar (within 1%) the cage-cage separation in the
Sn clathrate and the pseudo-Mackay cluster-cluster separation on the
-Al-Pd-Mn surface. Furthermore, the nucleation centers of the Sn adlayer on
the substrate are identified and these are shown to be a valid part of the Sn
clathrate structure. Thus, based on both experiment and theory, we propose that
4 nm thick Sn adlayer deposited on 5-fold surface of -Al-Pd-Mn substrate is
in fact a metastable realization of elemental, clathrate family quasicrystal.Comment: 10 figures in the Manuscript and the 8 figures in the Supplementary
materia
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