43 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
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
Phonon-phason coupling in icosahedral quasicrystals
From relaxation simulations of decoration-based quasicrystal structure models
using microscopically based interatomic pair potentials, we have calculated the
(usually neglected) phonon-phason coupling constant. Its sign is opposite for
the two alloys studied, i-AlMn and i-(Al,Cu)Li; a dimensionless measure of its
magnitude relative to the phonon and phason elastic constants is of order 1/10,
suggesting its effects are small but detectable. We also give a criterion for
when phonon-phason effects are noticeable in diffuse tails of Bragg peaks.Comment: 7 pages, LaTeX, uses Europhys Lett macros (included
Effect of Ca addition on interface formation in Al(Ca)/Al2O3 composites prepared by gas pressure assisted infiltration
The aim of the work is to study interface formation between Al2O3 particles and Al(Ca) matrix in dependence of Ca content. Aluminium matrix composites (AMC) subjected to investigation were prepared by gas pressure assisted infiltration of alumina beds with aluminium-calcium alloys. It is shown that alumina particles in the AMC are covered with a monocaldum aluminates layer whose coherence increases with increasing amounts of Ca in the aluminium-calcium alloys. Moreover, Al4Ca intermetallic phases are formed with increasing Ca content and interconnect alumina particles. XRD confirms the presence of both CaAl2O4 and CaAl4O2 ternary phases. However, HRTEM analysis confirmed CaAl2O4 with a rather complex structure containing a high density of stacking faults. It appeared that annealing at 735 degrees C does improve consistency of interface for Al 2 waCa/Al2O3 AMC, but do not affect the thickness of the interface in dependence on annealing time. (C) 2016 Elsevier Ltd. All rights reserved