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Combining inelastic neutron and x-ray scattering with atomic scale simulation, we report on a comprehensive study of the lattice dynamics and its relationship with the low thermal conductivity of the o-${\mathrm{Al}}_{13}{\mathrm{Co}}_{4}$ phase, a periodic approximant (about 100 atoms per cell) to a decagonal quasicrystal. The obtained experimental data, phonon lifetimes, and temperature-independent lattice thermal conductivity can only be fully described by molecular dynamics simulations, using oscillating pair potential, when the disorder of 12 Al sites is properly taken into account. Our results pave the way for a detailed understanding of materials where structural complexity and local disorder are at play.

International audience; We report the adsorption of lead adatoms on the pseudo-10-fold Al13Fe4(0 1 0) surface using low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). For the submonolayer regime, Pb adatoms remain highly mobile across the surface at 300K. STM analysis indicates the formation of irregularly shaped islands of monoatomic height. The latter do not coalesce with increasing coverage. At 0.95 MLE coverage, the LEED patterns are consistent with a pseudomorphic growth of the adatoms. This is confirmed by STM measurements which reveal local motifs qualitatively similar to those observed on the clean Al13Fe4(0 1 0) surface, i.e. prior to dosing. Apart from the absence of plasmons, the XPS measurements of Pb 4f and Al 2s core levels are comparable to those observed for the Pb/Al(1 1 1) system.

The oxidation of the Al4(Cr, Fe) quasicrystal approximant has been studied using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). STM data indicate that the initial oxidation on the (1 0 0) surface proceeds in a highly ordered fashion. Oxygen preferentially adsorbs onto the surface of terraces, and step edges remain intact. The STM images show ordered stripes after initial oxidation. XPS data indicate that only Al is oxidised, with at least two oxidation states present. The Cr and Fe peaks remain unchanged. The oxidation of Al, rather than Cr or Fe, is consistent with the enthalpies of formation for each oxide. The stripes visible on the STM images are therefore identified to be the initial stages of aluminium oxide formation. XPS performed after higher O2 exposure indicates that the (1 0 0) termination shows hindered Al oxide film thickness growth rates compared to the (0 1 0) and (0 0 1) surfaces.

Czochralski growth of cm size decagonal AlCoNi single crystals from Al-rich high-temperature solutions is described using native seeds oriented parallel to all symmetrically different crystal directions. Morphological observations allow first hints with respect to anisotropic growth rates. But only classical detachment experiments according to the original idea of Jan Czochralski reveal quantitative results of kinetically limited growth rates. Geometric conditions of wetted plane interfaces as well as aspects of constitutional supercooling affect the detachment experiments. Thus, in only one specific orientation of the decagonal quasicrystal quantitative data for the maximum growth rate could be obtained.

The Hall coefficient RH in monoclinic Y-Al-Ni-Co, orthorhombic Al13Co4 and monoclinic Al13Fe4 and Al13(Fe, Ni)4 single crystals has been investigated for all combinations of the electrical current and magnetic field directions and in the temperature interval from 90 to 370 K. In all three intermetallics, which belong to the Al13TM4 (TM = transition metal) class of approximants to the decagonal quasicrystals, the Hall coefficient exhibits well defined anisotropy. RH is positive hole-like or zero for the magnetic field parallel to the plane that corresponds to the quasiperiodic plane in decagonal (d) quasicrystals, and is negative electron-like or zero for the magnetic field perpendicular to this plane. The only exeption is RH in Al13Fe4 for the field parallel to the stacking direction, which changes the sign from positive to negative value with the increase of temperature. The results for the anisotropy of RH are correlated to the anisotropy of RH in d-Al-Ni-Co and d-Al-Cu-Co quasicrystals and a brief overview of the theoretical results is presented.

We compare the growth of a Bi thin film on the ten-fold surface of the decagonal Al-Ni-Co quasicrystal at different substrate temperatures and fluxes using scanning tunneling microscopy (STM) and l...

We present an experimental and theoretical study of the anisotropic Hall coefficient R H of the Y–Al–Ni–Co periodic approximant to the decagonal phase with composition Al76Co22Ni2. Performing ab-initio calculation of R H for the original Y–Al–Ni–Co structural model [B. Zhang, V. Gramlich, W. Steurer, Z. Kristallogr. 210 (1995) 498] and its relaxed version, we reproduced the experimentally observed anisotropy for all combinations of crystalline directions of the electric current and magnetic field, where the relaxed model yielded better quantitative matching to the experiment. The origin of the anisotropic Hall coefficient is the anisotropic Fermi surface, the anisotropy of which originates from the specific stacked-layer structure of the Y–Al–Ni–Co compound and the chemical decoration of the lattice. Due to the structural similarity of Y–Al–Ni–Co to the d -Al–Ni–Co-type decagonal quasicrystals, the same physical picture explains the universal R H anisotropy of this family of quasicrystals, where R H changes sign along different crystalline directions.

This paper addresses some issues related to the nanomechanical responses of decagonal quasicrystals using nanoindentation techniques. A Hysitron Triboscope with a Berkovich indenter was used to carry out the nanoindentation tests on single crystals of Al–Co–Ni decagonal phase. The anisotropy in terms of nanohardness at higher load was observed, whereas it was not possible to establish the same at lower loads. However, the Young's modulus was found to be indistinguishable in all these cases. Discontinuity in load displacement, which is known as a ‘pop-in’ effect, was observed frequently at various load regime. It was found that this discontinuity was not due to cracking or a phase transformation effect, but to a plastic yielding phenomena and nanodeformation of the material. Significant differences between the nanohardness and microhardness were noticed and are discussed based on various experimental parameters and the consequent mechanical response of materials.

Czochralski growth of decagonal AlCoCu was investigated using spontaneous nucleation as well as AlCoNi seeds of different orientations. Contrary to all knowledge gained from the growth of large single crystals in the Al–Co–Ni system, growth of AlCoCu quasicrystals proved to be much more complicated. Second-phase inclusions were found to be the main problem, resulting in various features affecting regular crystal growth. Even at extremely low pulling rates, this could only be solved by growing along the two-fold [10000]-direction. Finally, large-grain Al67.5Co20.0Cu12.3 single crystals, having a rather homogeneous elemental distribution, were successfully grown from the initial melt of composition Al66Co10Cu24.

Directional solidification of ternary melts in a series of Al-based systems (Al–Co–Cu, Al–Co–Ni, and Al–Cu–Fe) has been studied using the Bridgman technique at a very low growth rate. These systems have been chosen to investigate the phase equilibria of quasicrystals and structurally complex crystalline compounds in the vicinity of quasicrystal-forming composition regions. The Bridgman-grown ingots were characterized by optical microscopy, electron microprobe analysis, powder X-ray diffraction, and differential thermal analysis. Using this combination, it was possible to study phase equilibria and crystallization processes with respect to the liquidus surface projection. The solidification paths for the ternary quasicrystal-forming alloys and structurally complex crystalline phases traced in the Bridgman-grown specimens have been discussed within the framework of the Scheil–Gulliver solidification model. It has been proved that the Bridgman method is a feasible tool to study the complex solidification processes of ternary alloys.

In the ternary Al–Co–Ni system, decagonal quasicrystals occur that have proved thermodynamically stable. Large single crystals can be grown using rather conventional crystal growth techniques from incongruent melts. Decagonal quasicrystals combine two different types of order, periodicity along the 10-fold rotational axis and non-periodic order perpendicular to it, which makes them particularly interesting for solid state physics. Based upon X-ray structure analysis data obtained from AlCoNi single crystals, the question has been discussed whether or not inclined net planes exist that intersect periodic as well as non-periodic directions in these quasicrystals. Several inclined net planes were proposed and ranked with respect to their structure amplitudes. We have studied single crystal growth of large decagonal Al72Co9Ni19 quasicrystals using the Czochralski method from Al-rich melts. The crystal surfaces show a remarkable faceting. Exact angle measurements by optical goniometry revealed that the observed morphological faceting perfectly corresponds to some of the theoretically predicted inclined net planes. Besides the two different decagonal prisms { 1 0 0 0 0 } and {1 0 1¯ 0 0} and the pinacoid { 0 0 0 0 1 } we have found the inclined planes of two decagonal dipyramids {0 0 1¯ 1¯ 1} and {1 0 1¯ 0 2}.

The twofold decagonal Al71.8Ni14.8Co13.4(1 0 0 0 0) surface is investigated by using highly surface sensitive elastic helium atom scattering (HAS) and high resolution spot profile analyzing low energy electron diffraction (SPA-LEED). SPA-LEED and HAS data reveal diffraction patterns of the top most surface layer consistent with those of the surfaces generated by bulk truncations. Strong diffraction peaks corresponding to an 8 A periodicity are observed. These peaks are broadened along the quasiperiodic surface direction, reflecting a reduced correlation length of ≈80 A of the 8 A periodicity within the quasicrystalline planes. In addition very weak streaks corresponding to a 16 A periodicity are observed.

The solidification of a series of Bridgman-grown Al-Co-Cu alloys with compositions in the vicinity of the quasicrystal was studied by powder X-ray diffraction (XRD), differential thermal analysis (DTA), electron microprobe analysis, and optical microscopy. The phase equilibria and microstructure of solidified alloys are presented; the temperatures of the involved solidification reactions were determined. These experimental data were used to construct a solidification phase diagram as to understand the crystallization path. The decagonal (D) AlCoCu quasicrystals form incongruently, but they can be primarily solidified from off-stoichiometric melts.

Decagonal Al–Ni–Co quasicrystals were cleaved in ultrahigh-vacuum and the resulting surfaces were investigated by dynamic scanning force microscopy. The samples were cleaved perpendicular to the tenfold axis and perpendicular to one of the twofold axes. Both surfaces show a rough structure with lateral features on the nanometer scale and height differences of angstroms to nanometers. While the corrugation of the tenfold surface does not show any correlation to the quasiperiodic bulk structure, the twofold surface exhibits row-like corrugations, which indicate the existence of columnar structure motifs along the direction of the tenfold axis as expected from structure models. Images from surface regions tilted with respect to the twofold plane strongly indicate the existence of inclined netplanes. In addition, we studied surfaces of an n 0 -(Al–Pd–Mn) quasicrystal approximant, which was cleaved perpendicular to the pseudo-tenfold b-axis. These surfaces show a corrugated structure as well, similar to the results obtained from the decagonal Al–Ni–Co surfaces. There is no indication of a correlation to the periodicity or other structural features of this orthorhombic, crystalline material. � 2004 Elsevier B.V. All rights reserved.

The two-fold decagonal-Al71.8Ni14.8Co13.4(0 0 1 1 0) surface is investigated by highly surface sensitive He atom scattering. The diffraction pattern shows that the topmost surface layer preserves a perfect quasi-crystalline ordering expected from the bulk. The surface is found to develop facets with (1 0 0 0 0)-equivalent orientation, inclined by 18° to the original surface. The facets are of sufficient size to observe diffraction from them. The facet planes are identical to those identified in an as grown sample.

Powder samples of Al 70.7 Co 13.3 Ni 16.0 were investigated by X-ray diffraction in the temperature range 15–300 K. At T =150 K at least three additional reflections were observed. The onset temperature and hysteresis-like behaviour was monitored more closely. The reflections cannot be indexed using integers of reflections of the known decagonal lattice. Moreover, near this temperature lattice parameters of the decagonal phase showed deviations from a smooth curve. We suggest the occurrence of an intermediate phase, which, however, seems to be a property of the powdered sample, i.e. caused by milling single crystals. We will solely describe the experimental facts as they are and make a suggestion for the transition mechanism. It is assumed that the new reflections are due to a periodic superorder of the decagonal lattice as described for a Ni-rich alloy in the literature. From the distance of those satellite reflections to main (Bragg) reflections in reciprocal space we estimate the modulation length of the quasicrystalline substructure to be 10 a , i.e. ∼38 A.

In the ternary Al–Co–Ni system, decagonal quasicrystals occur that are thermodynamically stable and can be grown using rather conventional crystal growth techniques at near-equilibrium conditions. Detailed studies of the Al-rich part of the ternary phase diagram including the primary crystallization field of the decagonal phase are known from the literature. Using these data quasicrystal growth by the Czochralski method from off-stoichiometric (Al-rich) melts has proved to be successful. In a series of growth experiments with varying Co:Ni ratio along the 1050 and 1100°C isotherms, single-grain quasicrystals up to 1 cm 3 in size have been grown. Use was made of very low growth rates (0.2–0.5 mm/h) as to meet the needs of the constitutional supercooling criterion as well as those arising from the anisotropic growth kinetics of the decagonal phase. The segregation behaviour was studied by electron probe microanalysis (EPMA) measurements concerning the change in the Al concentration during growth and the shift in the Co:Ni ratio which marks the main extension of the decagonal stability region.

The quasicrystalline order of a decagonal, Co-rich AlNiCo alloy was investigated using spot profile analysis low energy electron diffraction (SPA-LEED). The surface gives rise to a tenfold diffraction pattern with the peak positions being related by the golden mean, indicating quasicrystallinity in the surface region. From the half-widths of the specular spot as a function of the electron energy, the step height was determined to be (2.05±0.05) A, a value which agrees with the bulk distance of the fivefold quasiperiodic planes of 2.04 A. Kinematic model calculations revealed that the cluster structure of the bulk is intact also in the surface region. The existence of broad features in the diffraction pattern points to a certain degree of lateral disorder within the terraces.

The single crystal growth of decagonal quasicrystals in the Al–Co–Ni system has been investigated using the Czochralski method. Using a ceramic tip instead of a native seed, single-grain quasicrystals of average composition Al71Co18Ni11 and approximately 1 cm3 in size have been grown. Very low pulling rates (0.25–0.5 mm/h) were used. Axial segregation and structural perfection were investigated by electron-probe microanalysis and neutron diffraction. The liquidus temperature of the starting melt and composition of the decagonal phase agree well with the published phase diagram data.

To address the questions on the anisotropy of bulk physical properties of decagonal quasicrystals and the intrinsic physical properties of the $d$-Al-Co-Ni phase, we investigated the anisotropic magnetic susceptibility, the electrical resistivity, the thermoelectric power, the Hall coefficient, and the thermal conductivity of a $d$-Al-Co-Ni single crystal of exceptional structural quality. Superior structural order on the local scale of atomic clusters was confirmed by ${}^{27}$Al nuclear magnetic resonance spectroscopy. The measurements were performed in the 10-fold periodic direction of the structure and in three specific crystallographic directions within the quasiperiodic plane, corresponding to the 2 and 2\ensuremath{'} twofold symmetry directions and their bisector. The specific heat, being a scalar quantity, was determined as well. The measurements of the second-rank bulk tensorial properties confirm the theoretical prediction that a solid of decagonal point group symmetry should exhibit isotropic physical properties within the quasiperiodic plane and anisotropy between the in-plane and the 10-fold directions. $d$-Al-Co-Ni is an anisotropic diamagnet with stronger diamagnetism for the magnetic field in the 10-fold direction. Electrical and thermal transport is strongly metallic in the 10-fold direction but largely suppressed within the quasiperiodic plane, the main reason being the lack of translational periodicity that hinders the propagation of electrons and phonons in a nonperiodic lattice. The third-rank Hall-coefficient tensor shows sign-reversal anisotropy related to the direction of the magnetic field when applied in the 10-fold direction or within the quasiperiodic plane. The observed anisotropy is not a peculiarity of quasicrystals but should be a general feature of solids with broken translational periodicity in two dimensions.

The crystallographic structures of decagonal quasicrystals and their periodic approximants are traditionally described as a periodic stacking of atomic planes. By performing a A 27 l NMR spectroscopic study of the Al13 M4 (M=transition metal) family of four-layer decagonal approximants, including the orthorhombic o-Al13 Co4, the monoclinic Al13 Fe4, its ternary derivative Al13 (Fe,Ni) 4, and the monoclinic Al13 Ru4, we show that all these phases contain structural detail of a nearly linear M-Al-M atomic group trapped inside an elongated cage, resembling the three-dimensional (3D) \"cage-compound\" structure of the intermetallic clathrates. We determined the electric-field-gradient- (EFG) and the magnetic-shielding tensors at the Al site of the M-Al-M groups. The asymmetry parameter of the EFG tensor was estimated theoretically by a point-charge model, taking into account the charges of both the M-Al-M atoms and the surrounding cage atoms. The calculations support ionic bonding of the M-Al-M group to the cage atoms and the existence of a 3D chemical bonding network in the Al13 M4 phases. The above results show that the traditional description of the Al13 M4 decagonal approximant phases in terms of two-dimensional (2D) atomic layers stacked along the pseudotenfold crystallographic direction is a convenient geometrical approach to describe their complex structures but is not appropriate for the description of their physical properties, which should be analyzed by taking into account the full 3D nature of the chemical bonding framework. This favors the 3D cage-compound structural description of the Al13 M4 phases over the pseudo-2D stacked-layer description.

We have investigated the magnetic susceptibility, the electrical resistivity, the specific heat, the thermoelectric power, the Hall coefficient, and the thermal conductivity of the ${\text{Al}}_{13}{\text{Fe}}_{4}$ and ${\text{Al}}_{13}{(\text{Fe},\text{Ni})}_{4}$ monoclinic approximants to the decagonal quasicrystal. While the ${\text{Al}}_{13}{\text{Fe}}_{4}$ crystals are structurally well ordered, the ternary derivative ${\text{Al}}_{13}{(\text{Fe},\text{Ni})}_{4}$ contains substitutional disorder and is considered as a disordered version of the ${\text{Al}}_{13}{\text{Fe}}_{4}$. The crystallographic-direction-dependent measurements were performed along the ${a}^{\ensuremath{\ast}}$, $b$, and $c$ directions of the monoclinic unit cell, where the $({a}^{\ensuremath{\ast}},c)$ atomic planes are stacked along the $b$ direction. The electronic transport and the magnetic properties exhibit significant anisotropy. The stacking $b$ direction is the most conducting direction for the electricity and heat. The effect of substitutional disorder in ${\text{Al}}_{13}{(\text{Fe},\text{Ni})}_{4}$ is manifested in the large residual resistivity $\ensuremath{\rho}(T\ensuremath{\rightarrow}0)$ and significantly reduced thermal conductivity of this compound, as compared to the ordered ${\text{Al}}_{13}{\text{Fe}}_{4}$. Specific-heat measurements reveal that the electronic density of states at the Fermi level of both compounds is high. The anisotropic Hall coefficient ${R}_{H}$ reflects complex structure of the anisotropic Fermi surface that contains electronlike and holelike contributions. Depending on the combination of directions of the current and the magnetic field, electronlike $({R}_{H}l0)$ or holelike $({R}_{H}g0)$ contributions may dominate, or the two contributions compensate each other $({R}_{H}\ensuremath{\approx}0)$. Similar complicated anisotropic behavior was observed also in the thermopower. The anisotropic Fermi surface was calculated ab initio using the atomic parameters of the refined ${\text{Al}}_{13}{\text{Fe}}_{4}$ structural model that is also presented in this work.

In this paper we present an investigation of the (100) Y-Al75.8 Ni2.1 Co22.1 surface, which is structurally and chemically related to the (10000) twofold symmetric surface of the decagonal Al-Ni-Co quasicrystal. The atomic surface structure was probed by low-energy electron diffraction (LEED) and by scanning tunneling microscopy (STM), revealing three different surface terminations and three different types of surface reconstructions. All three terminations were successfully assigned to the densest bulk layers and the origin of the surface reconstructions revealed by LEED could be identified locally by STM. The (100) Y-Al-Ni-Co surface shares with the related (10000) d-AlNiCo quasicrystal surface the strong tendency of reconstruction, which in the former case is occurring along the b axis and in the later along the periodic [00001] direction. Gaining a theoretical understanding of the reconstructions of the (100) Y-Al-Ni-Co surface might therefore help to understand the stability of bulk and surface atomic structures in decagonal quasicrystals. Furthermore, due to the strong relation to the quasicrystalline (10000) d-AlNiCo surface, the crystalline (100) Y-Al75.8 Ni2.1 Co22.1 surface is now a very promising candidate to study directional anisotropies in epitaxial thin film growth, friction, or the electronic structure, with the advantage of representing a much simpler structure (32 atoms/unit cell) which can be theoretically addressed with less effort compared to its quasicrystalline counter part.

We present a detailed study of the (100) surface of the orthorhombic Al13Co4 crystal using both experimental and ab initio computational methods. This complex metallic alloy is an approximant of the decagonal Al-Ni-Co quasicrystalline phase. After sputter-annealing preparation of the surface at 1073 K, the low-energy electron diffraction pattern recorded exhibits a pseudotenfold symmetry with lattice parameters consistent with those of the bulk model. At this stage, scanning tunneling microscopy (STM) measurements reveal two different types of surface terminations. A comparison between these two surface structures and the bulk planes indicate that the terminations correspond either to an incomplete puckered layer (P) or to an incomplete flat layer (F). At 1173 K, the majority of the surface consists of P layer terminations. STM images calculated from our proposed surface model are in good agreement with experimental images. X-ray photoelectron diffraction patterns and single scattering cluster calculations further confirm that the local atomic arrangements present in the bulk model are preserved within the near-surface region.

The electrical (σ) and thermal conductivity (λ) of Al76Co22Ni2 decagonal approximant has been measured along three orthogonal directions a *, b and cof the Al76Co22Ni2 unit cell, where (a, c) monoclinic atomic planes are stacked along the perpendicular b direction. Electrical conductivity is relatively high in all crystalline directions, appearing in the order σb >> σc > σa* . Thermal conductivity appears in the same order λb > λc > λa*, so that b is the most conducting direction for both electricity and heat. The results are compared to the reported anisotropic electrical and thermal conductivity of the d-Al–Ni–Co decagonal quasicrystal.

We have investigated anisotropic electrical resistivity and thermoelectric power of the ypsilon-phase Al–Ni–Co (Y–Al–Ni–Co) decagonal approximant with composition Al76Co22Ni2. The crystalline-direction-dependent measurements were performed along three orthogonal directions a*, b and c of the Y–Al–Ni–Co unit cell, where (a, c) monoclinic atomic planes are stacked along the perpendicular b direction. Anisotropic electrical resistivity is low in all crystalline directions, appearing in the order ρ a* > ρ c > ρ b and showing positive temperature coefficient typical of electron-phonon scattering mechanism. Thermopower shows electron-phonon enhancement effect. Anisotropic bare thermopower (in the absence of electron-phonon interactions) was extracted, appearing in the same order as the resistivity, ΙS a* bare/TΙ > Ι S c bare/T Ι > Ι S b bare/T Ι.

Crystal growth studies starting from spherical surfaces of singlecrystals can reveal morphological features that cannot be obtained inusual growth experiments. Especially, facets of a greater variety ofcrystallographic forms may occur that are otherwise hidden by theslowest growing faces which dominate the crystal’s morphology. We did asphere growth experiment with a decagonal Al(72)Co(9)Ni(19) quasicrystalprepared from a large single-grain sample formerly grown using theCzochralski method from an off-stoichiometric melt. By heating thepolished crystalline half-sphere to a temperature slightly higher thanits own growth temperature a thin liquid layer of self-flux formed. Apartial evaporation of Al being the solvent of this system causedre-crystallisation at the surface of the quasicrystalline sphere. Afterseveral times repeating this process at further increasing temperatures,tiny faces of four different crystallographic forms developed. By usingan optical goniometer and electron scanning microscopy indexing of thesefacets was possible. With the only exception of the decagonal prism5100006 they had not been observed so far in contact to the melt. Theresults are discussed with respect to the inclined net plane conceptestablished by Steurer and Cervellino [Acta Cryst. A57 (2001)333-340]. The experimental approach is not specific to sphere growthstudies of quasicrystals but can be applied to other high-temperaturesolution growth systems as well.

International audience; We present a study of the anisotropic electrical, magnetic and thermal transport properties of a single-crystalline Al80Cr15Fe5 approximant to the decagonal quasicrystal. Temperature-dependent electrical resistivity along the b and c crystalline directions shows nonmetallic behavior with a broad maximum, whereas it shows metallic positive temperature coefficient along the a direction perpendicular to the (b,c) atomic planes. The resistivity can be treated with the model of slow charge carriers, where the increased electron-phonon scattering upon raising the temperature induces transition from dominant Boltzmann (metallic) to dominant non-Boltzmann (insulatinglike) regime. Magnetic susceptibility and thermal conductivity show small anisotropy as well.

We present a comprehensive study of Bi thin-film growth on quasicrystal surfaces. The substrates used for the growth are the fivefold surface of icosahedral $(i)$-Al-Cu-Fe and $i$-Al-Pd-Mn and the tenfold surface of decagonal $(d)$-Al-Ni-Co quasicrystals. The growth is investigated at 300 and 525 K substrate temperatures and at different coverage $(\ensuremath{\theta})$ ranging from submonolayer to ten monolayers. The film is characterized by scanning tunneling microscopy, reflection high-energy electron diffraction, and x-ray photoelectron spectroscopy. At 300 K, the deposited Bi yields a quasicrystalline film for $\ensuremath{\theta}\ensuremath{\le}1$. For $1l\ensuremath{\theta}l5$, it forms nanocrystallites with (100) surface orientation. The islands have magic heights, which correspond to the stacking of four atomic layers (predominantly). The selection of magic heights is interpreted in terms of quantum size effects arising from the electron confinement within the film thickness. The islands establish rotational epitaxial relationship with the substrate. For higher coverage, the film grows with monatomic height, not with magic heights, and reflects the symmetry of the bulk Bi. When deposition is performed at 525 K, terrace diffusion is more effective, resulting in the aggregation of Bi adatoms developing into a smooth monolayer with quasiperiodic order. At this temperature, multilayers do not adsorb.

We have investigated anisotropic physical properties (magnetic susceptibility, electrical resistivity, thermoelectric power, Hall coefficient, and thermal conductivity) of Y-Al-Ni-Co decagonal approximant with composition ${\text{Al}}_{76}{\text{Co}}_{22}{\text{Ni}}_{2}$. The crystalline-direction-dependent measurements were performed along three orthogonal directions ${a}^{\ensuremath{\ast}}$, $b$, and $c$ of the Y-Al-Ni-Co unit cell, where $(a,c)$ monoclinic atomic planes are stacked along the perpendicular $b$ direction. Anisotropic magnetic susceptibility of conduction electrons is paramagnetic for the field lying within the $(a,c)$ atomic planes and diamagnetic for the field along the $b$ direction. Anisotropic electrical resistivity is low in all crystalline directions, appearing in the order ${\ensuremath{\rho}}_{{a}^{\ensuremath{\ast}}}g{\ensuremath{\rho}}_{c}⪢{\ensuremath{\rho}}_{b}$. Thermopower shows electron-phonon enhancement effect. Anisotropic bare thermopower (in the absence of electron-phonon interactions) was extracted, appearing in the same order as the resistivity, $|{S}_{{a}^{\ensuremath{\ast}}}^{\text{bare}}/T|g|{S}_{c}^{\text{bare}}/T|g|{S}_{b}^{\text{bare}}/T|$. Anisotropic thermal conductivity appears in the order ${\ensuremath{\kappa}}^{b}g{\ensuremath{\kappa}}^{c}g{\ensuremath{\kappa}}^{{a}^{\ensuremath{\ast}}}$, so that $b$ is the most conducting direction for both electricity and heat. Hall coefficient ${R}_{H}$ exhibits pronounced anisotropy, where the magnetic field in a given crystalline direction yields the same ${R}_{H}$ for the current along the other two crystalline directions in the perpendicular plane. These anisotropies are analyzed in terms of the anisotropic structure of the Y-Al-Ni-Co phase and the ab initio calculated anisotropic Fermi surface. The results are compared with the literature-reported anisotropy of the physical properties of the $d$-Al-Ni-Co decagonal quasicrystal.

We present a study of the anisotropic transport properties (electrical resistivity, thermoelectric power, Hall coefficient, and thermal conductivity) of a single-crystalline ${\mathrm{Al}}_{80}{\mathrm{Cr}}_{15}{\mathrm{Fe}}_{5}$ complex metallic alloy that is an excellent approximant to the decagonal quasicrystal with six atomic layers in one periodic unit. Temperature-dependent electrical resistivity along the $b$ and $c$ crystalline directions shows a nonmetallic behavior with a broad maximum, whereas it shows a metallic positive temperature coefficient along the $a$ direction perpendicular to the $(b,c)$ atomic planes. Ab initio calculations of the electronic density of states reveal that the nonmetallic transport occurs in the presence of a high density of charge carriers. The very different temperature-dependent electrical resistivities along the three crystalline directions can all be treated within the same physical model of slow charge carriers due to weak dispersion of the electronic bands, where the increased electron-phonon scattering upon raising the temperature induces transition from dominant Boltzmann (metallic) to dominant non-Boltzmann (insulatinglike) regime. The temperature dependence of the resistivity is governed predominantly by the temperature dependence of the electronic diffusion constant $D$ and the transition has no resemblance to the Anderson-type metal-to-insulator transition based on the gradual electron localization. Structural considerations of the ${\mathrm{Al}}_{80}{\mathrm{Cr}}_{15}{\mathrm{Fe}}_{5}$ phase show that the anisotropy of the transport properties is a consequence of anisotropic atomic order on the scale of nearest-neighbor atoms, suggesting that the role of quasiperiodicity in the anisotropic transport of decagonal quasicrystals is marginal. We also present a relaxed version of the ${\mathrm{Al}}_{4}(\mathrm{Cr},\mathrm{Fe})$ structural model by Deng et al. [J. Phys.: Condens. Matter 16, 2283 (2004)].

Epitaxial interfaces of commensurate periodic materials can be characterized by a locking into registry of their atomic structure. This characteristic is identified as a natural framework to capture the essence of epitaxy also for systems including quasicrystalline materials. The resulting general definition for epitaxy requires a matching of reciprocal lattice points. The consequences for the real space structure of an epitaxial interface between quasiperiodic and periodic materials are explored and an experimental realization of such an interface is presented. It is demonstrated that due to their higher number of reciprocal lattice basis vectors (exceeding three), quasicrystals can provide interlayers epitaxially linking incommensurate materials.

International audience; Large single crystals of the decagonal AlCoNi phase were reproducibly grown by the Czochralski method from Al-rich melts. By using native seeds, it was possible to grow crystals with well-defined orientations, e.g. along the 10-fold periodic axis or parallel to one of the 2-fold axes. Quasicrystals, that had been pulled in periodic or quasiperiodic directions, were characterized by X-ray transmission topography at the synchrotron source (ESRF) and at laboratory sources using the Lang technique. Because of the metallurgical homogeneity of the grown quasicrystals, lattice parameter differences did not contribute to the topographic contrast, instead, orientation contrast dominated. A column-like substructure along the periodic direction could be revealed. From this it is concluded, that crystal growth along a two-fold direction might be favourable with respect to minimizing defect spreading.

We have investigated the structure and morphology of the tenfold surface of decagonal ${\mathrm{Al}}_{71.8}{\mathrm{Ni}}_{14.8}{\mathrm{Co}}_{13.4}$ by highly surface sensitive He atom scattering (HAS), high resolution low energy electron diffraction(SPA-LEED), and low temperature scanning tunneling microscopy (STM). The SPA-LEED patterns reveal more than 500 individual diffraction spots in the $\mathbf{k}$-vector range of $\ensuremath{\mid}{\mathbf{k}}_{\ensuremath{\Vert}}\ensuremath{\mid}l3\phantom{\rule{0.3em}{0ex}}{\mathrm{\AA{}}}^{\ensuremath{-}1}$. The positions of all observed diffraction spots agree with the surface projections of the reciprocal lattice structure of the type-I bulk phase. HAS shows identical spot positions as SPA-LEED, thus demonstrating a top surface layer with long range quasicrystalline order and a reciprocal lattice structure consistent with that of a bulk truncated surface. SPA-LEED peak widths are found to vary between different diffraction orders. Based on an analysis of a randomized Fibonacci sequence, this is linked to the random nature of the tiling of the type-I structure. STM measurements reveal a surface morphology characterized by rough single-height steps separating terraces with widths on the order of $100\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. Two different surface terminations are observed, a coarse and a fine one, frequently coexisting on single terraces. The fine structure termination directly reflects the atomic structure of a bulk truncated surface, allowing a random rhombic tiling to be identified. In order to compare diffraction, real-space data, and atomic structure models, the Patterson function and autocorrelation of the surface structure, respectively, are studied. This allows an understanding of the coarse structure termination as consisting of subunits of a few atoms each arranged statistically on sites defined by the atomic tiling of the bulk tenfold planes.

Valence-band photoemission from the s-p region of the tenfold and the two inequivalent twofold surfaces of quasicrystalline decagonal Al 72.8Ni14.8Co13.4 reveals strongly dispersing bands. These exhibit a free-electron-like dispersion along quasiperiodic and periodic directions of the decagonal quasicrystal. The experimental photoemission maps are reproduced in detail by a model in which parabolic bands emanate from a set of reciprocal lattice vectors. A parity rule for the principal zone centers is observed.

Single element quasicrystalline monolayers were prepared by deposition of antimony and bismuth on the fivefold surface of icosahedral Al71.5Pd21Mn8.5 and the tenfold surface of decagonal Al71.8Ni14.8Co13.4. Elastic helium atom scattering and low energy electron diffraction of the monolayers show Bragg peaks at the bulk derived positions of the clean surfaces, revealing highly ordered quasicrystalline epitaxial films. Their adatom densities of (0.9+/-0.2)x10(15) cm(-2) and (0.8+/-0.2)x10(15) cm(-2) on Al-Pd-Mn and Al-Ni-Co, respectively, correspond to roughly one adatom per Al atom of the quasicrystalline substrate surfaces.

We have investigated the surfaces of decagonal AlNiCo single quasicrystals in UHV by Reflection High Energy Electron Diffraction (RHEED), Atomic Force Microscopy (AFM) and Auger Electron Spectroscopy (AES). The surfaces were prepared by Ar + -sputtering and annealing. We observe well-defined RHEED patterns directly after sputtering at room temperature, revealing a ten-fold symmetry with respect to the azimuthal angle of the incident beam. Two patterns occur, alternating every 18°. This symmetry behaviour is probably caused by a bcc overlayer consisting of multiple domains in different orientations. The RHEED patterns show a strong temperature dependence. A spontaneous phase transition into a quasiperiodic structure is observed upon annealing to about 350 °C. RHEED patterns of a flat, quasicrystalline surface of good structural quality are observed after sputtering at 400 °C. As indicated by AES, the influence of oxygen on the structural behaviour seems to be important.

Quasicrystals are metallic alloys that possess perfect long-range structural order, in spite of the fact that their rotational symmetries are incompatible with long-range periodicity. The exotic structural properties of this class of materials1 are accompanied by physical properties that are unexpected for metallic alloys. Considerable progress in resolving the geometric structures of quasicrystals has been made using X-ray and neutron diffraction, and concepts such as the quasi-unit-cell model2 have provided theoretical insights. But the basic properties of the valence electronic states—whether they are extended as in periodic crystals or localized as in amorphous materials—are still largely unresolved3. Here we investigate the electronic bandstructure of quasicrystals through angle-resolved photoemission experiments on decagonal Al71.8Ni14.8Co13.4. We find that the s-p and d states exhibit band-like behaviour with the symmetry of the quasiperiodic lattice, and that the Fermi level is crossed by dispersing d-bands. The observation of free-electron-like bands, distributed in momentum space according to the surface diffraction pattern, suggests that the electronic states are not dominated by localization.

The Hall effect in Y-Al-Ni-Co and o-Al13Co4 single crystals has been investigated for all combinations of the electrical current and magnetic field directions and in the temperature interval from 90 to 380 K. In these intermetallics the Hall coefficient RH exhibits well defined anisotropy and only weak temperature dependence. RH is negative electron-like or zero for the magnetic field perpendicular to the plane that corresponds to the quasiperiodic plane in decagonal quasicrystals, and is positive hole-like or zero for the magnetic field parallel to this plane. The results for the anisotropy of RH are well correlated to the anisotropy of RH in d-Al-Ni-Co and d-Al-Cu-Co quasicrystals. Therefore, the anisotropy in both crystalline and quasicrystaline samples originates from the specific stacked-layer structure and the chemical decoration of the lattice. The investigated anisotropic RH in Y-Al-Ni-Co was reproduced theoretically by calculation using Boltzmann transport theory, ab-initio calculated anisotropic Fermi surface, and isotropic relaxation time approximation.

Due to the lack of long-range periodic order in quasicrystals there are restrictions concerning a straightforward structure analysis on base of x-ray diffraction data. Computed reflection intensities in x-ray diffraction patterns, which are strongly affected by the shape and occupation of hypothetical atomic surfaces, need to be compared with experimental data. The interatomic short-range correlations as to be determined by means of DAFS and XAFS have to be in agreement with these structural models. The use of linearly polarised x-rays allows for orientation sensitive determination of otherwise averaged short-range order information. Additionally wave vector-selectivity of DAFS can yield short-range order information sensitive to sites occupied by atomic species within the coherently scattering structural units. Structural models of decagonal Al-Co-Ni were applied to compute theoretical DAFS and XAFS functions. Calculation of both, energy-dependent structure factors and pair distribution functions obtained within a hypothetical cluster of app. 81 000 atoms as the basis of computation of DAFS are outlined. Co-K and Ni-K DAFS and XAFS measurements at 3 independent reflections of a decagonal Al70Co9.5Ni20.5 single crystal were performed and the experimental data were subjected to quantitative evaluation. The reflections used involved wave vectors parallel and perpendicular to the periodic direction and in a third inclined direction. In agreement with our simulations DAFS of individual reflections and thereby corresponding shortrange orders exhibited no differences within the error limits. The same was found for the case of polarized Co-K and Ni-K XAFS results.

We have studied the e ect of leaching treatments on the surface microstructure and chemical composition on the two di erent twofold surfaces, (10000) and (10100), and the tenfold surface (00001) of a decagonal (d-) Al Ni Co quasicrystal. A sample exposing the three surfaces was leached with NaOH solution and then studied using atomic force microscopy and scanning electron microscopy combined with energy dispersive X-ray analysis. The leaching treatment preferentially removes the Al present in the crystal surface leaving a mixture of transition metal and their associated oxides. The high symmetry, tenfold axis was found to be less resistant to leaching than either of the twofold axes, providing further information regarding the e ect of crystallographic direction on leaching kinetic.

Low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) results are used to study the pseudo-6-fold nature of the (100) surface of the orthorhombic quasicrystal approximant Al4(Cr, Fe). LEED patterns are also presented from the pseudo-10-fold (010) surface of this material. In each case the results are compared with the known bulk structure of this complex metallic alloy.