Icosahedral Al Pd Mn Research Articles

Voids and pits on sputter-annealed fivefold terraces of icosahedral Al–Pd–Mn quasicrystals

Our scanning tunneling microscope studies on clean surfaces of fivefold icosahedral Al70.2Pd20.7Mn9.1 reveal that, after annealing at 900 K, two types of flat depressions are visible on terraces: voids (small, shallow, flat-bottomed depressions bordered by 2.5 Å steps) and pits (deeper, and often larger, depressions). They are different not only in their dimensions but in their behavior: voids preferentially nucleate on some of the terraces and, after annealing at 925 K, their coalescence and growth leads a new type of termination to be exposed. Pits, however, do not destroy the terraces on which they exist, under the experimental conditions. At the bottom of 4.1 Å-deep pits, 2.5 Å-deep voids nucleate, so that the total depth of the pit is 6.6 Å. We propose that 2.5 Å void-rich terraces are metastable terminations, and that these metastable terminations are also exposed at the bottom of the 4.1 Å-deep pits.

Scanning tunnelling microscopy with atomic resolution on the twofold surface of the icosahedral AlPdMn quasicrystal

We have investigated the 2-fold surface of an icosahedral Al70Pd21Mn9 quasicrystal by means of low-temperature scanning tunnelling microscopy (77 K). The sequence of the terrace step edge heights revealed a Fibonacci sequence with S = 2.40 Å and L = τS = 3.88 Å as fundamental terrace step heights. S and L can be related to the pentagon edge length of the below terrace Bergman polytope and the surface dissected Bergman polytope of the 5-fold surface. The atomic resolution STM images of the 2-fold surface have been compared to the M-model structure of the i-AlPdMn and the experimental STM surface structure of the 5-fold surface by analysis of the FFT spots. We could show that the positions of the FFT spots in the 2-fold surface are related to the FFT spots of the 5-fold surface by projections onto the fivefold axis and are in good correspondence to the model structure. We identified characteristic rectangular features on the 2-fold surface with edge length of 4.8, 7.7 and 12.6 Å corresponding to D2, τD2 and τ 2D2 with D2 being the pentagon edge length of the surface dissected Bergman polytope. We interpret our results as being a strong indication for the bulk termination of the 2-fold surface of the i-AlPdMn quasicrystal.

Low-temperature scanning tunneling spectroscopy on the 5-fold surface of the icosahedral AlPdMn quasicrystal

We report on scanning tunneling spectroscopy investigations at low temperature (5.3 K) on a bare 5-fold surface of an icosahedral Al70Pd21Mn9 quasicrystal. By averaging tunneling spectra recorded over several nm2 we could reproduce the typical line shape observed in point contact spectroscopy on oxidized surfaces. The averaged spectra show a large parabolic gap structure with a sharp dip of 20 meV FWHM at the Fermi energy. Whereas the averaged spectrum is relatively smooth and featureless, with exception of the sharp dip structure, the individual spectra show a rich structure of peaks and gaps. The distribution of the peak positions in energy shows a clear asymmetry between the occupied and the unoccupied states, in the sense that the peaks are mainly situated above the Fermi energy. The FWHM of these peaks is typically between 30 meV and 100 meV. The spectral features vary rapidly with tip position, where the spatial extent of a given peak in the spectrum is of the order of 0.5–1 nm. To the best of our knowledge, we will present for the first time LT-STS data in combination with high resolution STM images of a quasicrystalline surface to demonstrate the localization of electronic states and gaps and the asymmetry between the occupied and the unoccupied local density of states.

Tunneling spectroscopy and high electrical resistivity in quasicrystalline alloys

We report on a scanning tunneling spectroscopy investigation at low temperature on oxidized surfaces of highly resistive icosahedral AlPdMn, AlCuFe and AlPdRe phases. The tunneling conductance is found to vary significantly as a function of the scanning tunneling microscope tip location on the surface. A common feature of our quasicrystalline samples is the presence of a zero bias dip, with square root voltage dependence from 5 mV to 300 mV. This is indicative that the one electron density of states has a square root singularity at the Fermi level. More generally, we discuss how tunneling spectroscopy can be used to determine the electronic density of states and its limitation for our quasicrystalline alloys.

In situ observation of pore evolution during melting and solidification of Al–Pd–Mn quasicrystals by synchrotron X-ray radiography

It is now generally admitted that pores are intriguing special features of quasicrystals. Therefore, we have performed an ‘in situ’ and real time observation of the pore evolution during directional solidification and melting cycles of an icosahedral Al–Pd–Mn bi-grained sample, by synchrotron X-ray radiography. Rather surprisingly, no pore was observed to grow during the solidification stages. Nucleation and growth of pores were first seen during melting. These pores were subsequently shrinking either just before being absorbed or during resumption of directional solidification. It is concluded that the vacancy origin of pores is certainly valid whereas the vacancy supersaturation, needed thereby to explain their growth, would be more probably related to the peculiar structure of quasicrystal than to the destruction of the thermal equilibrium.

Extended investigation of porosity in quasicrystals by synchrotron X-ray phase contrast radiography—I: In icosahedral AlPdMn grains

The porosity was studied using coherent synchrotron X-ray radiography, firstly in several bulky icosahedral Al–Pd–Mn quasicrystals, grown in different laboratories by various techniques (Bridgman, Czochralski, Flux) and then in quasicrystal grains of many other alloys and structures (Part II). In this part I, we report a quantitative analysis which has been carried out in order to find some correlation of this porosity with some growth or after-growth cooling parameters (growth parameters). The size distribution of pores was observed to change drastically among samples grown with different growth parameters as well as in those extracted from the same ingot and no mode values were evidenced when we plotted the distribution of pores in all investigated as-grown samples. These changes point out the influence of the solidification process on the occurrence of porosity, especially the influence of the vacancy migration. These results are used to discuss the two main current hypotheses on the origin of porosity.

Extended investigation of porosity in quasicrystals by synchrotron X-ray phase contrast radiography: Part II, in grains of other alloys and structures than AlPdMn

The systematic study, by synchrotron X-ray phase sensitive radiography, of the porosity in the bulk samples of the icosahedral Al–Pd–Mn quasicrystal (i-Al–Pd–Mn) reported in the Part I of an extended investigation of porosity in quasicrystals (Part I) has shown that the origin of this porosity is difficult to identify. As it is not clear if this type of porosity is characteristic of icosahedral quasicrystals, in the present work, which is the Part II of the same investigation, we have studied the porosity in single grains of various alloys and structures (i-Al–Pd–Re, i-Zn–Mg-Rare-Earth, d-Al–Co–Ni, ξ ′ -Al–Pd–Mn) in order to attempt to elucidate this question. The results show that the presence of facetted micrometric pores is not restricted to icosahedral quasicrystals. Otherwise the part played by these pores to their actual existence and stability remains as questionable. The samples contain pores or not independently of the growth method, which does not seem to have an influence on the pores. Since pores were only observed in samples of the Al-rich phases this element could be an important ingredient for the formation of pores during the solidification process as it has been suggested in Part I.

Shear experiments under confining pressure conditions of Al–Pd–Mn single quasicrystals

Icosahedral Al–Pd–Mn quasicrystalline alloys are known to be extremely brittle at low and intermediate temperatures when deformed with conventional tests. Using specific deformation conditions, such as high confining pressure techniques, allowed plastic deformation of this non-periodic structure at temperatures in the brittle domain. Surprisingly, low temperature deformation microstructures revealed the presence of dislocations, whose motion essentially implicates climb mechanisms. The aim of this study is to use a shear experiment technique, specially designed to favour glide processes, in order to check if glide may occur solely, without any climb component. The results confirm the difficulty to promote pure glide processes and support the idea that both glide and climb are activated.

Ab initiostudy of quasiperiodic monolayers on a fivefoldi−AlPdMnsurface

We present a structural model for quasiperiodic monolayers formed on the fivefold surface of an icosahedral AlPdMn quasicrystal, based on ab initio density functional calculations. As a starting point we have investigated the relative stability of unsupported triangular, square, and quasiperiodic monolayers. The unsupported quasiperiodic monolayers are shown to be unstable upon relaxation by interatomic forces. This result indicates the important role of the adsorbate/substrate interaction for the stabilization of a quasiperiodic layer. The structural model of a monolayer adsorbed on the fivefold surface has been constructed on the basis of a mapping of the potential-energy landscape of an isolated adatom on the $i\text{\ensuremath{-}}\mathrm{Al}\mathrm{Pd}\mathrm{Mn}$ substrate. The structure of a clean fivefold $i\text{\ensuremath{-}}\mathrm{Al}\mathrm{Pd}\mathrm{Mn}$ surface is well described by a $P1$ tiling, with the vertices of the tiling located in the centers of $B$ (Bergman) clusters and occupied by Pd atoms. The potential-energy mapping emphasizes the important role of the $P1$ skeleton for the stabilization of a quasiperiodic adlayer: adsorption at the vertices of the $P1$ tiling leads to high binding energies of $\ensuremath{\simeq}4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\text{atom}$. The midedge positions of the $P1$ tiling and Mn atoms exposed at the surface are identified as further favorable adsorption sites. Altogether this leads to a structural model of the quasiperiodic adlayer with atoms at the vertices and midedge positions of the $P1$ tiling, centered pentagonal motifs decorating the pentagonal tiles, and one additional atom in the center of the pentagonal star and of the boat tile. For this structure we calculate a surface coverage of $\ensuremath{\simeq}0.09\phantom{\rule{0.3em}{0ex}}\text{atoms}∕{\mathrm{\AA{}}}^{2}$ in perfect agreement with experiment. The diffraction pattern of the adlayer exhibits (pseudo)decagonal symmetry. The structural stability of a $2∕1$ approximant to this model has been tested for Sn, Bi, and Sb monolayers via relaxation by the Hellmann-Feynman forces from ab initio density function theory calculations. The skeleton of adsorbed monolayers based on the $P1$ tiling was found to be stable, although the atomic decoration inside the tiles is partially distorted, albeit without violating the overall symmetry.

Continuum elastic sphere vibrations as a model for low lying optical modes in icosahedralquasicrystals

The nearly dispersionless, so-called ‘optical’ vibrational modes observed by inelasticneutron scattering from icosahedral Al–Pd–Mn and Zn–Mg–Y quasicrystals are foundto correspond well to modes of a continuum elastic sphere that has the samediameter as the corresponding icosahedral basic units of the quasicrystal. Whenthe sphere is considered as free, most of the experimentally found modes can beaccounted for, in both systems. Taking into account the mechanical connectionbetween the clusters and the remainder of the quasicrystal allows a completeassignment of all optical modes in the case of Al–Pd–Mn. This approach providessupport to the relevance of clusters in the vibrational properties of quasicrystals.

Transmission electron microscopy study of dislocation motion in icosahedral Al–Pd–Mn

Perfect and imperfect dislocations trailing phason faults in quasi-crystals are introduced using a simplified two-dimensional aperiodic structure. Then, on the basis of observations of deformed specimens as well as in situ experiments in a transmission electron microscope, the motion of dislocations in icosahedral Al–Pd–Mn is shown to take place exclusively by climb. Under such conditions, the very high brittleness of Al–Pd–Mn at low and medium temperatures is proposed to be a consequence of the difficulty of glide, which itself appears to be an intrinsic property of the quasi-crystalline structure.

Formation of Al4Cu9 on the 5 fold Surface of Icosahedral AlPdMn

The authors report about the formation of Al 4 Cu 9 from a thin Cu film evaporated on the five-fold surface of icosahedral AlPdMn. By heating up to 350°C Al diffuses from the quasicrystal into the Cu film forming well crystalline Al 4 Cu 9 present in 5 domains rotated by 72° with respect to each other and exposed in the (110) surface. The investigation was performed using Low Energy Electron Diffraction, X-ray Photoelectron Diffraction and X-ray Photoelectron Spectroscopy giving information about long range order, local structure and chemical composition.

Diffusion in Decagonal and Icosahedral Quasicrystals and a Related Hexagonal Phase

We present investigations of gallium diffusion in decagonal AlNiCo quasicrystals in both principal directions and of gallium diffusion in icosahedral AlPdMn quasicrystals. Gallium was used to approximate aluminium self-diffusion, which cannot be measured using conventional tracer techniques. The depth-profiles of the stable isotopes were analysed by SIMS profiling. We compare our results in decagonal AlNiCo with experimental data on self-diffusion of cobalt and nickel and with MD-simulations of Al self-diffusion from literature. Our experiments and the MD simulations show that the diffusion parallel to the decagonal axis is faster than perpendicular to it. In addition we carried out diffusion studies of the major component zinc in icosahedral ZnMgY, icosahedral ZnMgHo (fci-phases) and a related hexagonal ZnMgY composition (Z-phase) along both principal directions. We report the Arrhenius parameters and compare the diffusion behaviour of the various materials.

Investigation of the surface terminations of icosahedral AlPdMn quasicrystal based on amodified non-spherical model

The atomic positions are obtained from a modified non-spherical model of icosahedralAlPdMn quasicrystal (Fang et al 2003 J. Phys.: Condens. Matter 15 4947) by the cutmethod. The four-shell pseudo-Mackay clusters (PMCs) were searched for in a box of400 Å × 400 Å × 400 Å. The results show that the number of atoms in the fourth shell, an icosidodecahedron, ofthe pseudo-Mackay cluster can vary from 15 to 30 because of the cluster overlap, and about99.96% of the total atoms are included in such incomplete pseudo-Mackay clusters. Thecharacteristics of the atom distribution in the planes perpendicular to a fivefold axisindicate that the planes, which are 1.56 Å apart from their neighbouring planes, are expectedto be the terminal surfaces. If one such a plane and its closest neighbouring plane, betweenwhich the spacing is 0.48 Å, are considered as a thin layer or a corrugated surface, theselayers are also the layers with the maximum density. The pair of corrugated surfaces thatare 1.56 Å apart have almost identical chemical composition. These planes form terracesthat follow the rule of the Fibonacci sequence with two step heights, 6.60 and4.08 Å. On the corrugated surfaces perpendicular to a fivefold axis the pentagonalholes arise from the interspaces of adjacent incomplete PMCs. For the atomicplanes normal to a twofold axis, the planes with spacing of 1.48 Å from their adjacentplanes might be expected to be the terminal surfaces, which form terraces withstep heights of 6.28 and 3.88 Å following the rule of the Fibonacci sequence. For theatomic planes normal to a threefold axis, the planes with spacing of 0.86 Å fromtheir adjacent planes might be expected to be the terminal surfaces. No similarresults were found for the atomic layers perpendicular to a pseudo-twofold axis.

In-situ observation of dislocation motion in icosahedral Al–Pd–Mn quasicrystals

Dislocation motion in icosahedral Al–Pd–Mn quasicrystals has been observed in situ in a transmission electron microscope between 700 and 750°C. Contrast analyses show that it takes place by pure climb in twofold, threefold and fivefold planes. Moving dislocations exhibit polygonal shapes with edges parallel to twofold directions, in agreement with a difficult jog-pair nucleation. Dislocation multiplication, annihilation and local pinning are described and discussed, as well as the role of the phason faults trailed in the wake of dislocations, at the lowest investigated temperatures.

Dislocation climb in an Al–Pd–Mn quasicrystal deformed at low temperature

Abstract Dislocations and phason faults have been imaged in a transmission electron microscope in an icosahedral Al–Pd–Mn quasicrystal deformed at 300 °C. The contrast of these dislocations has been studied. The analysis of Burgers vectors indicates that their motion takes place by climb.

Plasticity of single-grain icosahedral Al–Pd–Mn quasi-crystals deformed at room temperature

Transmission electron microscopy observations have been performed on icosahedral Al–Pd–Mn quasi-crystal samples deformed at 20 °C under a high confining pressure. They reveal a large density of wavy walls from which several climbing dislocations are emitted. Near-screw dislocations have been found at the wall terminations with a Burgers vector contained in the wall plane. Careful plane determinations and dislocation analyses are not consistent with a glide and cross-slip mechanism. The results can be better interpreted as a deformation by cracks in mode III followed by a re-healing process.

Charge density determination in icosahedral AlPdMn quasicrystal using quantitative convergent beam electron diffraction

Charge density distribution in icosahedral AlPdMn quasicrystal has been studied on a single-crystal specimen by using quantitative convergent beam electron diffraction (QCBED) technique. The QCBED systematic row method was used in the refinement of structure factors. To refine the low-order structure factors, the wave-mechanical formulation of electron diffraction dynamical theory was used in the calculation of electron diffraction intensities for the quasicrystal in fitting the experimental intensity line scan profiles. The shapes of atomic surfaces (occupation domains) were described with symmetry-adapted series of surface harmonics. An iterative procedure was used in determination of structure factors of the quasicrystal. The structure factors of nine strongest symmetry inequivalent reflections according to X-ray diffraction experiment were refined with QCBED technique. The average of refinement results for a given reflection performed on several CBED patterns, which were slightly different in orientation and sample thickness, and on different line scans, was taken as the value of structure factor for the reflection. The obtained structure factors for electrons were transformed into X-ray structure factors with Mott formula. The bonding charge density map for the quasicrystal was constructed with the obtained nine structure factors. Assuming that the atoms are spheres, the gain or loss of electrons for different atoms were calculated. It shows that identical atoms can have different valences at different kinds of positions. The bonding charge is localized along certain directions.

SEM and AFM studies of a 5-fold surface of icosahedral AlPdMn

A 5-fold surface of the icosahedral AlPdMn (i-AlPdMn) quasicrystal (QC) was studied by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM) under ambient conditions. We show that a very complicated inhomogeneous surface morphology may develop on the 5-fold surface prepared by ion sputtering and annealing cycles in UHV. Sputtering and sublimation both contribute to material removal on the surface. The sublimation takes place at the annealing temperature at sites of surface defects induced by mechanical polishing and sputtering in UHV. The surface is characterized by nano-sized clusters of about 40 nm in size which form terraces. Results give further evidence for the cluster-based structure of the i-AlPdMn QC.

Initial oxidation of quasicrystals – an investigation by high-resolution RBS and ERDA

Surfaces of icosahedral AlPdMn with 5-fold orientation can be prepared either to be fully icosahedral or to form a cubic overlayer. When decagonal AlNiCo is cut perpendicular to the 10-fold axis, a fully quasicrystalline surface is obtained, however a ‘mixed-type’ (quasicrystalline-periodic) surface is obtained, when cut parallel to this axis. A comparison of the oxidation behavior of these different surfaces should therefore shed light on the suggestion by Rivier [N. Rivier, New Horizons in Quasicrystals: Research and Applications, in: A.I. Goldman, D.J. Sordelet, P.A. Thiel, and J.M. Dubois (Eds.), World Scientific, Singapore, 1997, p. 188] that quasicrystals are more corrosion resistant than their crystalline counterparts. The initial oxidation of the surfaces was studied by measuring oxygen depth profiles with high-resolution ERDA. The paper also reports about the preparation of the surfaces and the measurement of highly resolved depth profiles of the metallic components by RBS during the preparation. In both cases a high-resolution electrostatic spectrometer was used for the energy analysis. The results are in favor of an enhanced oxidation resistance of quasicrystalline surfaces.