Pd Mn Research Articles

Crystal structure, magnetism and bonding of the hexagonal compounds Pd1.63Mn0.37Si and Pd1.82Mn0.18Ge related to the Fe2P structure

We have used single crystal x-ray diffraction methods to establish the crystal structures of a compound in the Pd–Mn–Si system and in the Pd–Mn–Ge system. The title compounds have structures related to the Fe2P structure type and are ferromagnetic with Curie temperatures above the room temperature. Density functional electronic structure calculations help to understand the nature of the local moment ferromagnetism in these compounds. However, neither the electronic structure calculations nor the magnetic measurements provide any evidence of half-metallic behaviour.

Anomalous electronic transport inξ′-Al–Pd–Mn complex metallic alloy studied by spectral conductivity analysis

The ξ′ giant-unit-cell phase of the Al–Pd–Mn alloy system exhibits an almost temperature-independentelectrical conductivity with a value typical of metallic alloys, but an anomalously lowthermal conductivity comparable to that of thermal insulators. The origin of theT-independent conductivity is analysed by determining the spectral conductivity functionσ(E) from a combined analysis of the electrical conductivityσ(T) and thethermoelectric power S(T) using a phenomenological approach. It is found that theT-independence of the conductivity results from the specific form of the spectralconductivity, which shows very weak variation over an energy scale of severalkBT around the Fermi level, but exhibits fine structure that yields observable effects inσ(T) and S(T).

Al–Pd–Mn icosahedral quasicrystal: deformation mechanisms in the brittle domain

The extreme brittleness of Al–Pd–Mn quasi-crystalline alloys over a wide range of temperatures drastically restricts investigation of their plastic deformation mechanisms over a small high-temperature regime. Recently, plastic deformation of Al–Pd–Mn quasicrystal has been achieved in the brittle domain (20 ≤ T ≤ 690°C) using specific deformation devices, which combined a uniaxial compression deformation or a shear deformation with a hydrostatic pressure confinement (0.35–5 GPa). Results of these experimental techniques, which provide various deformation conditions giving rise to a range of Al–Pd–Mn plastic features in the brittle domain, are discussed. On this basis, we propose that low and intermediate temperature plastic properties of Al–Pd–Mn are controlled by non-planar dislocation core extensions specific to the non-periodic structure.

Ab initio study of a quasiperiodic Na monolayer on a five-fold i-Al–Pd–Mn surface

The structure and stability of a quasiperiodic Na monolayer formed on a five-fold surface of an icosahedral Al–Pd–Mn quasicrystal have been investigated using ab initio density-functional methods. The structural model of the adsorbed monolayer has been constructed on the basis of a mapping of the potential-energy landscape of an isolated adatom on the five-fold surface of i-Al–Pd–Mn. Na atoms adsorbed on the surface arranged to a highly regular quasiperiodic monolayer. The quasiperiodic ordering can be described by a tiling of decagons, hexagons, boats and pentagonal stars (DHBS). The coverage density of the adsorbed monolayer is 0.067 atoms/Å2.

Terrace-dependent nucleation of small Ag clusters on a five-fold icosahedral quasicrystal surface

Nucleation of Ag islands on the five-fold surface of icosahedral Al–Pd–Mn is influenced strongly by trap sites. Submonolayers of Ag prepared by deposition at 365 K and with a flux of 1 × 10−3 monolayers/s exhibit a variation in Ag island densities across different terraces. Comparisons with previous work and with rate equation analysis indicate that trap sites are not saturated under these experimental conditions and that the difference in island densities is not necessarily due to variation in trap densities. While it could have a number of different origins, our results point to a terrace-dependent value of the effective diffusion barrier for Ag adatoms.

Synchrotron X-ray diffractometry and imaging of strains and defects in icosahedral quasicrystal grains

In order to better understand the long-range propagation of quasicrystalline order, as well as quasicrystal stability, it is important to know if defects are generated in the quasicrystal grains during their growth. Previously, we studied the degree of perfection of about ten icosahedral quasicrystal grains of various alloys (Al–Pd–Mn, Al–Cu–Fe, Zn–Mg–Y), as grown or annealed, and we disclosed that some of them were much more perfect than the others. In this work we have concentrated on another slice of such a grain of the Al–Pd–Mn alloy. Similarly, we have performed an extensive synchrotron X-ray topographic investigation of strain and defects in this grain, combined with phase-contrast radiography and high resolution X-ray diffraction examinations. Very few two-lobe contrasts associated with pores and no loop-shaped contrasts were observed on the X-ray topographs, but straight line segments and band contrasts have been identified. Line segments could be considered as the result of the climbing of polygonal dislocation loops, as observed by TEM by Caillard and coworkers. This would indicate that most strains and defects observed in quasicrystal grains, at room temperature, are the result of stresses (external and internal) acting after growth.

The quasicrystal–crystal interface between icosahedral Al–Pd–Mn and deposited Co: Evidence for the affinity of the quasicrystal structure to the CsCl structure

We have evaporated Co on the pentagonal surface of an icosahedral Al–Pd–Mn quasicrystal kept at room temperature. For submonolayer Co deposits, five AlCo domains are formed exposing their (1 1 0) faces parallel to the surface and rotated by 72° with respect to each other. The orientational relationship between these domains and the substrate is determined by the optimum matching of the average quasicrystal structure and the CsCl structure with a lattice constant of about 2.88 Å. For further deposition, body-centred cubic Co domains grow epitaxially on the AlCo domains.

From phason planes to superstructures in an Al–Pd–Mn alloy

The atomic structures of four structurally complex phases of an Al–Pd–Mn alloy, namely the Ψ-, ξ1-, ξ2- and ξ3-phases, have been determined by means of high-resolution electron microscopy and by theoretical structure simulations based on the periodic stacking of two different types of phason planes. The structural characteristics of the Ψ-phase appears as a thin slab of ξ′-phase sandwiched between a pair of type-1 phason planes; in addition, three newly-found ξ i -phases (i = 1–3) were constructed by periodic stacking of the type-2 phason planes with an adjacent stacking (ξ1), centred by a flattened hexagon (ξ2), which are sandwiched around a thin slab of ξ-phase (ξ3). High-resolution electron microscopy image simulations of the Ψ-phase based on our structural model match very well with the experimental results and this agreement is also supported by electron diffraction and X-ray powder diffraction simulations based on the model. Our results show that structural models of periodic stacking of two types of phason planes can be used to construct the atomic structure of ξ′- and ξ-phase related superstructures in the Al–Pd–Mn alloy.

Elastic limit at macroscopic deformation of icosahedral Al–Pd–Mn single quasicrystals

Al 70.5Pd 21Mn 8.5 single quasicrystals were plastically deformed between 482 and 821 °C. The strain rate sensitivity of the flow stress was measured by stress relaxation tests. At several temperatures, the dislocation structures were imaged by diffraction contrast in a high-voltage electron microscope for determining the dislocation densities. At all temperatures, the plastic deformation starts with a range of very high work-hardening. The transition point between almost elastic and elastic–plastic deformation is called the elastic limit. At low temperatures, the deformation was stopped at about 1.5 GPa to prevent fracture. Above about 580 °C, the stress–strain curves bend down and show a yield point effect followed by a range of almost steady state deformation. At low temperatures, the elastic limit is much lower than the steady state flow stress or the maximum stresses reached without fracture. The activation parameters are different for the elastic limit, the range of high work-hardening and steady state deformation. The flow stresses are interpreted by the stress necessary to move individual dislocations and the athermal component due to the elastic interaction between dislocations. At low temperatures, a further component is necessary to explain the very high flow stresses reached by work-hardening.

Dynamics of phason diffusion in icosahedral Al–Pd–Mn quasicrystals

We present a direct experimental investigation of the dynamics of phason defects in an icosahedral quasicrystal. The activation parameters of the diffusion of phasons were determined as Δ H = 4.28 eV and t d 0 - 1 = 4 × 10 21 s - 1 .

Film growth arising from the deposition of Au onto ani-Al–Pd–Mn quasicrystal: a medium energy ion scattering study

The room temperature deposition of 7 ML of Au onto the fivefold symmetric surface oficosahedral Al–Pd–Mn leads to the formation of a several monolayers thick Au–Al alloyfilm. An AlAu film with 1:1 stoichiometry is formed, which shows no evidenceof ordered structure, being either amorphous or polycrystalline. Annealing to325 °C causes more Al to diffuse into the film, producingAl2Au but still with no indication of structure. Experiments using 0.5 ML of pre-deposited Indemonstrated a surfactant effect as the In ‘floated’ on the surface during growth andproduced a reduction in film roughness. However, contrary to previous findings the film wasstill either amorphous or polycrystalline, with no evidence of quasi-crystalline or aperiodicstructure. Experiments were also conducted using smaller doses of Au to look for theformation of an epitaxial layer and, if formed, determine the registry with the substrate.However, no change in the Pd blocking curves for the surface could be seen, suggesting thatthe Au does not adsorb in well defined sites. This result is not surprising whenconsidering that even for these low doses Al is drawn into the film, changing thecomposition and probably the structure of the topmost layers of the substrate,so that the potential adsorption sites on the clean surface may no longer exist.

Ni deposition on the pentagonal surface of an icosahedral Al–Pd–Mn quasicrystal

We have evaporated Ni on the pentagonal surface of an icosahedral Al–Pd–Mn quasicrystal kept at room temperature. At the initial stage of growth, Ni intermixes with the substrate surface. Subsequently, Al from the quasicrystal matrix migrates to growing layers. The modified chemical composition in an initially icosahedral region near the surface induces a structural transformation. An Al–Pd–Mn alloy is formed which consists of five cubic domains with dimensions in the nm-range exposing their (1 1 0) faces parallel to the surface. These domains are azimuthally rotated by 2π/5 with respect to each other and aligned with symmetry directions of the icosahedral substrate. Al–Mn–Ni, Al–Ni, and Ni overlayers adopt both structure and orientation of these domains which stabilises Ni in a novel body-centred cubic phase. Ni-rich overlayers exhibit out-of-plane magnetic ordering.

The clean and copper-dosed two-fold surface of the icosahedral Al–Pd–Mn quasicrystal

The two-fold surface of the icosahedral quasicrystal Al 70.2Pd 21.5Mn 8.3 prepared in UHV has been studied with LEED and STM. Two phases are observed: a quasiperiodic phase, in agreement with previous work, and a periodic phase tentatively identified as orthorhombic. Copper deposition on the two-fold surface leads to a multilayer exhibiting a quasiperiodic LEED pattern.

Determination of atomic structure of phason planes in the ξ′-Al–Pd–Mn phase

The atomic structures of specific types of linear defects (phason lines) and planar defects (phason planes) in the complex metallic alloy phase ξ′-Al–Pd–Mn have been determined by high-resolution electron microscopy (HREM) and theoretical HREM simulation. The results show that a representational atomic structural model for phason planes can be constructed by introducing a shift between two parts of the perfect crystalline structure using a translation vector of r = (1/2) a + (1/2τ) c . This typical phason plane is normally parallel to the (001) plane of the ξ′-Al–Pd–Mn phase and consists of phason lines, which are arranged side-by-side with their linear direction parallel to the [010] axis. HREM simulations, based on the structural model for both edge-on and inclined types of phason lines, agree well with the experimental results. Taking into account the fact that the structural difference between various curved phason planes arises from the variation in the arrangement of individual phason lines, the atomic structures of the edge-on and inclined phason lines can be used to explain the various curved phason planes frequently observed in the ξ′-Al–Pd–Mn phase.

Rapid Screening of Bimetallic Electrocatalysts for Oxygen Reduction in Acidic Media by Scanning Electrochemical Microscopy

Additional bimetallic electrocatalysts for the oxygen reduction reaction (ORR) in acidic media were designed using a previously reported thermodynamic selection guide. The electrocatalyst mixtures were prepared in large arrays on glassy carbon substrates and the electrocatalytic activity was screened using scanning electrochemical microscopy (SECM). Activities were measured for a range of bimetallic combinations that showed a synergetic electrocatalytic effect during screening, including Au–V, Ag–V, Pd–Mn, and Pd–V. Upon initial screening, a highly active electrocatalytic combination consisting of 60:40 Pd–V was identified. Using rotating disk electrode (RDE) experiments, the high activity of this combination for the ORR in acidic media was confirmed when the electrocatalysts were supported on Vulcan carbon. The electrocatalytic activity of Pd–V was close to that exhibited by Pt, the electrocatalyst of choice for the ORR in acidic media, and thus is another example of a nonplatinum catalyst with high activity that follows the previous strategy for catalyst design.

Thermodynamics of Hydrogen Solution and Hydride Formation in Pd−Mn Alloys. 2. Ordered Alloys

There are marked differences in H(2) solubilities between ordered and disordered Pd-Mn alloys with the largest difference found between the L1(2) and the disordered form of the Pd(3)Mn alloy. The thermodynamics of H(2) solution have been determined for the L1(2) form, the long-period superstructure (lps), and the disordered forms of the Pd(0.80)Mn(0.20) and Pd(0.75)Mn(0.25)(Pd(3)Mn) alloys. Relative partial molar enthalpies and entropies were determined mainly by reaction calorimetry over the range of H contents accessible from p(H)()2 approximately 10 Pa to approximately 0.3 MPa (303 K). The enthalpies for absorption of H(2) are more exothermic over most of the range of H contents for the L1(2) forms of the Pd(3)Mn and Pd(0.80)Mn(0.20) alloys than for their other forms. The reaction enthalpies are constant across a relatively wide range of H contents for the L1(2) form of the Pd(0.80)Mn(0.20) and Pd(3)Mn alloys indicating that there are two-phase coexistence regions (303 K). The H-H attractive interaction, which leads to hydride formation, is much greater for the L1(2) than for the other forms of the Pd(3)Mn alloy and for Pd itself. It has been found that the H-H interaction always decreases in magnitude and, accompanying this, the THS (terminal hydrogen solubility) always increases by alloying Pd.(1) The L1(2) ordered Pd(3)Mn alloy is an exception to this, and therefore, the generalization about THS must be restricted to disordered face centered cubic (fcc) Pd alloys.

Thermodynamics of Hydrogen Solution and Hydride Formation in Pd−Mn Alloys. 1. Disordered Alloys and a Correlation Effect

The thermodynamics of H(2) solution and hydride formation/decomposition have been determined by reaction calorimetry (303 K) for disordered face centered cubic (fcc) Pd-Mn alloys. This alloy system belongs to the expanded lattice category which predicts that and DeltaH(plat) for H(2) absorption should be more exothermic than those for Pd; the experimental results are that the former is more exothermic, at least at the higher Mn contents, but the latter is not. There is a regular decrease in the H capacity (at p(H)2 = 0.2 MPa) with atom fraction Mn. A linear dependence of log p(H)2 upon H content is found in the single hydride phase for all of these alloys suggesting that DeltaH(H) and DeltaS(H) are also linear functions of r in this region. This is confirmed using the Pd(0.875)Mn(0.125) alloy which has no two-phase region (303 K). It is shown for the Pd(0.875)Mn(0.125) alloy and for Pd that the changes of partial enthalpies and entropies with H content are correlated so as to minimize changes of mu(H).

Comparison of Surface Structure of Icosahedral Al–Pd–Mn Family Quasi-crystals

We present scanning tunneling microcopy (STM) studies of three different icosahedral (i) quasi-crystals of the same family, namely, i-Al–Cu–Fe, i-Al–Pd–Mn, and i-Al–Cu–Ru. STM images reveal that the fivefold surface of all three quasi-crystals is isostructural. Both step-height distribution and high-resolution images on terraces are consistent in all systems.

Hydrogen interactions with quasicrystalline Al–Pd–Mn surfaces

In this work we examine the interaction of molecular and atomic deuterium with the fivefold surface of an icosahedral (i-) Al–Pd–Mn alloy using angle-resolved low-energy ion scattering under ultrahigh vacuum conditions. i-Al–Pd–Mn has been studied extensively and is known to form a laterally-bulk-terminated surface that is Al-rich. The density of Al atoms on the clean surface of fivefold i-Al–Pd–Mn is about that of Al(111) and hence we compare our results to studies of molecular and atomic hydrogen adsorbed on Al(111). We find that molecular deuterium does not adsorb or dissociate on i-Al–Pd–Mn, as on Al(111). Atomic D, however, readily adsorbs on both surfaces and on i-Al–Pd–Mn attenuates the scattering signals from Al, Pd and Mn substrate atoms. It thus appears that D chemisorbs on the outer layer of i-Al–Pd–Mn and bonds with Al atoms.

On the concept of metadislocations in complex metallic alloys

Metadislocations are a novel type of structural defect in complex metallic alloys. We present an experimental characterization of metadislocation loops in ξ′-Al–Pd–Mn by means of transmission electron microscopy. Furthermore we discuss the mode of metadislocation motion (glide vs. climb) and the geometrical structure of metadislocations in other orthorhombic complex metallic alloys.