Γ-brass Phase Research Articles

Atomic control of active-site ensembles in ordered alloys to enhance hydrogenation selectivity.

Intermetallic compounds offer unique opportunities for atom-by-atom manipulation of catalytic ensembles through precise stoichiometric control. The (Pd, M, Zn) γ-brass phase enables the controlled synthesis of Pd-M-Pd catalytic sites (M = Zn, Pd, Cu, Ag and Au) isolated in an inert Zn matrix. These multi-atom heteronuclear active sites are catalytically distinct from Pd single atoms and fully coordinated Pd. Here we quantify the unexpectedly large effect that active-site composition (that is, identity of the M atom in Pd-M-Pd sites) has on ethylene selectivity during acetylene semihydrogenation. Subtle stoichiometric control demonstrates that Pd-Pd-Pd sites are active for ethylene hydrogenation, whereas Pd-Zn-Pd sites show no measurable ethylene-to-ethane conversion. Agreement between experimental and density-functional-theory-predicted activities and selectivities demonstrates precise control of Pd-M-Pd active-site composition. This work demonstrates that the diversity and well-defined structure of intermetallics can be used to design active sites assembled with atomic-level precision.

Structure and Spin-Glass Magnetism of the MnxNi2Zn11–x Pseudobinary γ-Brasses at Low Mn Contents

The pseudobinary MnxNi2Zn11–x γ-brass-type phases at low Mn dopant levels (x = 0.1–0.5) were investigated. Crystal structures were determined for the two loading compositions of x = 0.3 and 0.5. The structures were solved in the cubic space group of I43m and are described in close analogy to the Ni2Zn11 parent γ-brass that is based on the 26-atom cluster, consisting of inner tetrahedron (IT), outer tetrahedron (OT), octahedron (OH), and cuboctahedron (CO). The refined site occupancies of the MnxNi2Zn11–x (x = 0.3, 0.5) reveal that the cluster center, which is empty in the Ni2Zn11, shows a partial occupation by Zn, with a partial depletion of Zn at the IT sites. The OH sites show a mixed Zn/Mn occupation. The OT and CO sites remain intact with respect to Ni2Zn11. Magnetic properties were studied for the Mn0.3Ni2Zn10.7 composition. The temperature-dependent zero-field-cooled and field-cooled magnetization, the ac susceptibility, the M(H) hysteresis curves, the thermoremanent magnetization, and the memory effect demonstrate typical broken-ergodicity phenomena of a magnetically frustrated spin system below the spin freezing temperature Tf ≈ 16 K. The Mn0.3Ni2Zn10.7 γ-brass phase classifies as a spin glass, originating predominantly from the random distribution of diluted Mn moments on the octahedral partial structure.

Determination of Bulk and Surface Atomic Arrangement in Ni–Zn γ-Brass Phase at Different Ni to Zn Ratios

Previous attempts to characterize the γ-brass crystal structure of Ni–Zn (15.4–24% Ni) have failed to identify the location of the Ni and Zn atoms in the crystal lattice for more than 15.4% Ni content (Ni8Zn44) due to the similar X-ray diffraction cross sections of Ni and Zn. Ni8Zn44 is known to have a typical γ-brass crystal structure (space group 217, I43m, 52 atom unit cell with four distinct symmetry positions: inner tetrahedral, outer tetrahedral, octahedral, and cuboctahedral) where Ni atoms reside in outer tetrahedral sites completely isolated from each other and coordinated by 12 Zn atoms. We utilize neutron diffraction to identify the substitution positions of Zn by Ni when the Ni content is increased above 15.4% and up to 19.2% (Ni10Zn42). Upon increasing the Ni content above 15.4% (Ni9Zn43 and Ni10Zn42), Zn in the γ-brass octahedral positions are replaced by Ni leading to the formation of Ni–Ni–Ni trimers, which are absent in Ni8Zn44. Density functional theory (DFT) calculations confirm our ne...

Y-brass related composite structures : a (3+1)-dimensional space description

The γ-brass Hume-Rothery [1] phases which adopt at VEC values near about 21/13 presently attract attention due to their structural complexity and challenge for the understanding of the underlying stabilization mechanism. Morton, by electron microscopy studies, revealed that the γ-brass regions of Cu-Zn, Ni-Zn and Pd-Zn do not only accommodate the γ-brass phase but also a bundle of structurally related, complex phases with lower symmetry than that of the γ-phase. A bundle of γ-brass related phases in the Zn-rich region of the Ni-Zn, Pd-Zn, Pt-Zn phase diagram is investigated. Their structures have been refined from single crystal X-ray diffraction data in the conventional 3D space group using supercells [2,3]. In the course of a previous investigation of the Pd-Zn system, the structures of two γ-brass related composite compounds- Pd24.3Zn75.7 and Pd21.2Zn78.8 have been described with the (3+1) dimensional space description (superspace group Xmmm(00γ)0s0 with the following lattice parameters, a = 12.929(3) Å, b = 9.112(4) Å, c = 2.5631(7) Å, q = 8/13 c* and a = 12.909(3) Å, b = 9.115(3) Å, c = 2.6052(6) Å, q = 11/18 c*, respectively) [3]. The aim of this study is to represent the structure of these previously reported phases in a coherent, modulated description to make them more readily comparable. A refinement model with a variety of modulation vectors allows to refine any intergrowth structure in the Zn rich region of the M-Zn (M=Ni,Pd,Pt) system. In order to gain an insight into expressions, cause and mechanism and structure-composition relationship for such phases, we also study the impact of substitution on the evolution of the structure of ternary derivatives of M-Zn composite compounds by the use of (3+1) formalism. For instance, substitution of zero-valent palladium and bi-valent zinc by zero-valent platinum in the structure of Pd24.3Zn75.7. This presentation will discuss about the understanding of the complexity of the atomic arrangement through the various modulation which correlates with the variation of composition of the binary and ternary phases.

Molecular brass: Cu₄Zn₄, a ligand protected superatom cluster.

The first example of ligand protected Cu-Zn clusters is described. Reaction of [CpCu(CN(t)Bu)] with [Zn2Cp*2] yields [(CuCN(t)Bu)4(ZnCp*)4] (1a) and [(CuCN(t)Bu)4(ZnCp*)3(ZnCp)] (1b). According to DFT calculations, the [Cu4Zn4] unit fulfils the unified superatom model for cluster valence shell closing.

Synthesis and Indentation Behavior of Amorphous and Nanocrystalline Phases in Rapidly Quenched Cu–Ga–Mg–Ti and Cu–Al–Mg–Ti Alloys

Rapidly solidified ribbons of Cu–Ga–Mg–Ti and Cu–Al–Mg–Ti alloys show the formation of γ-brass type phase, which is an approximant of quasicrystals. The Cu–Ga–Mg–Ti alloy displays ordered γ-brass structure, whereas Cu–Al–Mg–Ti leads to the formation of amorphous and nanocrystalline disordered bcc γ-brass phase. The hardness values of as-synthesized ribbons of Cu–Ga–Mg–Ti and Cu–Al–Mg–Ti alloys are ~7.36 and ~9.01 GPa, respectively. The high hardness value of the Cu–Al–Mg–Ti alloy can be attributed to the combined effects of amorphous and nanocrystalline phases. The absence of cracks around the indentation up to 100 g of load of this alloy suggests better toughness in comparison with Cu–Ga–Mg–Ti alloy. The formation of shear bands is more prominent in case of Cu–Al–Mg–Ti alloy. The value of yield strength and Meyer exponent of the material has been estimated. It has been found that the indentation parameter seems to be sensitive to structural and microstructural variations.

Phase formation in rapidly quenched Cu-based alloys

In the present investigation, we report the formation of γ-brass type phase in the rapidly quenched Cu50Ga30Mg5Ti15 and Cu50Al30Mg5Ti15 alloys. Rapid solidification of Cu50Ga30Mg5Ti15 alloy shows the formation of simple cubic γ-brass type phase (a = 0.863 nm), which on annealing at 1,023 K for 60 h transforms to disordered type γ-brass phase (a = 0.879 nm). It has been observed that intensity modulation of electron diffraction spots corresponding to simple cubic γ-brass phase is similar to the intensity modulation observed in the mirror orientation of icosahedral quasicrystalline phase. Contrary to the crystalline phase formation in Cu–Ga–Mg–Ti alloy, rapid solidification of Cu50Al30Mg5Ti15 shows the formation of amorphous and nanocrystalline bcc γ-brass type phase (a = 0.870 nm), which on annealing transforms to ordered γ-brass type phase (a = 0.872 nm). The structural and microstructural characterization was done through X-ray diffraction and transmission electron microscopy.

Microstructural evolution in a Cu–24.5 at.% Al–19.4 at.% Mn alloy inside the miscibility gap

When a Cu–24.5 at.% Al–19.4 at.% Mn alloy was aged at 300 °C for 48 h, the microstructure was a mixture of (D0 3 + L2 1 + L–J) phases. However, as the present alloy was aged at 350 °C for moderate times, the microstructure revealed a coexistence of (D0 3 + L2 1 + γ-brass + L–J) phases. The γ-brass phase started to occur on the a/2〈1 0 0〉 anti-phase boundaries (APBs) initially as aged at 350 °C, and then grew along the 〈1 0 0〉 crystallographic directions after prolonged aging. Furthermore, the formation of L–J phase was found to be concerned with the redistribution of Mn and/or Al alloying elements during aging.

Stability of hume-rothery phases in Cu−Zn alloys at pressures up to 50 GPa

The crystal structure of the γ-brass phase Cu5Zn8 is confirmed with single-crystal X-ray diffraction and a charge-coupled device (CCD) detector to be cubic with 52 atoms in the unit cell, space group \(I\overline 4 3m\), and the refined atomic positions are in good agreement with previously reported data. The structural behavior of α-(fcc), β-brass (cI52) phases of the Cu−Zn alloy system has been studied under pressure using diamond anvil cells and powder X-ray diffraction with synchrotron radiation. The appearance of additional peaks in the diffraction patterns of α- and β-phases indicates the beginning of transitions to new phases at 17 and 37 GPa, respectively. The complex cubic γ-brass phase (52 atoms in the unit cell, space group \(I\overline 4 3m\) is observed to be stable up to at least 50 GPa. The bulk modulus K0 was determined as 140(4) GPa for α-, 139(5) for β-, and 121(2) for γ-phase assuming K0′=4. The structural stability of brass phases of the Cu−Zn system under pressure is discussed in terms of the Hume-Rothery mechanism.

Stability of Hume-Rothery phases in Cu–Zn alloys at pressures up to 50 GPa

The crystal structure of the γ-brass phase Cu5Zn8 is confirmed with single-crystal x-ray diffraction and CCD detector to be cubic with 52atoms in the unit cell, space group , and the refined atomic positions are in good agreement with previously reported data. The structuralbehaviour of α-(fcc), β-(bcc), and γ-brass (cI52) phases of the Cu–Zn alloy system has been studied under pressureusing diamond anvil cells and powder x-ray diffraction with synchrotronradiation. The appearance of additional peaks in the diffraction patterns ofα- andβ-phases indicates the beginning of transitions to new phases at 17 and 37 GPa, respectively. The complexcubic γ-brass phase is observed to be stable up to at least 50 GPa. The bulk modulusK0 was determinedas 140(4) GPa for α-, 139(5) GPa for β-,and 121(2) GPa for γ-phase assuming K0′ = 4. The structural stability of brass phases of the Cu–Zn system under pressure is discussedin terms of a Hume-Rothery mechanism.

Study of decagonal approximant and γ-brass-type compounds in Al–Cr–Fe thin films

This paper reports the preparation conditions and structure characteristics of Al–Cr–Fe very thin films (10–30 nm) obtained by the flash evaporation technique. The films are either amorphous or crystallized, depending on the thickness of the sample and temperature of the substrate. Annealing of amorphous films leads to crystallization of intermetallic phases that are all linked with quasicrystals. In particular, we have identified by transmission electron microscopy the following structures: body-centered-cubic (bcc) γ-brass phase, monoclinic λ–Al13(Cr,Fe)4phase, and orthorhombic O1-phase, all of them already observed in this system, together with four new structures, i.e., a face-centered-cubic (fcc) γ-brass phase (superstructure of the bcc phase), monoclinic λ′-phase (related to the λ-phase) and two orthorhombic phases (1/1/; 1/1) and (1/0; 2/1) approximants of the decagonal phase). In this study, we point out the occurrence of twin defects of the λ–Al13(Cr,Fe)4phase. Films prepared directly in the crystalline state comprise the O1approximant. Electron energy loss spectroscopy measurements show that all films are not oxidized except for the presence of a native oxide layer that forms in ambient atmosphere with a thickness that cannot exceed 0.3 nm. Optical properties were investigated and show that films need to be large enough (>30 nm) to reproduce the properties of bulk alloys. Finally, contact angle wetting measurements reveal that the presence of such films on a substrate, even at very low thickness, considerably decreases the wetting behavior by water.

Surface oxidation of Al–Cr–Fe alloys characterized by X-ray photoelectron spectroscopy

We present X-ray photoelectron spectroscopy (XPS) measurements of several Al–Cr–Fe samples which are mixtures of approximants of the decagonal phase. Some samples also contain a hexagonal γ-brass phase. Our purpose is to evaluate the effect of chemical composition, particularly Cr content, on the response of the surface to oxidation. Under mild conditions only aluminium oxidizes, but under extreme conditions (water immersion at room temperature, or oxygen exposure at high temperatures), chromium oxidizes as well. XPS data also provide a measure of the oxide thickness. Cr has no discernible effect on oxide thickness when the oxidizing environment is the gas phase, but provides significant protection against water immersion, where high concentrations of Cr reduce the thickness by as much as 40%. These results for the Al–Cr–Fe samples are compared with results for approximants and quasicrystals in other systems.

γ-Brass Crystallization in a Simple Monotomic Liquid

AbstractWe present a molecular dynamics simulation investigating the crystallization pattern of a simple monatomic liquid model which utilize an interatomic interaction potential. The shape of the potential imitates effective interionic interaction in metallic systems. The potentials is constructed to favour icosahedral arrangement in the first coordination shell. We observe that, the system forms a thermodynamically stable solid phase exhibiting cubic symmetry. Its diffraction pattern can be identified as that of γ-brass phase, a terahedrally packed crystalline structure with 52 atoms in the unit cell. We conclude that the monatomic version of γ-brass structure generated in this simultion can be regarded as a new archetype for the γ-base structures observed in many intemetllic compounds.

ION-INDUCED PHASE TRANSITION IN THE AU-IN SYSTEM

The Au–In equilibrium phase diagram exhibits a large number of intermetallic phases which have recently been explored via perturbed angular correlation spectroscopy, using alloyed and/or diffused 111In probes. In the present work we report on the phase transition from Au7In3 (hexagonal, γ'-phase) to Au9In4 (γ-brass phase) induced via Xe- or Ar-ion implantations at 80 K. We compare the hyperfine parameters with those obtained in thermal equilibrium and after quenching, and argue that ion beam mixing and fast cooling in the condensation phase of thermal (local) spikes are the predominant mechanisms to interpret our findings.