Crystal structures of three intermetallic phases in the Mo-Pt-Si system

The crystal structures of three ternary Mo-Pt-Si intermetallic compounds have been determined ab initio from powder X-ray diffraction data. All three structures are representative of new structure types. Both the X (MoPt₂Si₃, Pmc2₁, oP12, a=3.48438(6), b=9.1511(2), c=5.48253(8) Å) and Y (MoPt₃Si₄, Pnma, oP32, a=5.51210(9), b=3.49474(7), c=24.3090(4) Å) phases derive from PtSi (FeAs type) structure while the Z phase (ideal composition Mo₃₂Pt₂₀Si₁₆, refined composition Mo_29.9(2)Pt_21.0(3)Si_17.1(1), Cc, mC68, a=13.8868(3), b=8.0769(2), c=9.6110(2) Å, β=100.898(1)°) present similarities with the group of Frank-Kasper phases.     

Europium substitution into intermetallic phases grown in Ca/Zn flux

Replacement of calcium with europium in the phases Ca₂₁Ni₂Zn₃₆ and CaNi₂Zn₃ was attempted to explore the possibility of substitution in metal flux reactions and potential magnetic interactions between closely spaced Eu²⁺ ions. Limited substitution occurs when Eu is added to the reaction of nickel in a Ca/Zn flux mixture, up to stoichiometries of Eu_5.8(3)Ca_15.2(3)Ni₂Zn₃₆ and Eu_0.42(8)Ca_0.58(8)Ni₂Zn₃. Structural characterization and magnetic susceptibility studies on Eu_xCa_21−xNi₂Zn₃₆ phases indicate that the Eu and Ca ions do not form an even solid solution on their sites, but instead segregate in separate regions of the crystals. The europium-rich regions of the samples order ferromagnetically, with T_C dependent on the size of the clusters. If the concentration of Eu in the flux is raised above 20 mol%, a new compound Eu_1.63(1)Ca_1.37(1)Ni₂Zn₃ (Cmcm, a=4.1150(5) Å, b=16.948(2) Å, c=10.302(1) Å, Z=4, R₁=0.0396) is produced.     

Electrocatalytic activity of ordered intermetallic phases for fuel cell applications

The electrocatalytic activities of a wide range of ordered intermetallic phases toward a variety of potential fuels have been studied, and results have been compared to those of a pure polycrystalline platinum (Pt(pc)) electrode. A significant number of the ordered intermetallic phases exhibited enhanced electrocatalytic activity when compared to that of Pt, in terms of both oxidation onset potential and current density. The PtBi, PtIn, and PtPb ordered intermetallic phases appeared to be the most promising electrocatalysts tested thus far for fuel cell applications. PtPb, in particular, showed an onset potential that was 100 mV less positive and a peak current density approximately 40 times higher than those observed for Pt in the case of methanol oxidation. The ability to control the geometric and electronic structures of the electrocatalytic material by using ordered intermetallic phases has been shown to be a promising direction of inquiry in the search for superior electrocatalysts for fuel cell applications.     

Effect of oxide films on formation of Fe-rich intermetallic phases in aluminum alloys

Journal of Thermal Analysis and Calorimetry - The aim of the present work was to study the effect of the α-Al2O3 and MgAl2O4 oxide films, normally found in cast aluminum alloys, on formation...     

Growth of new ternary intermetallic phases from Ca/Zn eutectic flux

The eutectic 7.3:2.7 molar ratio mixture of calcium and zinc metal melts at 394 °C and was explored as a solvent for the growth of new intermetallic phases for potential use as hydrogen storage materials. The reaction of nickel in this molten mixture produces two new phases—the CaCu₅-related structure CaNi₂Zn₃ (P6/mmm, a=8.9814(5) Å, c=4.0665(5) Å) and a new cubic structure Ca₂₁Ni₂Zn₃₆ (Fd-3m, a=21.5051(4) Å). Palladium-containing reactions produced CaPd_0.85Zn_1.15 with the orthorhombic TiNiSi structure type (Pnma, a=7.1728(9) Å, b=4.3949(5) Å, c=7.7430(9) Å). Reactions of platinum in the Ca/Zn mixture produce Ca₆Pt₃Zn₅, with an orthorhombic structure related to that of W₃CoB₃ (Pmmn, a=13.7339(9) Å, b=4.3907(3) Å, c=10.7894(7) Å).     

Interfacial analyses for elucidation of the intergranular disintegration upon oxidation of intermetallic phases

Summary In a range of intermediate temperatures from 600 to 1000°C and in the presence of oxygen, numerous intermetallic phases show the phenomenon pest, an intergranular disintegration into small pieces. Thermo-gravimetric investigations and annealings in quartz ampoules have been performed, for annealing the oxygen pressures were established in the range 10^–30 to 10^–10 bar O₂ using metal/oxide mixtures. The specimens after annealing were fractured in UHV and the intergranular fracture faces were analyzed by AES. The Auger peaks of Al are markedly different for the intermetallic phase and for Al₂O₃, therefore it can be distinguished if oxygen has diffused into grain boundaries and not yet reacted or if Al₂O₃ was formed. The fracture face of NbAl₃ shows oxide precipitates near the surface and oxygen which had penetrated into the interior. Also in NiAl, Al₂O₃ was detected as well as oxygen penetrated into the grain boundaries. The pest obviously is a complex interplay of the processes: 1) penetration of oxygen through the outer oxide layer on the surface into grain boundaries of the intermetallic phase; 2) inward diffusion of oxygen along the grain boundaries into the interior of the intermetallic phase; 3) precipitation of Al₂O₃, beginning near the surface or (at low oxygen pressure) in the whole cross section, and cracking of the materials by the growth of Al₂O₃ into grain boundaries and cracks. Depending on the range of oxygen pressure different steps can be rate determining.     

Thermodynamics and defect structure of intermetallic phases with the B2 (CsCl) structure

Available theoretical models that describe the thermodynamic properties and the defect structures of intermetallic phases with the B2 structure are reviewed. The experimental methods for the determination of the Gibbs energy of alloys are briefly discussed and a summary of experimental data of lattice parameters, densities, activities, and enthalpies for nearly 30 B2 phases is given. It is shown that the data can be expressed in terms of a few simple model parameters. The importance of theoretical models for an understanding of the structural stability of intermetallic phases and for the evaluation and eventual prediction of binary and ternary phase diagrams is discussed.     

Preparation of nanocrystalline metal oxides and intermetallic phases by controlled thermolysis of organometallic coordination polymers

Organometallic coordination polymers of the super-Prussian blue type [(Me₃Sn)_nM(CN)₆] (Me=CH₃; n=3, 4; M=Fe, Co, Ru) were subjected to thermolysis in different atmospheres (air, argon, hydrogen/nitrogen). In air, oxides were found: Fe₂O₃/SnO₂ (crystalline and nanocrystalline), Co₂SnO₄ and RuO₂. In argon and in hydrogen, the intermetallic phases FeSn₂, CoSn₂, Ru₃Sn₇ and Fe₃SnC were obtained. A detailed mechanistic study was carried out using thermogravimetry (TG), X-ray diffraction (XRD), X-ray absorption spectroscopy (EXAFS) at Fe, Co, Ru and Sn K-edges, infrared spectroscopy (IR) and elemental analysis. Below 250°C, Me₃SnCN and (CN)₂ are released, whereas above 250°C oxidation or pyrolysis leads to the corresponding oxides or intermetallic phases. Polymeric cyanides containing at least two metals have turned out to be suitable precursors to prepare well-defined oxides and intermetallic phases at comparatively low temperature.     

Electrolytic D/H isotope separation efficiency and electrocatalytic activity of Mo–Pt intermetallic phases

The aim of this work was to investigate efficiency of the electrolytic separation of hydrogen isotopes (D/H), from alkaline 6 M KOH solution as standard electrolyte, on the Mo–Pt intermetallic phases as cathode materials. We measured the isotope separation factor (α) of the single-stage process, as a basic parameter determining the isotope separation efficiency. Furthermore, we have found that some combinations of activating compounds (ionic activators added directly into the electrolyte, assuming in situ activation) can reduce energy needs per mass unit of hydrogen produced, as well as the isotope separation efficiency, compared to those of the standard electrolyte. Also, the electrocatalytic ability of Mo–Pt intermetallic phases for the hydrogen evolution was investigated and evaluated at fixed overpotential (energy input) trough measured current density. Such investigations offer a possibility to optimize the electrolytic process, based on a significant upgrade of the efficiency regarding two needs: hydrogen production and isotope separation.     

X-ray fluorescence investigation of ordered intermetallic phases as electrocatalysts towards the oxidation of small organic molecules

The composition of ordered intermetallic nanoparticles (PtBi and PtPb) has been quantitatively studied by in situ X-ray fluorescence (XRF) during active electrochemical control in solutions of supporting electrolyte and small organic molecules (SOMs). Because the Pt L(β1,2) lines and the Bi L(α1,2) lines are only separated by 200 eV, an energy-dispersive detector and a multiple-channel analyzer (MCA) were used to record the major fluorescent emission lines from these two elements. The molar ratios of platinum to the less-noble elements (Bi, Pb) in the nanoparticles dramatically changed as a function of the applied upper limit potentials (E(ulp)) in cyclic voltammetric (CV) characterization. Similar to previous investigations for bulk intermetallic surfaces, the less-noble elements leached out from the surfaces of the intermetallic nanoparticles. For PtBi nanoparticles, the ratios of fluorescence intensities of Pt/Bi in the samples were 0.42, 0.96, and 1.36 for E(ulp)=+0.40, +0.80, and 1.20 V, respectively, while cycling the potential from -0.20 V to the E(ulp) value for 10 cycles. The leaching-out process of the less-noble elements occurred at more negative E(ulp) values than expected. After cycling to relatively positive E(ulp) values, nonuniform PtM (M=Bi of Pb) nanoparticles formed with a Pt-rich shell and intermetallic PtM core. When the supporting solutions contained active fuel molecules in addition to the intermetallic nanoparticles (formic acid for PtBi, formic acid and methanol for PtPb), kinetic stabilization effects were observed for E(ulp)=+0.80 V, in a way similar to the response of the bulk materials. It was of great importance to quantitatively explore the change in composition and structure of the intermetallic nanoparticles under active electrochemical control. More importantly, this approach represents a simple, universal, and multifunctional method for the study of multi-element nanoparticles as electrocatalysts. This is, to our knowledge, the first report of nondestructive, quantitative characterization of bimetallic or multi-elemental nanoparticles electrocatalysts under active electrochemical control.     

The intermetallic phases of gallium and alkali metals. Interpretation of the structures according to Wade's electron-counting methods

This is a survey of the preparation and structural properties of the intermetallic phases of gallium and alkali metals. Unlike the already known phases of gallium with lithium or sodium, the structures of the recently discovered phases Li₃Ga₁₄, Na₂₂Ga₃₉, KGa₃, K₃Ga₁₃, RbGa₃, RbGa₇, and CsGa₇ are characterized by stackings of coordinated gallium polyhedra such as icosahedra, octadecahedra, dodecahedra, and undecahedra. In these phases the alkali metals stabilize the gallium framework by giving their valence electrons. On this basis, the structures are interpreted according to Wade's electron-counting procedure, bringing the Zintl phases to a more general concept and enhancing the interest on the transition forms between metallic and ionic bonding.     

Arsolyl-supported intermetallic dative bonding

The first examples of late transition metal η⁵-arsolyls (L = CO, P(OMe)₃; R = Ph, Me, Et, SiMe₃; R′ = Ph, H, Me, Et, Me) serve as ditopic donors to extraneous metal centres (M = Pt^II, Au^I, Hg^II) through both conventional As → M and polar-covalent (dative) Co → M interactions.

Cobalt carbonyl reacts with arsoles to provide the first late transition metal η⁵-arsolyls. These serve as ditopic donors to extraneous metal centres (M = Pt^II, Au^I, Hg^II) through both conventional AsM and polar-covalent (dative) CoM interactions.     

Substitution of Au or Hg into BaTl2 and BaIn2. New ternary examples of smaller CeCu2-type intermetallic phases

The compounds BaAu(0.40(2))Tl(1.60(7)) (1), BaAu(0.36(4))In(1.64(4)) (2), and BaHg(0.92(2))In(1.08(2)) (3) have been prepared by high-temperature techniques. Single-crystal X-ray diffraction shows that these have the orthorhombic CeCu(2)-type structure, Imma, Z = 4 (a = 5.140(1), 5.104(1), 5.145(1) A; b = 8.317(2), 8.461(2), 8.373(2) A; c = 8.809(2), 8.580(2), 8.715(2) A, respectively). The structure consists of a four-linked honeycomblike polyanion (4(2)6(3)8) of infinity3[Tr2]2- (Tr = In or Tl) with encapsulated Ba2+ cations. The Au or Hg randomly replace Tr in a single type of site. The two gold phases exhibit appreciable nonstoichiometry ranges. Band calculations (EHTB) demonstrate that the three compounds are electron-poor and metallic, and the latter has been confirmed for 1 through resistivity and magnetic susceptibility measurements. The orthorhombic structure of 1 contrasts with the hexagonal structure of BaTl2 (CaIn2-type, P6(3)/mmc), a change that appears to be driven by substitution of the smaller Au atoms into the polyanion network. Relativistic effects for the heavier Au and Hg are evidently responsible for decreases in lattice parameters and bond lengths from BaIn2 to those in isostructural 2 and 3.     

Pressure-induced intermetallic valence transition in BiNiO3

The valence state change of BiNiO3 perovskite under pressure has been investigated by a powder neutron diffraction study and electronic-state calculations. At ambient pressure, BiNiO3 has the unusual charge distribution Bi(3+)(0.5)Bi(5+)(0.5)Ni(2+)O3 with ordering of Bi(3+) and Bi(5+)charges on the A sites of a highly distorted perovskite structure. High-pressure neutron diffraction measurements and bond valence sum calculations show that the pressure-induced melting of the charge disproportionated state leads to a simultaneous charge transfer from Ni to Bi, so that the high-pressure phase is metallic Bi(3+)Ni(3+)O3. This unprecedented charge transfer between A and B site cations coupled to electronic instabilities at both sites leads to a variety of ground states, and it is predicted that a Ni-charge disproportionated state should also be observable.     

Eutectoid decomposition of intermetallic CuZr

The eutectoid decomposition of intermetallic CuZr under isothermal and nonisothermal conditions was studied by differential scanning calorimetry, electrical resistivity measurements, X-ray diffraction analysis, optical microscopy, and dilatometry. The kinetic diagram of the eutectoid decomposition of intermetallic CuZr was determined for the first time. The rate of the eutectoid decomposition of the CuZr phase was found to be one to two orders of magnitude lower than that in many other copper-based systems. The kinetics of the eutectoid decomposition under isothermal conditions was investigated, and the kinetic parameters of the decomposition were determined (the activation energy E_a = 395 ± 24 kJ/mol and the Avrami constant n = 3.3 ± 0.1).     

Borido complexes via intermetallic metalloborylene transfer

Intermetallic borylene transfer has been applied to synthesise new borido complexes. Starting from [{(η(5)-C(5)Me(5))Fe(CO)(2))}(μ-B){Cr(CO)(5)}] (1) the borylene moiety has been transferred successfully to different transition metal fragments. In the manner described, two new compounds have been fully characterised in solution and by X-ray crystallography.     

Stabilization of new forms of the intermetallic phases beta-RENiGe2 (RE = Dy, Ho, Er, Tm, Yb, Lu) in liquid indium

Flux conditions using liquid indium bypass the thermodynamically stable structure and yield new forms of the phases RENiGe2 (RE = Dy, Er, Yb, Lu). The compounds crystallize in the orthorhombic Immm space group and possess the YIrGe2 structure type. Lattice parameters for ErNiGe2, DyNiGe2, YbNiGe2, and LuNiGe2 are a = 4.114(1) A, b = 8.430(2) A, c = 15.741(5) A; a = 4.1784(9) A, b = 8.865(2) A, c = 15.745(3) A; a = 4.0935(6) A, b = 8.4277(13) A, c = 15.751(2) A, and a = 4.092(1) A, b = 8.418(3) A, c = 15.742(5) A, respectively. These phases represent a new structural arrangement (beta) of the compound type RENiGe2 as another set of compounds with identical stoichiometry are known to adopt the orthorhombic Cmcm CeNiSi2 type structure (alpha). In this paper we report the crystal and electronic band structure of four new members of the YIrGe2 structure type, as well as an investigation of the relative thermodynamic stabilities of the two forms.     

Electrochemical investigation of the anodic hydrogen evolution on MgZn2, Mg2Si,and Al4Cu2Mg8Si7 intermetallic phases

Journal of Solid State Electrochemistry - The anodic hydrogen evolution (AHE) on Mg, MgZn2 (η-phase), Al4Cu2Mg8Si7 (Q-phase), and Mg2Si (β-phase) intermetallic compounds has been...     

Catalytic behavior of magnesium-copper intermetallic compound

Catalytic behavior of a magnesium-copper intermetallic alloy, Mg₂Cu, in the hydrogenation of acetone has been studied in the temperature range of 393–423 K. A continuous flow type reaction was employed. Mg₂Cu showed good catalytic activity with 2-propanol as the only product. A plausible mechanism for the mode of adsorption of acetone is proposed.