Unusual electronic and bonding properties of the Zintl phase Ca5Ge3 and related compounds. A theoretical analysis

Theoretical reasons for metallic behavior among diverse Zintl phases have generally not been pursued at an advanced level. Here, the electronic structure of Ca5Ge3 (Cr5B3 type), which can be formulated (Ca+2)5(Ge2-6)Ge-4 in oxidation states, has been explored comparatively by means of semiempirical and first-principles density functional methods. The FP-APW calculations show that alkaline-earth-metal and germanium orbitals, particularly the d orbitals on the cations and the p-pi orbitals of the halogen-like dimeric Ge2-6, mix considerably to form a conduction band. This covalency perfectly explains the unusual metallic properties of the nominally electron-precise Zintl phase Ca5Ge3 and its numerous relatives. Similar calculational results are obtained for Sr5Ge3, Ba5Ge3, and Ca5Sn3. Cation d orbitals appear to be a common theme among Zintl phases that are also metallic.     

溶胶-凝胶法设计与制备金属及合金纳米材料的研究进展

溶胶-凝胶法是常见的制备金属氧化物的方法之一。在溶胶-凝胶法中,各种反应物能达到分子级的均匀混合,因此能制备成份复杂的氧化物材料。目前,溶胶-凝胶法也应用于设计与制备金属纳米材料,特别是合金纳米颗粒。例如,溶胶-凝胶法能应用于制备CoPt、FePt等磁性纳米合金材料以及CoCrCuNiAl高熵合金纳米材料,以及物相结构为有序相的Cu3Pt合金纳米材料。本文综述溶胶-凝胶法设计制备金属纳米材料的研究进展,包括溶胶-凝胶法实施的基本步骤、该方法在制备金属纳米材料方面的具体应用,并着重论述采用热力学计算设计金属及化合物的基本原理。该基本原理包括计算金属氧化物与还原性气体如氢气的还原反应的吉布斯自由能的变化量、金属氧化物的标准电极电位(不同于金属离子的标准电极电位)。最后探讨溶胶-凝胶法设计制备金属纳米材料存在的问题以及后续可能的发展方向。     

Insulating or Metallic: Coexistence of Different Electronic Phases in Zinc Clusters

How many of the several attributes of the bulk metallic state persist in a nanoparticle containing a finite number of atoms of a metallic element? Do all those attributes emerge suddenly at a well‐defined cluster size or do they rather evolve at different rates and in a broad size range? These fundamental questions have been addressed through a conjoint experimental/theoretical investigation of zinc clusters. We report the observation of novel coexistence phenomena involving different electronic phases: for some sizes, metallic and insulating electronic states coexist within a single, Janus‐like, nanoparticle; for the rest of sizes, we report the coexistence of two weakly interacting metallic phases with different dimensionalities, localized at the shell and the core of the nanoparticle. These fascinating features are due to an anomalously long core–shell separation that equips the shell and core regions with largely independent structural, vibrational, and thermal properties.     

Low-coordination phases of both crystalline and fluid states of alkali metals under extreme conditions of high and low densities,respectively

After a brief summary of early work, involving the present authors, relating to low coordination phases of some alkalis in either dense crystalline states at high pressure (e.g. Li) or low density metallic fluids near criticality (Cs and Rb), contact is made with the very recent density functional study by Pickard and Needs (Phys. Rev. Lett. 102, 146401 (2009)). Whereas these authors predict three- and four-fold coordination numbers for extremely high pressure crystalline phases of Li, we stress here the remarkable behaviour of the heavy alkali metallic fluids Cs and Rb along the liquid–vapour coexistence curve towards the critical point. Coordination numbers ～8?10 near melting then reduce, as the density is lowered, to 2 at or near the critical point.     

Roles of cations, electronegativity difference, and anionic interlayer interactions in the metallic versus nonmetallic character of Zintl phases related to arsenic

A first-principles Density Functional Theory study of several layered solids structurally related to rhombohedral arsenic has been carried out. The electronic structures of rhombohedral arsenic, CaSi(2), CaAl(2)Si(2), KSnSb, and SrSn(2)As(2) are discussed in detail, emphasizing on the origins of their metallic or nonmetallic behaviours. It is found that all of these systems are metallic except KSnSb. Electronegativity differences between the elements in the anionic sublattice and/or direct interlayer interactions play the main role in controlling the conductivity behavior. CaSi(2) exhibits a peculiar feature since the cation directly influences the conductivity but is not essential for its appearance. Cation-anion interactions are shown to have an important covalent contribution, but despite this fact and the metallic character found for most of these phases, the Zintl approach still provides a valid approximation to their electronic structure.     

Concerning the different roles of cations in metallic Zintl phases: Ba7Ga4Sb9 as a test case

The question of the different roles of cations in metallic Zintl phases has been examined by taking Ba7Ga4Sb9, an electron-rich phase, as a test case. The electronic structure of this solid has been studied by means of a first-principles density functional theory approach and, indeed, the different Ba atoms are found to play very different roles in determining the structural and transport properties of this phase. It is also found that Ba7Ga4Sb9 should be an anisotropic metal with both one- and three-dimensional contributions to the Fermi surface so that the system could exhibit a potentially very interesting physical behavior while keeping the metallic properties down to very low temperatures. Suggestions in order to modify the band filling and the physical properties are examined. Although isostructural electron-precise phases may be envisioned, it is predicted that they would be essentially three-dimensional metals.     

Structure and conductivity of bilinear organic compounds

The relationship between crystal structures and metallic conductivities of linear organic materials such as TTF-TCNQ is explained in terms of strong lateral elastic interactions between chains. A microdomain model is presented in which at high temperatures there are, in general, two coexisting phases on each stacked molecular chain.     

The function of ruthenium oxides in Pt-Ru catalysts for methanol electro-oxidation at low temperatures

The electrocatalytic activities of different binary Pt-Ru(ox) catalysts have been investigated in half-cell experiments by cyclic voltammetry and stationary current–potential measurements. The materials have been prepared using a modification of the Adams method. X-ray analytical methods (X-ray diffraction, XRD, and energy dispersive X-ray spectroscopy, EDX) as well as thermogravimetric analysis (TGA) have been used to characterize the composition and the catalysts' content of the crystalline phases, and their surface areas have been determined by the BET method. It is found that the composition of the catalyst is strongly influenced by the synthesis temperature, which is varied between 400 and 600 °C. In contrast, the particle size of the metallic phases of the catalysts is not significantly affected for synthesis temperatures below 600 °C, as investigated by transmission electron microscopy. Synthesis temperatures of 500 °C favor the formation of crystalline RuO₂ phases, whereas at synthesis temperatures below 500 °C the presence of metallic alloy and of hydrous oxides was derived by the combined XRD and EDX measurements. The stationary current–potential curves show a correlation with the different synthesis temperatures. It can be concluded that both the presence of an alloyed metallic Pt-Ru phase as well as the presence of amorphous hydrated Ru oxides are favorable for the electrocatalytic oxidation of methanol.Dedicated to Prof. Wolf Vielstich on the occasion of his 80th birthday in recognition of his numerous contributions to interfacial electrochemistry     

Synthesis and Stability of Lanthanum Superhydrides

Recent theoretical calculations predict that megabar pressure stabilizes very hydrogen‐rich simple compounds having new clathrate‐like structures and remarkable electronic properties including room‐temperature superconductivity. X‐ray diffraction and optical studies demonstrate that superhydrides of lanthanum can be synthesized with La atoms in an fcc lattice at 170 GPa upon heating to about 1000 K. The results match the predicted cubic metallic phase of LaH₁₀ having cages of thirty‐two hydrogen atoms surrounding each La atom. Upon decompression, the fcc‐based structure undergoes a rhombohedral distortion of the La sublattice. The superhydride phases consist of an atomic hydrogen sublattice with H?H distances of about 1.1 Å, which are close to predictions for solid atomic metallic hydrogen at these pressures. With stability below 200 GPa, the superhydride is thus the closest analogue to solid atomic metallic hydrogen yet to be synthesized and characterized.     

生物材料用Mg-Y-Zn-Zr合金的微观结构及体外降解行为

通过向纯镁中添加微量的合金化元素（Y、Zn和Zr）,采用熔融浇铸法制备了名义成分为Mg1.5Y1.2Zn0.44Zr四元可降解镁合金生物材料,并对其进行了均匀化处理和挤压。 对这3种状态的镁合金进行了微观结构及在模拟体液(SBF)中体外降解行为的测试和分析,结果表明,合金主要由α-Mg基体和Mg12ZnY第二相组成;合金经过挤压后的组织得到了明显的细化;因此挤压后合金的降解性能得到明显改善;合金在模拟体液中生成的降解产物主要是含Ca和Mg的磷酸盐。     

Electronic structures of KNa3In9 and Na2In, two metallic phases with classical closed-shell electronic configurations

The cluster compounds KNa3In9 [K2Na6(In12)(In)6] and Na2In [(Na)8(In4)], which contain In12 icosahedra interbridged by 4-bonded In atoms and isolated In4 tetrahedra, respectively, both have classical closed-shell electronic configurations but show metallic transport properties. These contrasts have been studied by means of first-principles density functional methods (LMTO-ASA). Several bands cross the Fermi level in both compounds, consistent with their metallic properties. In KNa3In9, the metal atom framework alone is sufficient to generate a metallic characteristic. The alkali-metal s and indium p orbitals mix considerably in both phases, providing for substantial covalent contributions to their stabilities as well as bands crossing Ef. The participation of Na atoms in the 3D bonding networks is more striking in cation-richer Na2In than in KNa3In9.     

Characterization of metallic and ceramic high-temperature materials for energy systems

For the development of metallic and ceramic high temperature materials used, for example, in heat exchanger components, in turbine blades for stationary gas turbines, in ceramic industrial products and fusion reactor components, modern physico-chemical characterization methods are required. The formation stability of naturally formed protective scales is of prime importance in the successful application of metallic materials at high temperatures in aggressive atmospheres. For the characterization and investigation of the growth mechanisms of such surface scales, the main emphasis is placed on such modern spectroscopical methods as SIMS, SNMS, GDOS, EPMA and RBS. The morphology and composition of oxide scales have been investigated by imaging and diffraction techniques. The thermal and mechanical damage behaviour of high-temperature materials for application in fusion reactor components is of importance. Damage behaviour has been simulated by electron beam and laser irradiation experiments, especially by means of in situ techniques in a scanning electron microscope. By such techniques the material erosion, crack formation and crack propagation were studied for ceramic high temperature materials as a function of load parameters. The erosion and the crack formation processes are superim-posed by a redeposition of vaporized material and by thermally activated creep of the binder phases. The application potential for all methods discussed is outlined and available results are presented.     

Kinetically Controlled Solid State Reactions in the System InCl3/Sn

This paper reports on studies on the reactivity of metallic Sn in a solid state reaction in the system In‐Sn‐Cl. The reaction route is investigated by thermoanalysis and XRD measurements. The reaction of InCl₃ with Sn to form InSnCl₃ does not proceed directly but in several steps giving 4 intermediate phases. It can be interpreted as a kinetically controlled reaction where intermediate phases with a smaller degree of order and higher diffusivity are preferentially formed outside thermodynamic equilibria.     

Low‐Temperature Synthesis of Crystalline Inorganic/Metallic Nanocrystal‐Halloysite Composite Nanotubes

We have demonstrated a facile approach for the low‐temperature synthesis of crystalline inorganic/metallic nanocrystal‐halloysite composite nanotubes by employing the bulk controlled synthesis of inorganic/metallic nanocrystals on halloysite nanotubes. The halloysite clay nanotubes can adsorb the target precursor and induce inorganic/metallic nanocrystals to grow in situ. The crystalline phase and morphology of the composite clay nanotubes is tunable. By simply tuning the acidity of the titania sol, the crystalline titania‐clay nanotubes with tunable crystalline phases of anatase, a mixture of anatase and rutile or rutile are achieved. The approach is general and has been extended to synthesize the representative perovskite oxide (barium and strontium titanate)‐halloysite composite nanotubes. Metallic nickel nanocrystal can also be grown on the surface of halloysite nanotubes at low temperature. The traditional thermal treatment for crystallite transformation is not required, thus intact contour of halloysite nanotubes and the crystallinity structure of halloysite nanotubes can be guaranteed. The combined properties from inorganic/metallic nanocrystal (high refractive index, high dielectric constant and catalytic ability) and the halloysite clay nanotubes are promising for applications such as photonic crystals, high‐k‐gate dielectrics, photocatalysis and purification.     

Propene hydrogenation with PtAg intermetallic phases supported on SiO2 gels

Various intermetallic phases composed of Pt/Ag supported on SiO₂-gel were synthesized by reduction of Pt(allyl)₂ and Ag[cyclooctadiene]₂⁺ precursors anchored on the support surface. This method afforded highly dispersed metallic particles. X-ray analysis was used to ascertain the occurrence of alloy, for structural identification of Pt/Ag phases and degree of dispersion. The catalytic activity of these systems was studied in gas-phase hydrogenation of propene within the temperature range 10–90 °C. Kinetic parameters such as specific reaction rates, reaction orders and apparent activation energies were calculated as functions of the Pt/Ag ratio. Results are compared and discussed in terms of the structural and electronic features of the active metallic phase.     

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.     

First-principles study of K and Cs adsorbed on Pd(111)

The adsorptions of K and Cs on Pd(111) were studied by the density functional calculations within the generalized gradient approximation. The site preference, bonding character, work function, and electron structure of the system were analyzed. For K and Cs adsorption, the hcp hollow site was found to be preferred for all the coverages investigated. The calculated adsorption geometries for (2 x 2) and (square root 3 x square root 3)R30 degrees phases are both in reasonable agreement with the observed results. The decrease of the work function upon the adsorption of K and Cs can be attributed to a dipole moment associated with the polarized adsorbate atom, which is characterized by depletion of the electron charge in the alkali metal layer and a charge accumulation in the interface region. Our results indicate that the bonding of alkali metal with the Pd(111) surface has a mixed ionic and metallic bond character at low coverage and a metallic bond of covalent character at high coverage.     

Stable isotope dilution analyses of molybdenum in meteorites

Isotope dilution mass spectrometry is an ideal analytical technique to measure the elemental abundance of Mo in C1 carbonaceous chondrites and the metallic and troilite phases of iron meteorites. The mean abundance of Mo in two C1 meteorites is 0.909+/-0.040 microg/g which corresponds to a value of 2.55 atoms Mo with respect to Si equal to 10(6) atoms, which is identical to the currently accepted solar system abundance. The partitioning of Mo between the metallic and sulfide phases in the Mundrabilla iron meteorite was found to be 6.0+/-0.2 microg/g and 8.6+/-0.3 microg/g, respectively. A new, precise Mo concentration of 1.54+/-0.04 microg/g for the Geochemical Reference Standard BCR-1 is also reported.     

Physico-chemical characterization of crystalline phases in fly ashes

Fuel oil combustion in power plants, domestic heating systems and diesel engines, causes the emission in the environment of particles with a typical structure and composition: the cenospheres.These particles are produced during the microdrop fuel oil combustion, when air and fuel are injected into the combustion chamber; they have a spheroidal morphology and a spongy structure.Cenospheres are constituted by an amorphous component rich in C, S, Si, Fe and Al; phases composed by microcrystals of sulphates, oxides and pure metallic elements or their alloys, are frequently present in the cenospheres.These crystalline phases are important from environmental and toxicological points of view both because they are composed of heavy metals, and because they can play an important role in heterogeneous catalysis.We started to study these crystalline phases by analytical electron microscopy techniques and electron energy loss spectrometry to define and characterise their structure and composition.     

Structure, properties, and bonding of ZrTe (MnP type), a low-symmetry, high-temperature modification of ZrTe (WC type).

ZrTe (MnP) was synthesized by high-temperature methods at 1570 K in Ta ampules. The structure of the telluride was determined by means of single-crystal X-ray diffraction to be orthorhombic, Pnma (No. 62), Z = 4, Pearson Symbol oP8, a = 739.15(15) pm, b = 377.23(8) pm, c = 694.34(14) pm. The orthorhombic MnP type structure is a distorted variant of the NiAs type structure with pronounced metal-metal zigzag chains. Zigzag chains are typical for phases with a d(2) metal atom electron configuration. According to extended Hückel calculations, the homonuclear interactions in the zigzag chains make up for 2/3 of the Zr-Zr interactions in ZrTe (MnP) and contribute decisively to the stability of the structure. The emergence of the distorted MnP type structure instead of the high-symmetry NiAs type ZrTe at high temperatures can be understood as the result of an optimization of homonuclear Zr-Zr interactions arising from states close to the Fermi level. The hexagonal WC type ZrTe transforms above 1438 +/- 5 K into ZrTe (MnP) (DeltaH = 8.3 +/- 1.0 kJ mol(-1)). The phase transition is reversible, although at room-temperature the reverse reaction is kinetically inhibited. Zr5Te4 and Zr5Te6 are the phases next to ZrTe. ZrTe (MnP) exhibits temperature-independent paramagnetic properties (chi(mol) = 0.14 x 10(-3) cm(3) mol(-1)), as typical for a metallic conductor. Resistivity measurements on ZrTe (MnP) imply metallic behavior.