Struktur,Verzwillingung und Eigenschaften von Ce4Br3C4

Structure, Twinning, and Properties of Ce₄Br₃C₄ The new compound Ce₄Br₃C₄ can be prepared from Ce metal, CeBr₃ and C (3 : 3 : 2) at 1020 °C. It crystallizes in P 1 with a = 422.7(1) pm, b = 1103.4(3) pm, c = 1126.8(2) pm, α = 77.15(3)°, β = 90.13(2)° and γ = 84.42(3)°. The crystals are characteristically twinned, the twin law being (1 0 0, ¹/₂ –1 0, 0 0 –1). The crystal structure contains puckered layers of edge sharing Ce₆C₂ octahedra. The mean C–C distance in the C₂ units is 133(5) pm. Ce₄Br₃C₄ has at room temperature a specific resistivity of 100 mΩ cm and an effective magnetic moment of 2.55(3) μ_B (Ce³⁺).     

Solid State Metathesis Reactions in Various Applications

Solid state metathesis reactions have been studied in fused silica tubes, by differential thermal analysis, and by X‐ray powder diffraction. A selection of reactions between metal (La, Nb, and Ni) chlorides and lithium nitride or lithium acetylide were investigated to get more insight into reaction pathways and intermediate reaction stages that may be adopted on course of the formation of metal nitrides or carbides. Intermediate compounds are considered to be important because they can control the reactivity of a system. Such compounds were traced by changing the molar ratios of reaction partners away from the salt‐balanced binary metal nitride or carbide target compositions. New preparative perspectives are discovered when metal chlorides were reacted with lithium nitridoborate or lithium cyanamide. Due to their reductive nature towards several d‐block metal chlorides, (BN₂)^3‐ and (CN₂)^2‐ react to form metals or metal nitrides plus X‐ray amorphous BN, and probably C₃N₄. With lanthanum chloride they can react to form nitridoborates and nitridocarbonates. The metathesis reaction between lithium cyanamide and cyanuric chloride (C₃N₃Cl₃) instead of metal chloride was studied for the synthesis of C₃N₄.     

Superconductivity and Chemistry

The microscopic theory of superconductivity worked out by Bardeen, Cooper, and Schrieffer led to a fundamental understanding of the phenomenon. However, this theory does not provide a materials aspect. Starting from the special situation of chemical bonding in carbides and carbide halides of the rare earth metals a general view of the origin of superconductivity is developed, based on a tendency for (pairwise) localization of conduction electrons. This approach is tested in terms of special features in the electronic band structure and compared with existing physical models of electron pairing in real space. Its applicability to high-temperature superconductors discovered during the last decade is discussed.     

Ternary Alkali Metal Transition Metal Acetylides

The first alkali metal transition metal acetylides of general composition A₂M⁰C₂ (A = Na ? Cs, M⁰ = Pd, Pt) were obtained by solid state reactions of alkali metal acetylides with palladium and platinum. They are characterized by chains, which are separated by alkali metal ions. Analogous chains also separated by alkali metal ions are the characteristic structural feature of acetylides of composition AM^IC₂, which are accessible by reacting AC₂H with M^II in liquid ammonia (A = Li ? Cs, M^I = Cu, Ag, Au). Despite their structural similarities they possess different properties, as acetylides of composition A₂M⁰C₂ are semiconductors with very small indirect band gaps and slightly extended C–C distances compared to a C–C triple bond, whereas acetylides of composition AM^IC₂ show a typical salt‐like behavior with C–C distances close to the expected value for a C–C triple bond of 120 pm. But with the help of simple chemical models these differences can be made plausible. Furthermore, it is shown that only by a combination of different methods (powder diffraction with X‐rays and neutrons, solid state NMR spectroscopy, Raman spectroscopy) it was possible to characterize this new class of compounds structurally and chemically.     

The High‐Pressure Phase of MgB2C2

A high‐pressure modification of MgB₂C₂ was synthesized and structurally characterized. The compound crystallizes in the orthorhombic space group Pnnm, with the lattice parameters a = 7.19633(3) Å, b = 4.61791(13) Å and c = 2.77714(8) Å. The compound contains heterographene B–C nets, isoelectronic to graphite, just like the ambient pressure modification. The layers are intercalated by magnesium atoms, which are arranged in a chain‐like manner. According to Density Functional Theory (DFT) calculations, the high‐pressure form of MgB₂C₂ is a semiconductor with a band gap of 1.02 eV. The compound does not undergo a superconducting transition down to 2 K.     

Laser induction hybrid rapid cladding of WC particles reinforced NiCrBSi composite coatings

In order to investigate the microstructure characteristics and properties of Ni-based WC composite coatings containing a relatively large amount of WC particles by laser induction hybrid rapid cladding (LIHRC) and compare to the individual laser cladding without preheating, Ni60A + 35 wt.% WC composite coatings are deposited on A3 steel plates by LIHRC and the individual laser cladding without preheating. The composite coating produced by the individual laser cladding without preheating exhibits many cracks and pores, while the smooth composite coating without cracks and pores is obtained by LIHRC. Moreover, the cast WC particles take on the similar dissolution characteristics in Ni60A + 35 wt.% WC composite coatings by LIHRC and the individual laser cladding without preheating. Namely, the completely dissolved WC particles interact with Ni-based alloy solvent to precipitate the blocky and herringbone carbides, while the partially dissolved WC particles still preserve the primary lamellar eutectic structure. A few WC particles are split at the interface of WC and W₂C, and then interact with Ni-based alloy solvent to precipitate the lamellar carbides. Compared with the individual laser cladding without preheating, LIHRC has the relatively lower temperature gradient and the relatively higher laser scanning speed. Therefore, LIHRC can produce the crack-free composite coating with relatively higher microhardness and relatively more homogeneous distribution of WC particles and is successfully applied to strengthen the corrugated roller, showing that LIHRC process has a higher efficiency and good cladding quality.     

Some applications of analytical TEM to the characterisation of high temperature equipment

Much of the high temperature refinery and power generation equipment currently in use in Australia is typically quite old. In many cases plant has been operating for over 20 years, and in a few cases, for 40+ years. Understanding and predicting the behaviour of service-exposed equipment, which has operated at high temperatures and pressures for extended periods, requires a detailed understanding of the material microstructures and properties. Remaining life assessment of aged plant brings to bear a raft of techniques to measure mechanical properties and to look for damage effects. Analytical transmission electron microscopy (AEM), while not a traditional tool for looking at such problems, can provide some useful insights into the microstructural degradation processes which can occur during service, such as carbide coarsening, secondary precipitation and transformation. This paper will highlight some of the potentially useful information which AEM can provide, along with some of its limitations, with reference to refiner pressure vessels, turbine generating rotors, and superheater outlet headers.     

A Fully Encapsulated Acetylenediide in Ag2C2⋅8 AgF

The reaction of Ag ₂ C ₂ with a concentrated aqueous solution of AgF leads to the title compound, in which the dianion C₂²⁻ is completely enclosed in the Ag₉ cage of a chandelierlike arrangement of ten silver atoms.     

Location and Vibrations of Hydrogen in La2C3H1.5

We report on the incorporation of hydrogen into La₂C₃ and on the characterisation of its position. Temperature‐dependent X‐ray investigations show that the hydrogenation process is reversible and the product La₂C₃H_1.5 is amorphous. In principle, IR and Raman spectroscopies can provide some structural information, but in the present study, they are of only limited value. Inelastic neutron scattering (INS) has advantages for measuring the H‐vibrations in La₂C₃H_1.5 and these are compared with the results of molecular dynamics simulations. A structural model based on published pair‐potentials predicts that hydrogen (or deuterium) occupies the “tetrahedral” holes in the lanthanum sublattice. This model also accounts for the observed vibrational spectra.     

Surface structure of the missing-row reconstruction of VC0.8(1 1 0): a scanning tunneling microscopy analysis

Scanning tunnelling microscopy (STM) was used to study the (1 1 0) surface of a VC_0.8 sample. The surface shows a missing-row reconstruction, i.e., a grating structure with ridges and valleys oriented along the [0 0 1] direction and (1 0 0) and (0 1 0) facets. We did not find unreconstructed (1 1 0) terraces. The regular spacing of the ridges corresponds to a periodicity of (3 × 1) or (4 × 1), depending on preparation, presumably related to different concentrations of carbon vacancies. In the STM images, we can also observe apparent pairing of atoms in the rows, leading to the larger c(6 × 2) and (4 × 2) superstructure cells, which also show up in LEED. We attribute these additional periodicities to ordering of carbon vacancies in the surface rows.