Preparation of cobalt borides by solid-gas reactions.

The reaction of Co with gaseous BBr3 in a temperature range of 700 to 1000 °C was studied using the hot-wire method with an experimental set-up reminiscent of the van Arkel-de Boer method. The borides Co2B und CoB form as layers on the surface of elemental cobalt. The influence of pressure, temperature and time on the reaction rate and on the composition of the borides was investigated. The reaction rate is significantly decreased by small amounts of an inert gas. The adjustment of reaction conditions allows to obtain single-phase and well-crystallized bulk materials of Co2B or CoB.

Tungsten Borides: On the Reaction of Tungsten with Boron(III) Bromide.

The crucible-free reaction between heated tungsten wires and gaseous boron tribromide yields different tungsten borides. The experiments were carried out at various reaction temperatures and times, revealing the formation of W2 B, WB and WB2 phases. The underlying reactions were analyzed by using thermodynamic model calculations. The stability of the gaseous tungsten bromides was evaluated using quantum chemical methods. While the developed synthesis of phase-pure borides is only possible to a limited extent, it offers a potential route for a formation of protective coatings with high chemical and thermal resistance.

Crucible-Free Preparation of Transition-Metal Borides: HfB2.

This study gives an account of an innovative, crucible-free technique for the synthesis of single-phase borides at relatively moderate temperatures. A metal wire heated by an electrical current reacts with a chosen gaseous boron halide in a gas/solid reaction yielding a single-phase, oxygen- and carbon-free product, as evidenced by X-ray powder diffraction and chemical analysis. This method is demonstrated using the example of hafnium reacting with boron tribromide. Preliminary thermodynamic considerations show that this kind of crucible-free synthesis specifically enables the preparation of borides of transition metals and similar elements.

Unusual optical properties of Mn-doped ZnO: the search for a new red pigment-a combined experimental and theoretical study.

A pigment of your imagination: A range of polycrystalline solid solutions of a zinc-rich Zn(x-1)Mn(x)O system (see figure) have been prepared and studied in terms of their colour, diffuse reflectance spectra, Mn valence state and electronic structure. The intense optical absorption arises from Mn(2+) doping and is thought to be due to forbidden or partially forbidden transitions between the valence and the conduction band.We report an investigation of zinc-rich polycrystalline solid solutions of the Zn(1-x)Mn(x)O system concerning the colour, the diffuse reflectance spectra, the valence state of manganese and the electronic structure. Samples were prepared by a chemical-vapour-transport-assisted route and optimized with respect to colour strength. In agreement with previous experimental results, EPR studies showed that manganese is in the divalent charge state. The nature of the very intense optical absorption, which is caused by Mn(2+) doping and determines the colour of the material, is discussed. It is argued that the Mn(2+)-induced optical absorption is due to forbidden or partially forbidden transitions between the valence and the conduction band that involve Mn admixed states. This assignment is also confirmed by quantum chemical calculations using the semiempirical molecular orbital method MSINDO.

Band gap engineering of ZnO via doping with manganese: effect of Mn clustering.

The effect of Mn doping on optical properties of zinc oxide ZnO has been studied theoretically. The dependence of the Mn concentration and distribution on the optical band gap was investigated at density-functional level applying a hybrid functional. Supercells of varying size were used to model different Mn concentrations. Possible point defects such as oxygen vacancies and zinc interstitials were taken into account. The thermodynamic stability of defect clustering in ZnO was studied. The magnetic coupling between the Mn ions was studied in dependence of the Mn-Mn distance and the distance to lattice defects. As a main result, we find that Mn clustering in the ZnO host lattice is energetically preferred, and leads to pronounced changes in the electronic structure. In agreement with previous theoretical studies we obtain antiferromagnetic ground states in the absence of point defects. The energy difference between ferromagnetic and antiferromagnetic coupling decreases if electron donating defects such as interstitial Zn are close to Mn ions. The strong dependence of the optical band gap from the Mn-Mn and Mn-defect distances is in line with earlier experiments.

Identification and characterization of monomeric, volatile SiCl3NH2 as product of the reaction between SiCl4 and NH3: an important intermediate on the way to silicon nitride?

We studied the reaction of SiCl(4) with NH(3) by mass spectrometry and IR spectroscopy. By means of mass spectrometry, SiCl(3)NH(2) was for the first time identified as an intermediate generated in significant amounts in the course of the reaction. In additional experiments, SiCl(3)NH(2) was formed as a stable gaseous product of the ammonolysis of SiCl(4), and the product was identified and characterized in detail by IR spectroscopic methods (gas phase and matrix isolation) in combination with quantum-chemical calculations. The calculations also gave access to important thermodynamical data.