No abstract

No abstract

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.

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.

The formation of the gaseous species LiCl, Li2Cl2, AlCl3 and LiAlCl4 was shown by mass spectrometric studies of the reaction of solid LiCl with gaseous AlCl3 at 575 °C. Besides AlCl3 and Al2Cl6, the gas complexes LiAlCl4, Li2AlCl5 and LiAl2Cl7 were formed during the evaporation of liquefied LiAlCl4. The structures of the molecules under discussion were computed by quantum chemical DFT studies. Thermodynamic data of these molecules were determined by experimental methods (mass spectrometry), and the results were confirmed by theoretical calculations. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)