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.

Crystalline solids have become a subject of growing interest for both experimentalists and theorists. In particular their defect properties are of fundamental importance in modern and future technical applications. The efficiency of fuel cells and batteries strongly depends on the mobility of ions in the lattice which is affected by various kinds of point defects and the local crystal structure. Fundamental understanding of processes involved in ion migration at atomic scale can be achieved by combined spectroscopical and theoretical investigation. During the last decades theoretical methods have become an indispensable tool for studying solid state materials. A broad variety of methods and models are available, all of them with peculiar benefits. In this review article an overview of some state-of-the-art methods and model types is given with a focus on their applicability to studies of defects and ion mobility.

MSINDO calculations were performed to elucidate the effect of doping and defects on the electronic properties of zinc oxide. The cyclic cluster Zn48O48 Served as a model for the bulk. Band gaps and stabilities of spin states were determined for doping with Mn, Fe, and Co. It was found that a substantial lowering of the band gap occurs for Mn and Co doping. In contrast, the Fe doping has a rather insignificant effect on the band gap. Additional defects such as oxygen vacancy, zinc interstitial, or zinc vacancy were also studied. In the case of substitution of zinc by two transition metal atoms, various spin states arise which can be classified as anti ferromagnetic, ferrimagnetic, or ferromagnetic. We find that the spin state with the lowest multiplicity, anti ferromagentic or ferrimagnetic, is more stable than the high-multiplicity ferromagnetic state in most considered cases, but additional zinc vacancies or mixed doping may reverse this trend.

Cyclic cluster studies with the quantum chemical method MSINDO were performed to elucidate the key properties of brucite concerning the magnesia expansion. A model is presented that allows the calculation of the compressibility of brucite under hydrostatic pressure. In this way, the bulk modulus of brucite can be determined via the binding energy. The elastic constants, Poisson's ratios, volume compressibility, and finally the bulk modulus were calculated and compared with the experimental values. Except for the component K-a of the compressibility, there is good agreement with the experimental values. Based on the bulk modulus, an expression for the volume change of brucite under pressure is derived. The hydration pressure of the periclase reaction with water to form brucite is related to the partial water vapor pressure. In this way, the maximum hydration pressure was determined.

The effects of oxygen vacancies and zinc interstitials on the structure and energy of zinc oxide were studied with the semiempirical MO method MSINDO. Cyclic clusters were chosen as model systems. Single and multiple removal of oxygen atoms and zinc interstitials in zinc oxide served to determine the defect formation energy and the band gap. The interaction between two and three oxygen vacancies was investigated. The vacancies cause a decrease of the band gap, which originates from an occupied defect level. This is also found for zinc interstitials under zinc rich conditions. The defect formation energy of such zinc interstitials is found to be lower than that of oxygen vacancies at 0 K but decreases for oxygen vacancies and increases for zinc interstitials with increasing temperature.

Cyclic cluster calculations were performed with the quantum chemical method MSINDO to elucidate the relative stabilities of c(4×2), p(3×2) and (1×1) overlayer structures of water molecules on the MgO(100) surface. For the c(4×2) and p(3×2) structures both molecular adsorption and partially dissociated adsorption were considered. In agreement with earlier theoretical studies partial dissociation was found to be more stable than molecular adsorption. For the c(4×2) structure both monolayer and double layer coverage were studied. Adsorption was found to be more stabilized with increasing degree of dissociation until 50% of the water molecules were dissociated. In the case of 50% dissociated water molecules we found that one quarter of the Mg atoms were pulled out of the MgO surface when surface relaxation was taken into account. A new structure for the fully dissociated (1×1) water monolayer was found which is considerably more stable than previously studied arrangements. In all cases surface relaxation was found to be important. The most stable structures of c(4×2), p(3×2) and (1×1) symmetry have adsorption energies which differ by no more than 13kJ/mol. This offers an explanation of phase transitions of overlayer structures found by experiments between 180 and 300K.

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Molecular dynamics (MD) calculations were performed to determine the vibrational contribution to the entropy of mixing and its importance for the mixing of ZnO/ZnS and ZnS/Zn3P2. These systems were modeled by cyclic clusters Zn48O48, Zn32S32, and Zn48P32. The mixed cyclic clusters considered were Zn48O47S, Zn32S31O, Zn33S30P2, and Zn47S2P30. For each of the clusters, the entropy was calculated in the range of the experimental temperature of the mixing process. The convergence of the entropy with respect to the number of MD steps was studied. Finally, the thermal part of the entropy of mixing was determined, and its dependence on the number of MD steps was investigated. It was found that the thermal entropy is important for the Gibbs free energy of mixing near the miscibility gaps.

Solid solutions in the system zinc sulfide/zinc phosphide (Zn(2+)(x)S(2-2xP(2x)) were investigated using the cyclic cluster model within the semiempirical MSINDO method. Results of cyclic cluster calculations for binding energies of the perfect ZnS and Zn(3)P(2) are presented and compared with the experimental data. The miscibility of ZnS and Zn(3)P(2) over the whole composition range of 0 < x < 1 was investigated by calculating the Gibbs free energy of mixing Delta(M)G for different values of x. A miscibility gap was found at both ends of the composition range and compared with experimental data.

The electronic and magnetic structure of the low‐temperature phase of ScMnO3 was studied theoretically from first principles. The solid phase was modeled with periodic supercells using three different methods: unrestricted Hartree–Fock, B3LYP and BLYP. The magnetic coupling constants obtained with these methods were compared with experimental values. The effect of lattice relaxation on the coupling constant was investigated. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

This paper presents dipole moment (μ), static mean polarizability (α) and mean first hyperpolarizability (β) of thiophene-substituted stilbene derivatives calculated in the framework of density functional theory. The calculations were performed using a finite field approach implemented in the density functional program allchem. All-electron type basis sets optimized for the calculation of the polarizabilities and hyperpolarizabilities using a local exchange-correlation functional were employed. The molecular structures have been fully optimized using the semiempirical program MSINDO. The calculated mean first hyperpolarizability trends are in agreement with the experimental trends obtained from hyper-Rayleigh scattering technique. This work shows that the increased of hyperpolarizability of the studied thiophene-substituted stilbene derivatives is due to a geometrical effect of their trans and cis isomers.

Anion substitution effects on the structure and energy of zinc chalcogenides were studied with the semiempirical molecular orbital method MSINDO. Cyclic clusters of different sizes were chosen as model systems. The convergence of the bulk properties of the perfect clusters with increasing cluster size was tested. Single and multiple substitution of oxygen atoms in zinc oxide by sulfur and of sulfur atoms in zinc sulfide by oxygen served to determine the energetics of substitution for these two cases. It was found that the substitution of oxygen by sulfur in ZnO is easier than the substitution of sulfur by oxygen in ZnS in agreement with experimental results. The interaction between two oxygen atoms vs. two selenium atoms in zinc sulfide was investigated. Oscillations of the cluster energy in dependence of the distance between the two doping atoms were observed. These are explained by the relative sites of the doping atoms in the crystal lattice. The magnitude of the oscillations is smaller in ZnS:Se than in ZnS:O, because the difference between the anion radii of S2- and Se2- is smaller than between S2- and O2-. This is also reflected in the band gap.

Born–Oppenheimer molecular dynamics (MD) simulations were performed in the framework of the semi-empirical molecular orbital method MSINDO to study water adsorption on rutile surfaces. Monolayer and doublelayer water coverage was considered on the rutile (110) and (100) surfaces and the adsorbate structures were determined. Vibrational density of states of hydrogen atoms were calculated by constant temperature MD simulations at 100K. These were used to interpret the experimental vibrational spectrum by assigning all peaks to the particular types of hydrogen atoms.

MgO hydration is an expansive process. It is used in cement to compensate for shrinkage, but it can be harmful at higher concentrations. In this quantum-chemical study, the first steps of the hydration of MgO have been explored. The semiempirical MO method MSINDO with the cyclic cluster model, including long-range electrostatic interactions, has been applied to investigate water adsorption on the (001) surface of MgO. Both molecular adsorption and dissociative adsorption have been found to occur. The latter is stabilized by hydrogen bonding from neighbouring water molecules and surface oxygen. Several energy minima exist for the experimentally low temperature p(3×2) monolayer H2O observed on the MgO (001) surface which consists of a mixture of dissociated and molecular water. One third of the water molecules were dissociated, which is consistent with previous DFT studies. A stable fully hydroxylated MgO (001) surface with OH bridging two Mg ions and the hydrogen bound to the surface oxygen was discovered. This structure appears to promote the expansive, topotactic nucleation of brucite on the MgO (001) surface.

The formation of a solid by the reaction of small gaseous molecules during a CVD or CVT experiment is a highly complicated process. In the case of the combustion of silicon(IV) chloride in oxygen, which leads to solid silicon dioxide, hundreds of intermediate chlorosiloxanes have been detected. On the basis of their compositions, structures, and stabilities, growth mechanisms of Si-O networks can be derived. The stepwise formation of solid SiO2 can be described in terms of three basic reactions: (i) formation of highly reactive O=SiCl2, (ii) insertion of O=SiCl2 into an Si-Cl bond, and (iii) thermally induced elimination of SiCl4 from the resulting chlorosiloxanes.

Oxygen vacancy diffusion in rutile was studied by Born-Oppenheimer molecular dynamics techniques in the framework of the semiempirical molecular orbital method MSINDO. Migration of an oxygen vacancy from the rutile (110) surface towards the bulk was simulated. The metadynamics technique was employed to accelerate the diffusion processes. In this way, transition state structures and activation energies for the diffusion processes were obtained. Rate constants and the time scale of diffusion processes were estimated for different temperatures using the calculated activation energy. It was found that the vacancies in the bulk are less stable than on the surface. The feasibility of oxygen vacancy diffusion under experimental conditions is discussed.

The reaction of surface hydroxyl groups with adsorbed VO4H3 on TiO2-anatase surfaces was investigated by means of the semiempirical molecular orbital method MSINDO. The (101), (001) and (100) surfaces of anatase were considered. They were modeled by appropriate two-dimensional cyclic clusters of TiO2. Stable structures of VO4H3 species on the (101), (001) and (100) surfaces were obtained by Born–Oppenheimer molecular dynamics simulations at 300K and 600K. The reaction of the hydrated vanadia species with the surface hydroxyl groups was studied using metadynamics techniques. It was found that the reaction between these species is only favorable on the (100) surface. In consequence the formation of hydrated polymeric chains (VO4H)n should take place on this surface.

Structures of hydrated vanadia species on the TiO2-anatase surfaces were investigated using the semiempirical molecular orbital method MSINDO. The (101), (001), and (100) surfaces of anatase were considered. They were modeled by appropriate two-dimensional cyclic clusters of TiO2. Monomeric and dimeric hydrated vanadia species on the anatase surfaces were simulated by adsorbing VO4H3 and V2O7H4 molecules, respectively. Different adsorption structures were considered, and their stabilities at 300 and 600 K were tested by constant-temperature Born-Oppenheimer molecular dynamics simulations in the framework of MSINDO. Structural features of the vanadia-titania catalysts found in extended X-ray absorption fine structure, secondary ion mass spectrometry, IR, Raman, and NMR spectroscopy and conductivity experiments can be explained by the present calculations.

The adsorption of small molecules NO, NH3 and H2O on V2O5/TiO2 catalysts is studied with the semiempirical SCF MO inethod MSINDO as pre-stage for the selective catalytic reduction of NO. The mixed catalyst is represented by hydro-en-terminated cluster models. The local arrangement of the cluster atoms is in accordance with available experimental information. partial relaxation Of cluster atoms near the adsorption sites is taken into account. calculated adsorption energies are compared with experimental literature data. Rapid convergence of computed properties with cluster size is observed. A possible reaction mechanism for the catalytic reduction of NO with NH3 and O-2 is outlined.