Search results for "init"
showing 10 items of 6629 documents
Electronic properties of Co2FeSi investigated by X-ray magnetic linear dichroism
2014
We present experimental XMLD spectra measured on epitaxial (001)-oriented thin Co$_{2}$FeSi films, which are rich in features and depend sensitively on the degree of atomic order and interdiffusion from capping layers. Al- and Cr-capped films with different degrees of atomic order were prepared by DC magnetron sputtering by varying the deposition temperatures. The local structural properties of the film samples were additionally investigated by nuclear magnetic resonance (NMR) measurements. The XMLD spectra of the different samples show clear and uniform trends at the $L_{3,2}$ edges. The Al-capped samples show similar behavior as previous measured XMLD spectra of Co$_2$FeSi$_{0.6}$Al$_{0.4…
Ab initio DFT+U study of He atom incorporation into UO(2) crystals.
2009
We present and discuss results of a density functional theory (DFT) study of a perfect UO2 crystals and He atoms in octahedral interstitial positions. We have calculated basic bulk crystal properties and He incorporation energies into the low temperature anti-ferromagnetic UO2 phase using several exchange-correlation functionals within the spin-polarized local density (LDA) and generalized gradient (GGA) approximations. In all these DFT calculations we included the on-site correlation corrections using the Hubbard model (DFT+U approach). We analysed a potential crystalline symmetry reduction and confirmed the presence of the Jahn-Teller effect in a perfect UO2. We discuss also the problem o…
Ab initiolattice dynamical studies of silicon clathrate frameworks and their negative thermal expansion
2014
The thermal and lattice dynamical properties of seven silicon clathrate framework structures are investigated with ab initio density functional methods (frameworks I, II, IV, V, VII, VIII, and H). The negative thermal expansion (NTE) phenomenon is investigated by means of quasiharmonic approximation and applying it to equal time displacement correlation functions. The thermal properties of the studied clathrate frameworks, excluding the VII framework, resemble those of the crystalline silicon diamond structure. The clathrate framework VII was found to have anomalous NTE temperature range up to 300 K and it is suitable for further studies of the mechanisms of NTE. Investigation of the displa…
Ab initio modeling of oxygen impurity atom incorporation into uranium mononitride surface and subsurface vacancies
2011
The incorporation of oxygen atoms has been simulated into either nitrogen or uranium vacancy at the UN(001) surface, sub-surface or central layers. For calculations on the corresponding slab models both the relativistic pseudopotentials and the method of projector augmented-waves (PAW) as implemented in the VASP computer code have been used. The energies of O atom incorporation and solution within the defective UN surface have been calculated and discussed. For different configurations of oxygen ions at vacancies within the UN(001) slab, the calculated density of states and electronic charge re-distribution was analyzed. Considerable energetic preference of O atom incorporation into the N-v…
Chemisorption of a molecular oxygen on the UN(001) surface: Ab initio calculations
2010
The results of DFT GGA calculations on oxygen molecules adsorbed upon the (0 0 1) surface of uranium mononitride (UN) are presented and discussed. We demonstrate that O2 molecules oriented parallel to the substrate can dissociate either (i) spontaneously when the molecular center lies above the surface hollow site or atop N ion, (ii) with the activation barrier when a molecule sits atop the surface U ion. This explains fast UN oxidation in air.
High-pressure structural phase transitions in CuWO4
2010
We study the effects of pressure on the structural, vibrational, and magnetic behavior of cuproscheelite. We performed powder x-ray diffraction and Raman spectroscopy experiments up to 27 GPa as well as ab initio total-energy and lattice-dynamics calculations. Experiments provide evidence that a structural phase transition takes place at 10 GPa from the low-pressure triclinic phase (P-1) to a monoclinic wolframite-type structure (P2/c). Calculations confirmed this finding and indicate that the phase transformation involves a change in the magnetic order. In addition, the equation of state for the triclinic phase is determined: V0 = 132.8(2) A3, B0 = 139 (6) GPa and = 4. Furthermore, experim…
New Insights into the Structure of the Vapor/Water Interface from Large-Scale First-Principles Simulations
2015
We present extensive ab initio simulations of the molecular arrangements at the vapor/water interface, which provide valuable insights into the interface structure. In particular, the simulations address the controversy of whether there is a significant amount of nondonor configurations at this prototypical interface, using a novel Car-Parrinello-like ab initio molecular dynamics approach. The interface is modeled by a system of 384 water molecules for 125 ps in a two-dimensional periodic slab, the most extensive ab initio molecular dynamics simulation to date. In contrast to previous theoretical simulations and X-ray absorption spectroscopy, but consistent with sum-frequency generation exp…
Metal Cluster — Surface Interaction: Simple Models and Ab Initio Calculations
1999
We review recent ab initio atomistic calculations on interactions between metal clusters and electronically inert (insulating) substrates. The model system is sodium clusters on the sodium-chloride (001) surface. This system provides an example of weak cluster-support interaction (physisorption) which can however be easily modified by introducing color centers at the surface, resulting in chemisorption of sodium adatom or cluster. The results obtained from atomistic calculations can be used for constructing simple jellium-type models for the adsorbed cluster. These models allow for systematic investigations in a large size-range of clusters on the shell structure, dimensionality, and stabil…
Quantum Mechanical Modelling of Pure and Defective KNbO3 Perovskites
2000
Ab initio electronic structure calculations using the density-functional theory (DFT) are performed for KNbO3 with and without defects. Ferroelectric distortive transitions involve very small changes in energies and are therefore sensitive to DFT-approximations. This is discussed by comparing results obtained with the local density approximation (LDA) to those where generalized gradient approximations (GGA) are used. The results of ab initio calculations for F-type centers and bound hole polarons are compared to those obtained by a semiempirical method of the Intermediate Neglect of the Differential Overlap (INDO), based on the HartreeFock formalism. Supercells with 40 and 320 atoms were us…
Lattice Dynamics in Wurtzite Semiconductors: The Bond Charge Model of CdS
1999
An extension of the adiabatic bond charge model of Rustagi and Weber is used to study the lattice dynamic properties of wurtzite-type compounds. The model has been applied to the description of the phonon dispersion of CdS, which has been recently measured by neutron scattering. The agreement with the neutron data is excellent with a small set of physically meaningful parameters. The eigenvector admixture of the E2 modes, calculated at the G-point, agrees with the experimental values obtained through the isotopic mass dependence of the optical modes and ab initio calculations.