Karsten Albe

Extracting dislocations and non-dislocation crystal defects from atomistic simulation data

We describe a novel method for extracting dislocation lines from atomistic simulation data in a fully automated way. The dislocation extraction algorithm (DXA) generates a geometric description of dislocation lines contained in an arbitrary crystalline model structure. Burgers vectors are determined reliably, and the extracted dislocation network fulfills the Burgers vector conservation rule at each node. All remaining crystal defects (grain boundaries, surfaces, etc), which cannot be represented by one-dimensional dislocation lines, are output as triangulated surfaces. This geometric representation is ideal for visualization of complex defect structures, even if they are not related to dislocation activity. In contrast to the recently proposed on-the-fly dislocation detection algorithm (ODDA) Stukowski (2010 Modelling Simul. Mater. Sci. Eng. 18 015012) the new method is extremely robust. While the ODDA was designed for a computationally efficient on-the-fly analysis, the DXA method enables a detailed analysis of dislocation lines even in highly distorted crystal regions, as they occur, for instance, close to grain boundaries or in dense dislocation networks.

Analytical interatomic potential for modeling nonequilibrium processes in the W-C-H system

A reactive interatomic potential based on an analytical bond-order scheme is developed for the ternary system W–C–H. The model combines Brenner’s hydrocarbon potential with parameter sets for W–W, W–C, and W–H interactions and is adjusted to materials properties of reference structures with different local atomic coordinations including tungsten carbide, W–H molecules, as well as H dissolved in bulk W. The potential has been tested in various scenarios, such as surface, defect, and melting properties, none of which were considered in the fitting. The intended area of application is simulations of hydrogen and hydrocarbon interactions with tungsten, which have a crucial role in fusion reactor plasma-wall interactions. Furthermore, this study shows that the angular-dependent bond-order scheme can be extended to second nearest-neighbor interactions, which are relevant in body-centered-cubic metals. Moreover, it provides a possibly general route for modeling metal carbides.

Modelling of compound semiconductors: analytical bond-order potential for gallium, nitrogen and gallium nitride

An analytical bond-order potential for GaN is presented that describes a wide range of structural properties of GaN as well as bonding and structure of the pure constituents. For the systematic fit of the potential parameters reference data are taken from total-energy calculations within the density functional theory if not available from experiments. Although long-range interactions are not explicitly included in the potential, the present model provides a good fit to different structural geometries including defects and high-pressure phases of GaN.

Diffusion of zinc vacancies and interstitials in zinc oxide

The self-diffusion coefficient of zinc in ZnO is derived as a function of the chemical potential and Fermi level from first-principles calculations. Density functional calculations in combination with the climbing image-nudged elastic band method are used in order to determine migration barriers for vacancy, interstitial, and interstitialcy jumps. Zinc interstitials preferentially diffuse to second nearest neighbor positions. They become mobile at temperatures as low as 90–130K and therefore allow for rapid defect annealing. Under predominantly oxygen-rich and n-type conditions self-diffusion occurs via a vacancy mechanism.

Molecular-dynamics simulations of steady-state growth of ion-deposited tetrahedral amorphous carbon films

Molecular-dynamics calculations were performed to simulate ion beam deposition of diamond-like carbon films. Using the computationally efficient analytical potentials of Tersoff and Brenner we are able to simulate more than 103 carbon atom impacts on {111} diamond, so that steady-state film properties can be computed and analyzed. For the Tersoff potential, we achieve sp3 fractions approximately half of the experimentally observed values. For the more refined hydrocarbon potentials of Brenner the fraction of tetrahedrally coordinated atoms is much too low, even if structures with densities close to diamond are obtained. We show, that the sp3 contents calculated with Tersoff’s potential are an artifact related to the overbinding of specific bonding configurations between three- and fourfold coordinated sites. On the other hand, we can prove, that the range for the binding orbitals represented by the cutoff function is too short in Brenner’s parametrization. If an increased C–C interaction cutoff value is c...

Analytic bond-order potential for bcc and fcc iron—comparison with established embedded-atom method potentials

A new analytic bond-order potential for iron is presented that has been fitted to experimental data and results from first-principles calculations. The angular-dependent functional form allows a proper description of a large variety of bulk, surface and defect properties, including the Bain path, phonon dispersions, defect diffusivities and defect formation energies. By calculating Gibbs free energies of body-centred cubic (bcc) and face-centred cubic (fcc) iron as a function of temperature, we show that this potential is able to reproduce the transitions from α-iron to γ-iron and δ-iron before the melting point. The results are compared to four widely used embedded-atom-method potentials for iron.

Modelling of boron nitride: Atomic scale simulations on thin film growth

Molecular-dynamics simulations on ion-beam deposition of boron nitride are presented. A realistic Tersoff-like potential energy functional for boron nitride, which was specially fitted to ab initio-data, has been used. The impact of energetic boron and nitrogen atoms on a c-BN target is simulated with energies ranging from 10 to 600 eV. The structural analysis of the grown films shows that a loose, dominantly sp 2 -bonded structure arises at high ion flux. In no case the formation of a sp 3 -bonded phase is observed, but the obtained films partially reveal textured basal planes as found in experiment. Two different growth regimes are identified for ion energies above and below 100 eV.

Size-Dependent Lattice Expansion in Nanoparticles: Reality or Anomaly?

Size-dependent lattice expansion of nanoparticles is observed for many ionic compounds, including metal oxides, while lattice contraction prevails for pure metals. However, the physical origin of this effect, which is of importance for the thermodynamic, chemical and electronic properties of nanoparticles, is discussed controversially. After a survey of the experimental literature, revealing a wide variety of materials with size-dependent lattice expansion, we show that the negative surface stress is the key reason for lattice expansion, while the excess of lattice sums or point defects of various charge states can be excluded as general explanations. Ab initio calculations of surface stresses for various surface structures of metal oxides confirm the model of a surface-induced lattice expansion.

Thermodynamics of mono- and di-vacancies in barium titanate

The thermodynamic and kinetic properties of mono- and di-vacancy defects in cubic (para-electric) barium titanate BaTiO3 are studied by means of density-functional theory calculations. It is determined which vacancy types prevail for given thermodynamic boundary conditions. The calculations confirm the established picture that vacancies occur in their nominal charge states almost over the entire band gap. For the dominating range of the band gap the di-vacancy binding energies are constant and negative. The system, therefore, strives to achieve a state in which, under metal-rich (oxygen-rich) conditions, all metal (oxygen) vacancies are bound in di-vacancy clusters. The migration barriers are calculated for mono-vacancies in different charge states. As oxygen vacancies are found to readily migrate at typical growth temperatures, di-vacancies can be formed at ease. The key results of the present study with respect to the thermodynamic behavior of mono- and di-vacancies influence the initial defect distribu...

Association of oxygen vacancies with impurity metal ions in lead titanate

Thermodynamic, structural, and electronic properties of isolated copper and iron atoms as well as their complexes with oxygen vacancies in tetragonal lead titanate are investigated by means of first principles calculations. Both dopants exhibit a strong chemical driving force for the formation of