Paul Erhart

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.

Atomistic Shock Hugoniot simulation of single-crystal copper

Planar shock waves in single-crystal copper were simulated using nonequilibrium molecular dynamics with a realistic embedded atom potential. The simulation results are in good agreement with new experimental data presented here, for the Hugoniot of single-crystal copper along ⟨100⟩. Simulations were performed for Hugoniot pressures in the range 2 GPa – 800 GPa, up to well above the shock induced melting transition. Large anisotropies are found for shock propagation along ⟨100⟩,⟨110⟩, and ⟨111⟩, with quantitative differences from pair potentials results. Plastic deformation starts at Up≳0.75km∕s, and melting occurs between 200 and 220 GPa, in agreement with the experimental melting pressure of polycrystalline copper. The Voigt and Reuss averages of our simulated Hugoniot do not compare well below melting with the experimental Hugoniot of polycrystalline copper. This is possibly due to experimental targets with preferential texturing and/or a much lower Hugoniot elastic limit.

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.

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

Atomistic modeling of shock-induced void collapse in copper

{M}_{\mathrm{Ti}}\ensuremath{-}{V}_{\mathrm{O}}

Analytic bond-order potential for atomistic simulations of zinc oxide

(M=\mathrm{Cu},\mathrm{Fe})