A reactive interatomic potential based on an analytic 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, like 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, that have a crucial role in fusion reactor plasma-walls. 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.

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 M Ti − V O ͑M =Cu,Fe͒ defect associates. The most stable configurations correspond to a local dipole aligned along the tetragonal axis parallel to the spontaneous polarization. Local spin moments are obtained and the calculated spin densities are discussed. The calculations provide a simple and consistent explanation for the experimental findings. The results are discussed in the context of models for degradation of ferroelectric materials.

The defect structure of hard copper-modified polycrystalline PbTiO3 ferroelectrics is investigated by means of electron paramagnetic resonance and hyperfine sublevel correlation spectroscopy, as well as density functional theory calculations. Special emphasis is put on the 207Pb-hyperfine couplings, which are resolved up to the third coordination sphere. The results prove that copper is incorporated at the octahedrally coordinated Ti site, acting as an acceptor. Because of charge compensation the formation of Cu impurity-oxygen vacancy pairs is energetically very favorable. The corresponding (CuTi''-VO)x defect dipole is found to be orientated along the [001] axis.