Göran Wahnström

Structure and thermodynamic stability of hydrogen interstitials in BaZrO3 perovskite oxide from density functional calculations

Density functional calculations have been used to study the electronic structure, preferred sites in the lattice, formation energies and vibrational frequencies for hydrogen interstitials in different charge states in the cubic phase of perovskite-structured BaZrO3. By combining ab initio results with thermodynamic modeling, defect formation at finite temperature and pressure has been investigated. We demonstrate how the site selectivity and spatial distribution of dopant atoms in the lattice can be affected by changes in the environmental conditions (atomic chemical potentials, oxygen partial pressure and temperature) used during processing of the material. In addition, we have calculated the thermodynamic parameters of the water uptake reaction for an acceptor-doped BaZrO3 crystal in equilibrium with a humid atmosphere. The interaction energies between a protonic defect and the investigated Ga, Gd, In, Nd, Sc, and Y dopants were found to be attractive, and we show that a simple model of defect association may reproduce an experimentally observed trend in the hydration enthalpy.

Motion of ‘‘hot’’ oxygen adatoms on corrugated metal surfaces

We have investigated the likelihood of a high transient mobility for hyperthermal adatoms resulting from dissociative adsorption of a diatomic molecule, in particular O2 on Al(111), using dynamical simulations on model potentials fitted to available first principle data. We find no evidence for a large transient mobility, compatible with the conclusions by Brune et al. [Phys. Rev. Lett. 68, 624 (1992)], for hyperthermal O atoms moving on the chemisorption potential energy surface for the O/Al(111) system. Our findings are more compatible with the STM results for O2 on Pt(111). We have also examined the possibility that one of the O atoms moves further away from the surface as a neutral species. That could result in that some O atoms leave the surface as neutral species (abstraction) or extended trajectories along the surface if a weakly bound neutral state for the O atom exists with sufficiently long lifetime.

Transient hyperthermal diffusion following dissociative chemisorption: a molecular dynamics study

Abstract The non-equilibrium diffusion of highly hyperthermal atoms resulting from dissociative adsorption of a diatomic molecule is investigated using molecular dynamics and an effective-medium model potential. The parameters of the potential are chosen to describe O2 on Al(111) and the simulation results are discussed in relation to recent STM measurements for this system [Brune et al., Phys. Rev. Lett. 68 (1992) 624]. The transient displacement along the surface is found to be about one order of magnitude larger than the distance to the nearest adsorption site. The traveling distance is limited mainly by the rapid randomization of the adsorbate motion, and not primarily by the rate of energy transfer to the substrate degrees of freedom.

The calculation of the thermal rate coefficient by a method combining classical and quantum mechanics

We propose and test a method for computing flux–flux correlation functions (and thermal rate coefficients) which divides the degrees of freedom in two groups, one treated classically and the other quantum mechanically. The method is tested by applying it to a simple model for which we can also obtain exact results. The approximate method gives good results if the mass associated with the classical degrees of freedom exceeds 16 a.u.

Diffusion of an adsorbed particle: theory and numerical results

Abstract A theory for the motion of an atom adsorbed on a solid surface is developed, based on the Zwanzig-Mori approach and on nonlinear mode coupling ideas. It generalizes the Fokker-Planck equation by incorporating a general frequency, wavevector, as well as position dependent friction coefficient. The purpose of this paper is primarily to investigate the validity of the Fokker-Planck equation when applied to the motion of an adsorbed atom coupled to the phonon excitations of the substrate. This is the first in a series of papers dealing with this problem. The main conclusion drawn here is that for the situation when the adatom is identical with the substrate atoms the Fokker-Planck equation gives quite accurate results, provided one uses a proper position dependent friction coefficient. The position dependence found from the theory is such that it supports in this case the absolute rate theory for the diffusion constant.

An interaction model for OH+H2O-mixed and pure H2O overlayers adsorbed on Pt(111)

A model potential for the adsorbate-adsorbate interaction among OH and H2O molecules adsorbed on a Pt(111) surface has been developed solely based on first-principle calculations. By combining this directional-dependent model potential for the lateral interaction with a lattice model of Ising type, large length scale structure calculations can be made. The strength of different hydrogen bonds can be analyzed in detail from this model potential. It is found that the hydrogen bond between OH and H2O molecules is stronger than that between two H2O molecules (0.4 eV per pair as compared to 0.2 eV per pair, respectively). Via the computed chemical potential for water in mixed OH + H2O overlayers the water uptake as a function of oxygen precoverage on Pt(111) has been determined. The results compare very well with recent experiments.

Oxygen vacancy segregation and space-charge effects in grain boundaries of dry and hydrated BaZrO3

A space-charge model is applied to describe the equilibrium effects of segregation of double-donor oxygen vacancies to grain boundaries in dry and wet acceptor-doped samples of the perovskite oxide BaZrO3. The grain boundary (GB) core vacancy concentrations and electrostatic potential barriers resulting from different vacancy segregation energies were evaluated. Density-functional calculations on vacancy segregation to the mirror-symmetric Σ3 (112) [-110] tilt grain boundary are also presented. Our results indicate that oxygen vacancy segregation can be responsible for the low grain boundary proton conductivity in BaZrO3 reported in the literature.

Hydrogen motion on a rigid Cu surface: the calculation of the site to site hopping rate by using flux-flux correlation functions

We use the quantum flux–flux correlation function theory to calculate the rate coefficient for site‐to‐site hopping by a single hydrogen atom absorbed on a rigid Cu(100) surface. We investigate hydrogen dynamics during barrier crossing and determine the time scales on which the hydrogen atom crosses or recrosses the barrier, as well as the time scale on which double jumps occur. We define two kinds of transition state theory rate coefficients: one (Miller and Tromp) which assumes that only the short time dynamics contributes to the rate coefficient and another which includes the effect of the earliest recrossing. We examine numerically the accuracy of these approximations and compare them to other transition state theory calculations and to our ‘‘exact’’ calculations. The simulations are also used to study the contribution of multiple jumps to the diffusion coefficient, to calculate the isotope effect on the rate coefficient and to determine the role of dimensionality in modeling surface diffusion. We fin...

Atomistic simulations and Peierls–Nabarro analysis of the Shockley partial dislocations in palladium

Abstract A detailed study of Shockley partials in Pd is presented, using a full-scale atomistic simulation of the structure and energetics of the dislocation motion. The interatomic potential is derived by fitting to data from first-principles electron-structure calculations. The outcome from the atomistic simulation is compared with the corresponding results using the Peierls–Nabarro (PN) model. The required input to the PN model is determined from the interatomic potential used in the atomistic simulation in order to make the comparison as decisive as possible. We find that the core, obtained in the atomistic simulation, is much wider and extends over a larger region compared with the PN model. A modification of the PN model is suggested to make it more consistent with the atomistic description. Using the modified version of the PN model improves the core structure considerably. The magnitude of the Peierls barrier from the atomistic simulation is in reasonable agreement with both experimental data and the corresponding values obtained using the PN model.

Motion of nanometer sized magnetic particles in a magnetic field gradient

Using magnetic particles with sizes in the nanometer range in biomedical magnetic separation has gained much interest recently due to their higher surface area to particle volume and lower sedimentation rates. In this paper, we report our both theoretical and experimental investigation of the motion of magnetic particles in a magnetic field gradient with particle sizes from 425 nm down to 50 nm. In the experimental measurements, we monitor the absorbance change of the sample volume as the particle concentration varies over time. We also implement a Brownian dynamics algorithm to investigate the influence of particle interactions during the separation and compare it to the experimental results for validation. The simulation agrees well with the measurements for particle sizes around 425 nm. Some discrepancies remain for smaller particle sizes, which may indicate that additional factors also influence the separation for the smaller size range. We observe that the separation process includes the formation of...