Hydrogen Dissociation and Diffusion on Transition Metal(=Ti,Zr,V,Fe,Ru,Co,Rh,Ni,Pd,Cu,Ag)-doped Mg(0001) Surfaces

The kinetics of hydrogen absorption by magnesium bulk is affected by two main activated processes: the dissociation of the H$_2$ molecule and the diffusion of atomic H into the bulk. In order to have fast absorption kinetics both activated processed need to have a low barrier. Here we report a systematic ab-initio density functional theory investigation of H$_2$ dissociation and subsequent atomic H diffusion on TM(=Ti,V,Zr,Fe,Ru,Co,Rh,Ni,Pd,Cu,Ag)-doped Mg(0001) surfaces. The calculations show that doping the surface with TM's on the left of the periodic table eliminates the barrier for the dissociation of the molecule, but the H atoms bind very strongly to the TM, therefore hindering diffusion. Conversely, TM's on the right of the periodic table don't bind H, however, they do not reduce the barrier to dissociate H$_2$ significantly. Our results show that Fe, Ni and Rh, and to some extent Co and Pd, are all exceptions, combining low activation barriers for both processes, with Ni being the best possible choice.Comment: accepted in the International Journal of Hydrogen Energ

The first principles calculation of transport coefficients

We demonstrate the practical feasibility of calculating transport coefficients such as the viscosity of liquids completely from first principles using the Green-Kubo relations. Results presented for liquid aluminum are shown to have a statistical error of only ca. 5%. The importance of such calculations is illustrated by results for a liquid iron-sulfur alloy under Earth's core conditions, which indicate that the viscosity of the liquid outer core is not substantially higher than that of typical liquid metals under ambient conditions.Comment: four pages, including four figure

The energetics of water on oxide surfaces by quantum Monte Carlo

Density functional theory (DFT) is widely used in surface science, but gives poor accuracy for oxide surface processes, while high-level quantum chemistry methods are hard to apply without losing basis-set quality. We argue that quantum Monte Carlo techniques allow these difficulties to be overcome, and we present diffusion Monte Carlo results for the formation energy of the MgO(001) surface and the adsorption energy of H$_2$O on this surface, using periodic slab geometry. The results agree well with experiment. We note other oxide surface problems where these techniques could yield immediate progress.Comment: 5 pages, 2 figure

Ab initio statistical mechanics of surface adsorption and desorption: II. Nuclear quantum effects

We show how the path-integral formulation of quantum statistical mechanics can be used to construct practical {\em ab initio} techniques for computing the chemical potential of molecules adsorbed on surfaces, with full inclusion of quantum nuclear effects. The techniques we describe are based on the computation of the potential of mean force on a chosen molecule, and generalise the techniques developed recently for classical nuclei. We present practical calculations based on density functional theory with a generalised-gradient exchange-correlation functional for the case of H$_2$O on the MgO~(001) surface at low coverage. We note that the very high vibrational frequencies of the H$_2$O molecule would normally require very large numbers of time slices (beads) in path-integral calculations, but we show that this requirement can be dramatically reduced by employing the idea of thermodynamic integration with respect to the number of beads. The validity and correctness of our path-integral calculations on the H$_2$O/MgO~(001) system are demonstrated by supporting calculations on a set of simple model systems for which quantum contributions to the free energy are known exactly from analytic arguments.Comment: 11 pages, including 2 figure

Thermodynamics of hexagonal-close-packed iron under Earth’s core conditions

The free energy and other thermodynamic properties of hexagonal-close-packed iron are calculated by direct ab initio methods over a wide range of pressures and temperatures relevant to the Earth’s core. The ab initio calculations are based on density-functional theory in the generalized-gradient approximation, and are performed using the projector augmented wave approach. Thermal excitation of electrons is fully included. The Helmholtz free energy consists of three parts, associated with the rigid perfect lattice, harmonic lattice vibrations, and anharmonic contributions, and the technical problems of calculating these parts to high precision are investigated. The harmonic part is obtained by computing the phonon frequencies over the entire Brillouin zone, and by summation of the free-energy contributions associated with the phonon modes. The anharmonic part is computed by the technique of thermodynamic integration using carefully designed reference systems. Detailed results are presented for the pressure, specific heat, bulk modulus, expansion coefficient and Grüneisen parameter, and comparisons are made with values obtained from diamond-anvil-cell and shock experiments

The reconstruction of Rh(001) upon oxygen adsorption

We report on a first-principles study of the structure of O/Rh(001) at half a monolayer of oxygen coverage, performed using density-functional theory. We find that oxygen atoms sit at the center of the black squares of a chess-board, $c(2\times 2)$, pattern. This structure is unstable against a rhomboid distortion of the black squares, which shortens the distance between an O atom and two of the four neighboring Rh atoms, while lengthening the distance with respect to the other two. We actually find that the surface energy is further lowered by allowing the O atom to get off the short diagonal of the rhombus so formed. We predict that the latter distortion is associated with an order-disorder transition, occurring below room temperature. The above rhomboid distortion of the square lattice may be seen as a rotation of the empty, white, squares. Our findings are at variance with recent claims based on STM images, according to which it is instead the black squares which would rotate. We argue that these images are indeed compatible with our predicted reconstruction pattern.Comment: 14 pages (inclusive of 5 figures). To appear on Surface Scienc

Oxygen in the Earth's core: a first principles study

First principles electronic structure calculations based on density functional theory have been used to study the thermodynamic, structural and transport properties of solid solutions and liquid alloys of iron and oxygen at Earth's core conditions. Aims of the work are to determine the oxygen concentration needed to account for the inferred density in the outer core, to probe the stability of the liquid against phase separation, to interpret the bonding in the liquid, and to find out whether the viscosity differs significantly from that of pure liquid iron at the same conditions. It is shown that the required concentration of oxygen is in the region 25-30 mol percent, and evidence is presented for phase stability at these conditions. The Fe-O bonding is partly ionic, but with a strong covalent component. The viscosity is lower than that of pure liquid iron at Earth's core conditions. It is shown that earlier first-principles calculations indicating very large enthalpies of formation of solid solutions may need reinterpretation, since the assumed crystal structures are not the most stable at the oxygen concentration of interest.Comment: 21 pages, 12 figure

Ab-initio chemical potentials of solid and liquid solutions and the chemistry of the Earth's core

A general set of methods is presented for calculating chemical potentials in solid and liquid mixtures using {\em ab initio} techniques based on density functional theory (DFT). The methods are designed to give an {\em ab initio} approach to treating chemical equilibrium between coexisting solid and liquid solutions, and particularly the partitioning ratio of solutes between such solutions. For the liquid phase, the methods are based on the general technique of thermodynamic integration, applied to calculate the change of free energy associated with the continuous interconversion of solvent and solute atoms, the required thermal averages being computed by DFT molecular dynamics simulation. For the solid phase, free energies and hence chemical potentials are obtained using DFT calculation of vibrational frequencies of systems containing substitutional solute atoms, with anharmonic contributions calculated, where needed, by thermodynamic integration. The practical use of the methods is illustrated by applying them to study chemical equilibrium between the outer liquid and inner solid parts of the Earth's core, modelled as solutions of S, Si and O in Fe. The calculations place strong constraints on the chemical composition of the core, and allow an estimate of the temperature at the inner-core/outer-core boundary.Comment: 19 pages, two figure

Melting curve of MgO from first-principles simulations

First-principles calculations based on density functional theory, both with the local density approximation (LDA) and with generalized gradient corrections (GGA), have been used to simulate solid and liquid MgO in direct coexistence in the range of pressure 0 <= p <= 135 GPa. The calculated LDA zero pressure melting temperature is T-m(LDA)=3110 +/- 50 K, in good agreement with the experimental data. The GGA zero pressure melting temperature T-m(GGA)=2575 +/- 100 K is significantly lower than the LDA one, but the difference between the GGA and the LDA is greatly reduced at high pressure. The LDA zero pressure melting slope is dT/dp similar to 100 K/GPa, which is more than 3 times higher than the currently available experimental one from Zerr and Boehler [Nature (London) 371, 506 (1994)]. At the core mantle boundary pressure of 135 GPa MgO melts at T-m=8140 +/- 150 K