Ultra-thin film (UTF) electronic structure calculations are a common tool for investigating surface properties. In this work, electronic structure calculations for A1(111) films ranging from one to twelve atoms thick are used to illustrate some of the difficulties that can arise when one attempts to determine surface properties of metals with UTF calculations.

Computer modeling of atomic processes on surfaces and bulk materials has seen noticeable growth due to the availability of new, powerful semiempirical techniques that provide both the numerical simplicity, physical foundation and computational efficiency necessary for the study of complex systems. In an attempt to partially summarize the current status of this work, we apply one of these techniques, the BFS method for alloys, to the study of several basic topics in surface structure and analysis: surface energies of alloy surfaces, multilayer relaxation of ordered alloy surfaces, temperature-dependent segregation profiles, surface alloying and thin film growth of multicompo-nent systems.

First-principles density-functional calculations were used to study the effects of Ca impurities on the Σ=5 (310) <001> tilt grain boundaries in MgO. An equilibrium structure and two metastable structures of the grain boundaries in pure MgO have been established. The calculations further demonstrated that Ca impurities segregate at particular sites in the metastable grain boundary and induce a structural transformation. This result is consistent with atomic resolution Z-contrast imaging. The calculations also found that the impurities at the grain boundaries do not induce states in the band gap. The mechanism of the transformation is also discussed.

The oxidation of the Si(001) surface is an important process on both technological and theoretical grounds. Experimental studies have not provided a clear picture of even the relevant atomic structures during the initial stages of oxidation, while previous theoretical studies of these processes have yielded contradictory results. Using careful first principles total-energy calculations based on density functional theory, we study several mechanisms of incorporating a sub-monolayer coverage of oxygen into the characteristic p(2 × 1) dimer reconstruction of the Si(001) surface. Our recently developed Adaptive Coordinate Real-space Electronic Structure (ACRES) method allows us to obtain results that are adequately converged with respect to the numerous computational parameters associated with this difficult system. We compare our results with previous theoretical work and propose a physically motivated two step pathway for the initial incorporation of an oxygen atom into the dimerized surface. Based on our results, we can explain what formerly appeared to be puzzling Ultraviolet Photoelectron Spectroscopy measurements which indicated that each initial oxygen atom saturates two surface dangling bonds.

A theoretical atomic process model of the interfacial oxidation reaction on a silicon substrate is proposed based on first-principles calculation. In the calculation, H-terminated Si(100) surfaces are used for the first approximation of the silicon-oxide/silicon interfaces. The proposed atomic process model is based on the plausible assumption that the remaining stress in the oxidized region is kept at a minimum and does not break the grown Si-O-Si network when the oxidation proceeds. As a natural consequence, Si atoms are emitted from the interface as the oxidation proceeds to release the stress due to substitution of Si-Si bonds by Si-O-Si bonds. This emission is consistent with well-known experiments of oxidation-induced stacking faults and oxidation-enhanced diffusion. Our model presents a microscopic approach for understanding the silicon oxidation process, whereas the widely accepted Deal-Grove model presents a macroscopic one.

ab initio theoretical calculations have been performed to reveal the etching mechanism of HF with Al metal oxide on Si surfaces. The following sequential process has been revealed to be the most probable reaction path. HF + AlO-Si surface → HO-Si surface + AlF (1), HF + HO-Si surface → F-Si surface + H2O (1'). Both of the reactions have potential energy barriers of 0.8∼1.0eV, and are exothermic by 0.3 and l. leV, respectively. Another reaction path that proceeds with one-step process, HF + AlO-Si surface → F-Si surface + AlOH (2), is also found to be probable for AlO etching.

We present a method for the calculation of the binding and rotational energies of neutral (H2S) and charged (HS-) molecules impinging upon a charged (Cu <100>) surface in the presence of an electrolyte. A molecular surface is constructed surrounding the H2S and HS- molecules forming boundary elements. A coupled Schrödinger-Poisson-Boltzmann iterative procedure treats the electronic structure of the molecules at the 6–31G**/MP2 level of theory and includes solvation effects through the single and double layers of charge induced by the electronic distribution. The molecule, together with its charged layers, forms a Molecular Single and Double Layer (MSDL), an object which then interacts with a Gouy-Chapman plane within the electrolyte. The additional induced charge at the molecular surface resulting from this electric field is obtained by solving a second set of boundary element equations. Repulsive interactions between the atoms of the molecule and those of the surface are obtained using a rigid-ion Hartree-Fock method. Binding energies of the molecule to the surface are determined as a function of the real surface charge imposed and also the ionic strength of the solution. It is found that surface charges can completely (180°) reorient these molecules and that the counterions in the solution can completely screen binding effects of even large surface charges.

Work supported by the United States Department of Energy under contract #DE-AC04–94AL85000.

An extended Hiickel calculation was employed to calculate the total electronic energy and the electron density during the breakage of the Si-O networks of a (F3SiO)2FSiOSiF(OSiF3)2 cluster modelled on the a-cristobalite structure. The Si-O networks are opened by the attack of F- ions on the silicon atoms, and the reaction is exthothermic by 2.7 eV through the attack of H+ ions on the oxygen atoms. Although the hydrogen termination of the oxygen atoms is an early reaction, the fluorine termination of the silicon atoms is a late reaction. The atomic bond population on the Si-O bonds decreases to zero by opening the Si-O networks. We conclude that the a-cristobalite and a-quartz SiO2 are dissolved in HF solutions since the Si-O networks are easily opened by the attack of F- ions. Our conclusion indicates that both cosite and stishovite SiO2, which are not dissolved in the HF solutions, are composed of Si-O networks that can be hardly opened by the attack of F- ions. Moreover, we propose the continual breakage of Si-O networks without the desorption of H2O molecules as an etching mechanism of SiO2 films.

Corrosive behavior of commercial organopolysulfides of the type R-(S)n-R is known to increase dramatically towards copper-based alloys when n ≥ 4, whereas di- and trisulfides are essentially inert. Three possible reasons for this behavior are examined using local and nonlocal density-functional theory as well as post-Hartree-Fock theory at the MP2 level. The hypotheses are: the shorter chains are protected from the surface by steric hindrance of the terminal organic groups, longer polysulfides can chelate copper atoms and remove them from the surface, and S-S bonds become weaker and more reactive as the polysulfide chains become longer. We find the predominant reason for the increase in corrosive behavior to be a dramatic decrease in S-S bond dissociation energy when n ≥ 4 due to stabilization of the unpaired electron via delocalization in RSS• thiyl radicals. The thiyl radicals are thus the species which is capable of attacking and corroding the copper surface. Chelation is eliminated as a possible mechanism, and steric protection and oil solubility afforded by the organic terminal groups are found to play a minor role.

Copper ions exchanged into the zeolite, ZSM-5, are known to catalyze the reduction of NO to N2 in the oxidizing environment of lean automotive exhaust, but do require trace hydrocarbons. The interface of this catalyst provides a locally reducing environment even when the surrounding exhaust has molecular oxygen concentrations as high as 10%. The mechanism of this reduction is still unclear; however, structure calculations of the active sites and dynamics simulations of species diffusion within the zeolite pores provide insight into the mechanism of reduction. One active site is proposed to be Cu++ ionically bound to a bridge oxygen in a nonsymmetric site within a zeolitic pore. Another proposed site has the copper ion centrally (symmetrically) bound within a six member zeolitic ring. Evidence suggests that the copper ions cycle between Cu++ and Cu+ during the reduction of NO to N2. The nonsymmetric copper ions are shown here to be hydrated ions attached to the Brønsted acid sites in the zeolite. The calculations here show a four member, first hydration shell for Cu++ and a first shell of 2–3 oxygens for the Cu+ ion. An examination of the pore size in ZSM-5 indicates sufficient room for a first and second hydration shell for most of the possible acid sites. The conclusion that the copper ions are typically hydrated suggests that the catalytic mechanism may have much in common with homogeneous catalysis which is sometimes termed heterogenized homogeneous catalysis.

For a long time, understanding the mechanisms of impurity-promoted embrittlement in iron and the consequent cohesion(decohesion) effects has been a challenge for materials scientists. The role alloying elements play in impurity-promoted embrittlement is important due to either their direct intergranular cohesion(decohesion) effects or effects upon embrittling potency of other impurities. Some alloying elements like Pd and Mo are known to be helpful for intergranular cohesion in iron and some other alloying elements like Mn are known to segregate to and weaken iron grain boundaries dramatically[1]. There have been intensive investigations on these mechanisms for a long time and especially, with the progress in computing techniques in recent years, calculations on more realistic models have become possible[2–4]. In this paper we briefly present our studies on some selected alloying-element/iron grain boundaries(GB) and free surface(FS) systems. The effects of Pd, Mo, Mn and Cr on the Fe Σ5 (031) grain boundary and its corresponding (031) free surface are examined, using a combination of molecular dynamics(MD) and first-principles electronic structure calculations. Section 2 gives a brief introduction to the methods used and Section 3 gives the main results.

Materials that undergo intergranular degradation processes, such as Stress Corrosioi Cracking (SCC) or certain types of Liquid Metal Embrittlement (LME), show variations in sus ceptibility with grain boundary character Samples with high fractions of “special” boundaries i.e. highly periodic boundaries with low-S Coincidence Site Lattices (CSL), exhibit higher resis tance to these types of attack[l,2]. Very useful comparisons have been drawn between low periodicity boundaries and high-periodicity boundaries; however, few attempts have been mad to date to explain variations in behavior between various “general” boundaries. The presen study employs the LME of aluminum by liquid gallium in an attempt to elucidate the variation in structure and properties of general grain boundaries.

In this paper we analyze the effect of the surplus of titanium in the bulk on γ/γ interfaces. Monte Carlo calculations using a central force many-body potential suggest that in Ti rich alloys titanium segregates to the 120° rotational fault and the pseudotwin. This leads to the formation of a thin region of the DO19 Ti3Al at these interfaces. While titanium does not segregate to the ordered twin, it does to the ordered twin with the APB. But in this case the interface dissociates into the 120° rotational fault and the pseudotwin. The calculations further show that there are two types of atomic sites at the interfaces. One is the same as in the ideal L10 the other, to which the segregation takes place, is specific for interfaces parallel to {111} planes. The specific distribution of the sites favored for segregation is the reason why segregation leads to the formation of a narrow region of the DO19 Ti3Al in the 120° rotational fault and the pseudotwin.

We provide a recipe for the evaluation of pre–factors for various diffusion processes relevant to crystal growth on Ag(100), based on explicit calculations of appropriate vibrational density of states using realistic many-body interaction potentials. Our calculated diffusion coefficients show that while hopping processes are favored for terrace self-diffusion, exchange processes dominate step-edge diffusion. We also find the characteristics of adatom diffusion along the < 100 >-step-edge to be strikingly different from the < 110 >-step-edge with interesting consequences for interlayer transport on this surface.

Multi-phase particle reinforced heat conducting composites are considered. We treat the case when there is an interfacial thermal barrier between phases. We indicate how the particle size distribution and particle geometry influences the overall properties of the suspension. These results apply immediately to the mathematically analogous case of DC electrical conductivity in the presence of electrical contact resistance between phases.

We have investigated adsorption and diffusion processes of a Si atom on hydrogen-terminated Si(001)-(2×1) surfaces using first-principles total-energy calculations. The Si adatom segregates H atoms from surface Si dimer atoms on the monohydride terminated Si(001) surface. The migration of the Si adatom is assisted by the mobility of H atoms, that is, the Si adatom migrates on the surface by repeating release and capture of H atoms. The effects of a single H-terminated Si dimer on the Si migration are also examined. Calculated results are in good agreements with the variation in morphology of Si homoepitaxial films induced by hydrogen termination.

We report molecular dynamics simulations of the martensitic phase transition from bcc to hep in zirconium. We show the evolution of a laminated twinned microstructure, in which some plastic deformation has occurred to rotate the twins manifested as basal stacking faults. This rotation is such as to alter the twinning angle from the 60° between the hep variants to the 61.5° angle of the low energy (1011) twins. These are thus identified as a cause of microscopic irreversibility in the transition.

Silica, SiO2, is one of the most widely studied substance because of its complex and unusual properties. We have used a recently developed 2-body interaction force field [1] to study the structural phase transformations in silica under various pressure loading conditions. The specific transformations we studied are the a-quartz to stishovite, coesite to stishovite and fused glass to a dominantly six-coordinated dense glassy phase. Molecular dynamics simulations are performed under constant loading rates ranging from 0.1 GPa/ps to 1.0 GPa/ps, with final pressures upto 100 GPa and at temperatures of 300, 500, and 700 K. We observe the crystal to crystal transformations to occur reconstructively, whereas it occurs in a smooth and displacive manner from glass to a stishovite-like phase confirming earlier conjectures.[2] We studied the dependence of transition pressure on the loading rate and the temperature to elucidate the shock loading experiments. We also studied the unloading behavior of each transformation to assess the hysterisis effect.

We present a detailed molecular-dynamics study of the coalescence of gold nanoclusters within the framework of the embedded-atom method. We find macroscopic sintering theories to be unable to describe the coalescing behavior of two small clusters, a failure we attribute to the fact that the nanocrystals are faceted; this has important consequences for the morphology of cluster-assembled materials. We also examine the static and dynamic properties of a 249-atom gold cluster on nickel surfaces.

First-principles electronic structure studies based on local spin density functional theory and performed on extremely complex simulations of ever increasingly realistic systems, play a very important role in explaining and predicting surface and interface magnetism. This has led to solving even more challenging problems like the embrittlement of the Fe grain boundary, discussed here. Now, a major issue for first-principles theory is the treatment of the weak spin-orbit coupling (SOC) in magnetic transition metals and their alloys and its subsequent effects: (i) A major breakthrough in eliminating the numerical randomness for the determination of the magneto-crystalline anisotropy was made with the state-tracking and torque approaches. This now enables us to treat magnetostriction and its inverse effect, strain-induced magnetic anisotropy in transition metal bulk, thin films and alloys, (ii) The magneto-optical Kerr effects and x-ray magnetic circular dichroism are now directly calculated and compared with experiment. In all this work, and more recently, on the first-principles calculations of giant magneto-resistance in multilayers, extensive first-principles calculations and model analyses provide simple physical insights and guidelines to search for new magnetic recording and sensor materials.