G. Brizuela

The electronic structure and bonding of an hydrogen pair near a FCC Fe stacking fault

Abstract The bonding of H to Fe is analyzed using qualitative electronic calculations in the framework of the atom superposition and electron delocalization orbital cluster method (ASED–MO). Upon introduction of a stacking fault in FCC Fe, the changes in the electronic structure are studied when a pair of hydrogen atoms are located near the fault. A cluster of 180 metallic atoms distributed in five layers represents the solid. The interstitial H atoms are positioned in accordance with previous studies. The Fe–H interaction decreases the Fe–Fe bond strength and the H effect is limited to the first Fe neighbor. An analysis of the orbital interaction reveals that the Fe–H bonding involves mainly the Fe 4s and H 1s orbitals. The orbital population analysis reveals some H–H association, which is more important than that obtained in dislocated Fe. A comparison is given with H pairs located near BCC Fe dislocations or vacancies.

The electronic effect of carbon and hydrogen in an () edge dislocation core system in bcc iron

Abstract The Fe–C–H interaction in a dislocated bcc structure was studied using qualitative structure calculations in the framework of the atom superposition and electron delocalisation molecular orbital (ASED-MO) theory. Calculations were performed using Fe 85 cluster to simulate a dislocated bcc structure. The cluster geometry and atomic parameters were optimised to make a better approximation to the repulsive energy terms. The most stable position for C atom inside the cluster was determined. Therefore, and H atom was approximated to the minimum energy region where the C atom was previously located. The total energy of the cluster decreases with the C atom near the dislocation core. The a /2[ 1 1 1 ] dislocation creates an energetically favourable zone for accumulation of C. The presence of C in the dislocation core make no favourable H accumulation. The C acts such as an expeller of H and could reduce the weakening of Fe–Fe bonds. In addition, a sort of Fe–C–Fe “bridge” could prevent dislocation displacement if a shear stress would be applied.

The electronic structure and bonding of a H-H pair in the vicinity of a BCC Fe bulk vacancy

Abstract The H–Fe interaction near a bcc Fe vacancy is analysed using a semi-empirical theoretical method. Calculations were performed using a Fe 86 cluster with a vacancy. Hydrogen atoms are positioned in their local energy minima configurations. Changes in the electronic structure of Fe atoms near a vacancy were analysed for the system without H, with one H and with two H atoms. Fe atoms surrounding the vacancy weaken their bond when hydrogen is present. This is due to the formation of H–Fe bonds. Hydrogen influences only its nearest-neighbour Fe atoms. The H–H interaction was also analysed. For H–H distance of 0.82 A an H–H association is formed, while H–Fe interaction and Fe–Fe weakening is markedly reduced, when compared with other H–H interactions.

Quantum chemical study of C and H location in an fcc stacking fault

Abstract The H–C–Fe interaction has been investigated in γ -Fe with a stacking fault using a cluster model. The energy of the system was calculated by the atom superposition and electron delocalization molecular orbital method. The electronic structure was studied using the concept of density of states and crystal orbital overlap population curves. By modifying the geometrical positions of the impurity within the cluster we have found that C occupies nearly octahedral sites on the stacking fault plane. The H does not reside in the vicinity of the C. The presence of C could reduce the detrimental effect of H on the Fe–Fe bonds. The present paper provides detailed energy mapping for C–Fe and H–C–Fe subsystems in the fault region. C–Fe and H–C equilibrium distances are reported and the orbital contributions to the bonding are also addressed.

THE ADSORPTION AND BONDING OF H2S ON THE α-FeOOH(110) SURFACE

The electronic structure of H2S adsorbed on the goethite (110) surface has been studied by ASED-MO cluster calculations. We have studied both the perpendicular and the parallel H2S molecular adsorption on the FeOOH(110) surface. We have analyzed the adsorption configuration energies including rotation. The parallel species does not rotate during adsorption and corresponds to the most stable configuration. We have also studied the bonding contributions for the minimum energy configuration and the density of states plots.

The electronic structure and bonding of hydrogen near a fcc Fe stacking fault

The atom superposition and electron delocalization molecular orbital (ASED-MO) semiempirical method is used to analyse the atomic hydrogen-Fe interaction. The face centred cubic (fcc) Fe model contains a stacking fault and as a comparison the H-fcc Fe (normal) system is also studied. The solid is represented by a cluster of 180 metallic atoms distributed in five layers. The interstitial atoms localized in different geometric positions found an energetic minimum in a zone close to octahedral interstitial holes in the stacking fault. The electronic structure shows that the H-Fe bond involves mainly the Fe 4s and 4p orbitals and the 1s H orbital. The Fe-Fe bond near H is destabilized, to approximately 27% of its original value.

HYDROGEN ADSORPTION AND DIFFUSION ON A Pt(111) CLUSTER

The interaction of hydrogen with a platinum (111) cluster using the atom superposition and electron delocalization–higher binding ASED-TB quantum calculation method was studied. The metal surface was represented by a Pt cluster of seven layers. The effect of hydrogen on this metal substrate was studied by the analysis of density of states and crystal orbital overlap populations curves. The energy surface plots allow us to find a possible diffusion path through the cluster from one side to the other. The Pt–Pt metal bond is weakened during H adsorption and diffusion. The main components in the Pt–H bond are the Pt 6s (31%), 6p (26%), and 5 dxz (16%) orbitals.

A Theoretical Study of a H–H Pair on the BCC Fe(100) Surface

Fil: Gonzalez, Estela Andrea. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Bahia Blanca; Argentina. Universidad Nacional del Sur. Departamento de Fisica; Argentina

Multiple hydrogen location in a vacancy region of a FCC iron–nickel-based alloy

The interaction between four-hydrogen atoms and a FCC FeNi-based alloy ideal structure having a vacancy (V) was studied using a cluster model and a semi-empirical theoretical method. The energy of the system was calculated by the atom superposition and electron delocalisation molecular orbital (ASED-MO) method. The electronic structure was studied using the concept of density of states (DOS) and crystal orbital overlap population (COOP) curves. After a sequential absorption, the hydrogen atoms are finally positioned at their energy minima configurations, near to the vacancy. The energy difference for H agglomeration was also computed. The vacancy–H n complexes become less stable for n > 3. The changes in the electronic structure of Fe and Ni atoms near to the vacancy were also analysed. The interactions mainly involve Fe and Ni, 4s and 4p atomic orbitals. The contribution of 3d orbitals is much less important. The Fe–Fe, Fe–Ni and Ni–Ni bonds are weakened as new Fe–H, Ni–H and H–H pairs are formed. The effect of the H atoms is limited to its first neighbours. The detrimental effect of H atoms on the metallic bonds can be related to the decohesion mechanism for H embrittlement.

THE CO-ADSORPTION OF BENZENE AND CO ON Co(0001)

The co-adsorption of carbon monoxide and benzene on Co(0001) has been studied using density functional calculations. We used the ordered