Carlo Massobrio

Structural and Electronic-Properties of Small Copper Clusters - a First Principles Study

Equilibrium geometries and electronic properties of neutral Cu-n (n = 2, 3, 4, 6, 8, 10) clusters are determined via first principles calculations which treat s and d electrons on an equal footing. Overall, we find ground state and local minimum structures similar to those of Na-n. However, Cu-n clusters tend to prefer more compact arrangements. Electronic states with atomic s-character are strongly hybridized with d-states and located mostly at the band edges. Angular decomposition of the electronic wavefunctions in Cu-n clusters shows that their shell model character is significantly less pronounced than in Na-n clusters.

Breakdown of intermediate-range order in liquid GeSe2 at high temperatures

The structure of Liquid GeSe2 at T = 1373 K has been investigated by first-principles molecular dynamics. The calculated total neutron structure factor is in good agreement with recent experimental data. We found that the disappearance with increasing temperature of the first sharp diffraction peak (FSDP) in the total neutron structure factor is due to an increase of short-range chemical disorder. At T = 1373 K various bonding configurations coexist in close amounts, such as the Ge-GeSe3, Ge-GeSe2 and Se-SeGe2 motifs. This contrasts with the behaviour of liquid GeSe2 at T = 1050 K, for which more than half of the Ge atoms are four-fold coordinated to Se atoms in regular GeSe4 tetrahedra. Our result correlates the appearance of the FSPD in disordered AX(2) network-forming materials to the predominant presence of AX(4) subunits.

Neutral and anionic CuO2 : an ab inito study

Abstract By using first-principles calculations within density functional theory via the local density approximation (LDA) and the generalized-gradient approximation (GGA) of Perdew and Wang for exchange and correlation, we calculate the equilibrium structures of CuO 2 and CuO 2 − clusters. In the case of CuO 2 , three isomers (OCuO linear and two CuO 2 complexes, side-on and bent) lie within 0.5 eV, while the negatively charged cluster is most stable as a linear molecule. Our assignment of measured photo-electron spectra features on the basis of the electronic density of states (EDOS) suggests that the bent structure is the most stable among the two forms of CuO 2 − complexes.

The role of chemical disorder and volume expansion in crystal-to-amorphous transitions: simulation results for NiZr2 and Cu3Au

Abstract We report the results obtained in a molecular dynamics simulation study of the behavior of two different systems, NiZr 2 and Cu 3 Au, subjected to a chemical disordering process, under different thermodynamical conditions. We underline the relevant phenomena which, in the first case, trigger the onset of the crystal-to-amorphous transition but in the latter do not. We focus on the dominant role played by volume expansion and elastic softening as precursor effects on the onset of the transition. Finally we draw attention to the percolation model (C. Massobrio and V. Pontikis, Phys. Rev. B, 45 (1992) 2484) which correlates elastic instability to percolation of locally expanded regions located around the defects.

Structure of liquid GeSe: A first principle study

We have investigated liquid GeSe by first-principle molecular dynamics simulations performed in the framework of density functional theory. Our partial structure factors are in good agreement with recent experimental data. In particular, we do not find any evidence of intermediate range order, as indicated by the absence of a first sharp diffraction peak for low magnitudes of the momentum transfer in the partial structure factors. Comparison between experimental and calculated partial pair correlation functions gαβ(r) yields a less satisfactory agreement, non-negligible differences being found particularly in the shape and peaks position of the gGeGe(r) and gSeSe(r) for short distances (<4 A). The origin of these discrepancies are discussed in the light of recent experimental and theoretical findings on other GexSe1−x disordered systems. Analysis of the atomic configurations reveals that this liquid GeSe does not exhibit a regular network structure. A variety of bonding configurations are found and, in pa...

Concentration fluctuations on intermediate range distances in liquid GeSe2: the critical role of ionicity

In the attempt to identify the physical origin of a first sharp diffraction peak (FSDP) in the concentration-concentration partial structure factor, we perform first-principle molecular dynamics of liquid GeSe2. We compare the structural properties of two set of simulations, which differ by their treatment of the electronic structure. This is achieved by considering different energy cutoffs defining the plane-wave basis sets, the higher energy cutoff leading to a moderate enhancement of the ionicity in the system. Both schemes give a neutron scattering factor in excellent agreement with experiment. The higher ionicity leads to small but perceptible modifications in some structural features related to short and intermediate range order, slightly improving the agreement with the experimental partial structure factors. In particular, a FSDP becomes discernable in the concentration-concentration structure factor. Although the magnitude of this peak is small compared to the experiment, this observation suggests that a good description of this peak could be obtained with an energy functional inducing an even higher ionicity

DEPOSITION OF MASS-SELECTED Ag7 ON Pd(100): FRAGMENTATION AND IMPLANTATION

Mass-selected silver-cluster ions with an incident energy of 2.86 eV/atom and of 13.6 eV/atom are directed on a well-prepared Pd(100) surface, which is probed with thermal-energy atom (helium) scattering (TEAS), before, during, and after the deposition, yielding information on the collision process. We find that part of the cluster atoms are implanted into the surface layer, the fraction depending on the impact energy. Considerable fragmentation is present at both impact energies. Molecular dynamics calculations based on embedded atom method (EAM) potentials are used to model the collision process. These calculations confirm qualitatively the experimental results.

Embedded atom method computations of structural and dynamical properties of Cu and Ag clusters adsorbed on Pd(110) and Pd(100): evolution of the most stable geometries versus cluster size

The structural stability of one- and two-dimensional arrangements of Cu and Ag clusters adsorbed on Pd(110) and Pd(100) is investigated as a function of cluster size by using embedded atom method potentials. In Cu/Pd(110), we find a remarkable stability of one-dimensional chains over two-dimensional compact arrangements, which increases with cluster size N while the opposite trend (decreasing stability with increasing size) is found in the case of Ag/Pd(110). On Pd(100) two-dimensional structures are the most stable. The diffusion mechanisms for the adatom movement on Pd(110) are elucidated via static computations and described in detail for migration along the [001] direction

A density functional study of copper hydroxonitrate: size effects and spin density topology

Abstract Copper hydroxonitrate has been investigated within density functional theory and a plane-wave approach. In order to elucidate the topology of the spin density distribution and the microscopic origins of the antiferromagnetic character of this compound, systems with an equal number of unit cells differently oriented in space have been considered. Parallel and antiparallel alignment of spin densities occur between Cu centers, the coupling pattern varying from one plane to the other. Depending on their coordination with the Cu atoms, O atoms carry non-negligible spin densities. These are larger for oxygens on hydroxo groups bridging those Cu atoms which have spin densities aligned in a parallel fashion. The spatial distribution of spin densities is far from being unique, suggesting that the S=0 global spin state is compatible with different local spin structures very close in energy.

Low energy impact of silver atoms on Pd(100): comparison between helium scattering and microscopic scale simulation results

Molecular dynamics simulations of the impact of Ag atoms at low energies (20 eV and 95 eV in the direction perpendicular to the surface) on Pd(100) are performed using embedded atom (EAM) potentials. The results are compared with recent helium scattering data relative to the impact of Ag/sub 1//sup +/ ions on a Pd(100) surface. Overall, the simulation results are in favorable agreement with the experiments and allow the outcome of the collision to be described in terms of a defect population consisting of the impinging Ag atom, Pd adatoms and substrate vacancies.