Energetics and electronic structure of carbon doped aluminum clusters

The energetics and the electronic structure of AlnC clusters (n=3, 4, 5; 11, 12, 13) have been studied by a global optimization of their geometry without any symmetry constraint. The total energies of these clusters both in neutral and charged states are calculated using an all-electron basis and the generalized gradient approximation to the density functional theory. While Al4C and Al12C clusters share some characteristic features of closed shell structures, namely enhanced stability and low electron affinity compared to their neighboring sizes, their ionization potentials exhibit different behavior. These decrease steadily from Al3C to Al5C while that of Al12C is higher than its neighbors. Carbon is found to form planar structures in small AlnC clusters (n=3, 4, 5) irrespective of their charge state although neutral Al4C possesses a nearly degenerate tetrahedral isomer lying slightly higher in energy from the planar configuration. The results agree well with experimental and previous theoretical data. In larger AlnC (n=11, 12, 13) clusters, carbon occupies an interior site. In Al12C, carbon occupies the center of an icosahedron while it is off-centered in Al11C and Al13C. As an electron is attached, the near degeneracies of the neutral Al4C is lifted whereas nondegenerate isomers of neutral Al12C yield nearly degenerate anions. Both these features produce complicated photoelectron spectra making identification of their adiabatic electron affinity a difficult problem. With the exception of neutral Al12C, the bonding of carbon to aluminum atoms is governed primarily by covalent interaction. The above calculations were also performed with a simplified basis by freezing the atomic cores of aluminum. In most cases, this simple basis yields results in good agreement with all electron calculations

Evolution of the electronic structure and properties of neutral and charged aluminum clusters: A comprehensive analysis

Density-functional theory with generalized gradient approximation for the exchange-correlation potential has been used to calculate the global equilibrium geometries and electronic structure of neutral, cationic, and anionic aluminumclusters containing up to 15 atoms. The total energies of these clusters are then used to study the evolution of their binding energy, relative stability, fragmentation channels, ionization potential, and vertical and adiabatic electron affinities as a function of size. The geometries are found to undergo a structural change from two dimensional to three dimensional when the cluster contains 6 atoms. An interior atom emerges only when clusters contain 11 or more atoms. The geometrical changes are accompanied by corresponding changes in the coordination number and the electronic structure. The latter is reflected in the relative concentration of the s and p electrons of the highest occupied molecular orbital. Aluminum behaves as a monovalent atom in clusters containing less than seven atoms and as a trivalent atom in clusters containing seven or more atoms. The binding energy evolves monotonically with size, but Al7, Al+7, Al−7, Al−11, and Al−13 exhibit greater stability than their neighbors. Although the neutral clusters do not conform to the jellium model, the enhanced stability of these charged clusters is demonstrated to be due to the electronic shell closure. The fragmentation proceeds preferably by the ejection of a single atom irrespective of the charge state of the parent clusters. While odd-atom clusters carry a magnetic moment of 1μB as expected, clusters containing even number of atoms carry 2μB for n⩽10 and 0 μB for n\u3e10.The calculated results agree very well with all available experimental data on magnetic properties,ionization potentials,electron affinities, and fragmentation channels. The existence of isomers of Al13 cluster provides a unique perspective on the anomaly in the intensity distribution of the mass spectra. The unusual stability of Al7 in neutral, cationic, and anionic form compared to its neighboring clusters is argued to be due to its likely existence in a mixed-valence state

Caging of Ni clusters by benzene molecules and its effect on the magnetism of Ni clusters

Global optimization of the geometry of small Ni clusters interacting with benzene molecules yields equilibrium structures where the Ni atoms find themselves caged between the benzene molecules. This leads to quenching of the Ni magnetic moment in most of the complexes even though the structure of the caged Ni clusters remain relatively unchanged from their otherwise free gas phase geometry. The accuracy of these predictions, based on density functional theory with generalized gradient approximation for exchange and correlation, is established by the good agreement achieved between the calculated and available experimental dissociation energies and ionization potentials

Alkalization of aluminum clusters

Equilibrium geometries, binding energies, ionization potentials, and electron affinities of neutral and charged Aln clusters (n⩽8) decorated with alkali atoms such as Li and K have been calculated using molecular orbital theory based on density functional formalism and generalized gradient approximation. While the electron affinities and the ionization potentials depend on size, no clear signatures of shell closings are found in this size range. Similar studies on Al5Xm  (X=Li, K, 1⩽m⩽4) also fail to provide any indication consistent with shell closings. On the other hand, the ionization potentials and electron affinities of aluminum clusters decrease with the addition of alkali atoms. The results are in good agreement with available experimental data

Spectroscopy of Ni-n(benzene)(m) anion complexes

Total energy calculations based on the generalized gradient approximation to the density functional theory reveal that the Ni(benzene) and Ni(benzene)2 anions are unstable against autodetachment of the additional electron while other anion complexes containing more than one Ni atom are stable. Although the adiabatic electron affinities increase with Ni content, they are significantly smaller than those in pure Ni clusters containing the same number of Ni atoms. The difference between adiabatic electron affinities and vertical detachment energies are around 0.2 eV in most cases, indicating that the equilibrium geometries of Nin(benzene)−m are not significantly altered from their corresponding neutral geometries. The vertical transitions from the anion to the neutral provide new insight into the magnetic moment of these organometallic complexes

Molecular view of the interfacial adhesion in aluminum‐silicon carbide metal‐matrix composites

The binding energies, electron charge transfer,bond lengths, and core level shifts of Al‐Al, Al‐Si, Al‐C, and Si‐C dimers have been calculated self‐consistently using the linear combination of atomic orbitals‐molecular orbital theory. The exchange interactions are treated using the unrestricted Hartree–Fock theory and correlation corrections are included through the Möller–Plesset perturbation scheme up to fourth order. The results are used to understand the nature and strength of bonding at the interface of Al and SiC crystals. The strong bonding of Al‐C dimers compared to Al‐Al and Al‐Si is shown to be responsible for the aluminumcarbide formation at the interface. The charge transfer between the constituent atoms in the dimer and the accompanying core level shifts are also shown to be characteristic of what has been observed at the Al/SiC interface

Warm Asymmetric Nuclear Matter and Proto-Neutron Star

Asymmetric nuclear matter equation of state at finite temperature is studied in SU(2) chiral sigma model using mean field approximation. The effect of temperature on effective mass, entropy, and binding energy is discussed. Treating the system as one with two conserved charges the liquid-gas phase transition is investigated. We have also discussed the effect of proton fraction on critical temperature with and without $\rho$-meson contribution. We have extended our work to study the structure of proto-neutron star with neutron free charge-neutral matter in beta-equilibrium. We found that the mass and radius of the star decreases as it cools from the entropy per baryon S = 2 to S = 0 and the maximum temperature of the core of the star is about 62 MeV for S = 2.Comment: 25 pages, 16 figure