Budda V. Reddy

Electronic, Magnetic, and Geometric Structure of Metallo-Carbohedrenes

The energetics and the electronic, magnetic, and geometric structure of the metallocarbohedrene Ti8C12 have been calculated self-consistently in the density functional formulation. The structure of Ti8C12 is a distorted dodecahedron with a binding energy of 6.1 electron volts per atom. The unusual stability is derived from covalent-like bonding between carbon atoms and between titanium and carbon atoms with no appreciable interaction between titanium atoms. The density of states at the Fermi energy is high and is derived from a strong hybridization between titanium 3d and carbon sp electrons. Titanium sites carry a small magnetic moment of 0.35 Bohr magneton per atom and the cluster is only weakly magnetic.

Electronic structure and transport properties of Fe–Al alloys

Abstract Electronic structure of iron-aluminides (Fe 1− x Al x ) has been calculated for a range of aluminum concentration (0⩽ x ⩽0.5) by using first principles density functional theory to explain the variation of electrical resistivity with increasing Al content. The Fe–Al intermetallics are modeled by a cluster of 15 atoms confined to their bulk geometry. The location of Al atoms as a function of concentration, x was determined by minimizing the total energy of the clusters. The electronic structure was determined by calculating the total as well as partial density of states around each of the Fe and Al atoms. With increasing Al concentration, the transfer of Al 3p electrons into the minority 3d orbital of Fe not only has a profound effect on the magnetic properties of these intermetallics, but affects their transport properties as well. For example, the observed anomaly in the electrical resistivity of Fe 1− x Al x that peaks at x =0.33 is found to be a direct consequence of the filling of the Fe 3d orbital with Al valence electrons. The density of states is characterized by three distinct features: a narrow 3d band just below the Fermi energy originating from the Fe atoms, an Al s-band lying deeper in energy, and an Al p-band above the Fermi energy. The energy gap between Al 3p and Fe 3d density of states decreases with increasing Al concentration and for x =0.40, the density of states at the Fermi energy is a strongly hybridized p–d state giving Fe 1− x Al x metallic-like properties. These features are consistent with the recent photoemission studies carried out at the synchrotron facility at Lawrence Livermore National Laboratory. An anomaly in the temperature dependence of electrical resistivity is also explained in terms of the unique electronic and magnetic structure of these intermetallics.

Structure and properties of Ni7 cluster isomers

Abstract First principles calculations based on molecular orbital theory as well as molecular dynamics simulation using a many body interatomic potential reveal the existence of two nearly degenerate isomeric forms of Ni 7 -a capped octahedron and a pentagonal bipyramid. Contrary to expectations, the magnetic moments of the two isomers are the same. Although the reactivity of the isomers with N 2 is different, it is argued that the current experimental studies of N 2 saturation coverage cannot confirm the coexistence of the two isomers. Further experiments are necessary to achieve this goal.

Formation and stability of dodecahedral and fcc structures in metal—carbon clusters

Abstract The electronic structure and stability of the met—cars and the fcc TiC fragments has been investigated using the self-consistent molecular orbital calculations within the density functional theory. The fcc fragments are shown to be more stable than the dodecahedral met—cars. The formation of met—cars or the fcc clusters in beams is shown to be governed by the relative composition of Ti and C atoms. The stability of met—cars formed from elements across the transition metal series is proposed to be related to the transition metal ionization potential.

Electronic structure and magnetism in (FeAl)n(n⩽6) clusters

Abstract Theoretical electronic structure calculations on (FeAln)(n⩽6) clusters have been carried out to examine the structure, nature of bonding, and the magnetic behavior in small clusters. It is shown that the ground state geometrical arrangements are different from the bulk ordering and are dominated by the Fe–Fe clustering. As opposed to the bulk FeAl that is non-magnetic, the clusters are highly magnetic with a magnetic moment that initially increases with size and then decrease. The electronic states in small clusters and the nature of magnetic interaction in reduced sizes are discussed.

Signature of crystalline order in ultra-small metal-oxide clusters

Abstract First principles calculations of the equilibrium geometries, binding energies, electronic structure and ionization potentials of Sb x O y clusters reveal that clusters as small as Sb 5 O 7 bear the fingerprint of their crystal structure. Electronic structure of even smaller clusters mimics the bulk behavior. A partial ionic bonding between metal and oxygen atoms leads to a new family of magic numbers for ( x ,  y )=(2, 3), (3, 4), (4, 5) and (5, 7). Simple rules are provided for constructing geometries of larger magic clusters.

N2 adsorption around small Nin (n=2−4) clusters

Abstract The nature of N 2 adsorption and its effect on the geometries, electronic structure and magnetism of Ni n ( n =2−6) clusters has been studied using a linear combination of atomic orbitals-molecular orbital approach within the local density functional scheme. The adsorption sites in small clusters are found to be similar to those on bulk surfaces. The principal effect of N 2 adsorption is a demagnetization upon saturation coverage. The NiNi bonds are dilated but the geometrical structures are left intact. The studies show that the recent chemical methods for determining the geometrical structure of clusters can provide information on structure of bare clusters.

Chemically induced changes in the magnetic moments in transition metal monomers and dimers

Abstract Self-consistent molecular orbital calculations based on the generalized gradient approximation to the density functional theory reveal that dinitrogen interacting with transition metal monomers and dimers can substantially alter their underlying spin multiplicity. In particular, the coupling between spins of two Cr atoms in Cr 2 can change from anti-ferromagnetic to ferromagnetic with the atomic moment almost intact even when N 2 is molecularly adsorbed. This effect, caused by the dilation of the metal–metal bond, can have an important impact in the study of magnetism in general.

Magnetic properties of Al, V, Mn, and Ru impurities in Fe–Co alloys

Theoretical studies on the magnetic properties of impurities in Fe–Co alloys have been carried out using a molecular-orbital approach within a gradient corrected density functional formalism. The defected alloy is modeled by a large cluster and the calculations on the ordered alloy are used to show that a cluster containing 67 atoms can provide quantitative information on the local magnetic moment. It is found that although bulk Al, V, and Ru are nonmagnetic, all the impurities carry finite moments. While Al and V impurities couple antiferromagnetically, Ru impurities couple ferromagnetically to the host sites. It is shown that the observed composition dependence of the rate of increase of magnetic moment of FexCo1−x alloys upon addition of Mn impurities is due to the change in the magnetic moment of Mn impurities with composition. The reasons for this change and the possibility of stabilizing a higher Mn moment at all concentrations are discussed.

Magnetic properties of transition metal superlattices

Abstract The magnetic coupling between Fe layers separated by spacer layers consisting of up to two atomic planes of 3d transition metal elements (Sc, Ti, V, Cr, Fe, Co, Ni and Cu) has been studied systematically by using two complimentary theories based on cluster and band structure methods. The Fe layers are found to be ferromagnetically coupled in all cases except for Cr where this coupling alternates from ferro- to anti-ferromagnetic depending on whether the spacer layers are odd or even. Furthermore the spacer layers involving Sc, Ti and V are anti-ferromagnetically coupled to Fe while Co and Ni layers are coupled ferromagnetically.