Structure, fragmentation patterns, and magnetic properties of small nickel oxide clusters.

We report a comprehensive theoretical study of the structural and electronic properties of neutral and charged nickel oxide clusters, NinOm0/± (n = 3-8 and m = 1-10), in the context of recent experiments of photodissociation and Ion Mobility Mass Spectrometry. By means of density functional theory calculations in the generalized gradient approximation for exchange and correlation, we determined the putative ground states as well as the low-energy structural- and spin-isomers which were then used to explore the favorable fragmentation channels of the nickel oxide cationic clusters, and the resulting most abundant products, in good qualitative agreement with photodissociation measurements. Apart from stoichiometries different from those of their nickel oxide macroscopic counterparts, we found a tendency to form compact Ni subclusters, with reentrance of low-coordinated structures close to the equiatomic Ni-O concentration, taking the form of alternating Ni-O rings in the smaller sizes, in good qualitative agreement with Ion Mobility Mass Spectrometry measurements. This structural pattern is manifested in a drop of the total spin magnetic moment close to the equiatomic concentration due to the formation of antiparallel magnetic couplings. Although antiparallel couplings are found to a more or less extent in most clusters, especially in the oxygen rich phase, we identified certain clusters of special interest in the context of magnetic grains because of their large total magnetic moment and abundance. There are even some nickel oxide clusters with a higher total moment than their pure Ni counterparts, due to parallel magnetic couplings and the contribution of the oxygen atoms to the total moment.

New structural and electronic properties of (TiO2)10.

We present, based on state of the art density functional theoretic calculations, a new putative ground state (GS) for the cluster (TiO2)10, which results more than 1 eV lower in energy than all those previously reported in the literature. The geometric and electronic properties of this new cluster are discussed in detail and in comparison with the rest. We analyze the implications of the new GS in the context of recent experiments of reactivity regarding oxygen exchange with gaseous CO2 in TiO2 nanostructures, and also in connection with a recent interpretation of photoelectron spectroscopic measurements of the band gap of gas phase TiO2 (-) clusters.

Structure, fragmentation patterns, and magnetic properties of small cobalt oxide clusters.

The favorable stoichiometry of Co(n)O(m)(+) clusters has been recently determined by means of multiphoton dissociation of oxide cluster beams coming from laser evaporation of metal rods seeded with 0.5-5% oxygen and selected by time of flight mass spectroscopy. It was observed that the prominent stoichiometry is n = m, and that the preferred dissociation channel is the loss of O2 molecules. The Co4O4(+) cluster is found to be particularly abundant, an indication of its high stability. In this work we present density functional calculations, within the generalized gradient approximation, for the geometric, electronic, and magnetic properties of neutral and cationic Co(n)O(m)(0/+) clusters with n = 3-8 and m = 1-10. The ionic structures were determined after optimizing several initial geometries selected from previous calculations of pure Co clusters, with consecutive adsorbed oxygen atoms, as well as geometries constructed by assembling several CoO units and adding subsequent oxygen atoms. The fragmentation patterns were studied by comparing the energy separation of O2, CoO, Co2O, CoO2, and Co fragments. We obtain that the preferred fragmentation channel is the loss of O2, that the favourable stoichiometry is 1 : 1, and that Co4O4(+) is especially stable, in full agreement with the experiments. In addition the magnetic properties related to spin isomeric configurations of (CoO)n(+) clusters are studied in detail.

Titanium embedded cage structure formation in Al(n)Ti+ clusters and their interaction with Ar.

Recently, Ar physisorption was used as a structural probe for the location of the Ti dopant atom in aluminium cluster cations, Al(n)Ti(+) [Lang et al., J. Am. Soc. Mass Spectrom. 22, 1508 (2011)]. As an experiment result, the lack of Ar complexes for n > nc determines the cluster size for which the Ti atom is located inside of an Al cage. To elucidate the decisive factors for the formation of endohedrally Al(n)Ti(+), experimentalists proposed detailed computational studies as indispensable. In this work, we investigated, using the density functional theory, the structural and electronic properties of singly titanium doped cationic clusters, Al(n)Ti(+) (n = 16-21) as well as the adsorption of an Ar atom on them. The first endohedral doped cluster, with Ti encapsulated in a fcc-like cage skeleton, appears at nc = 21, which is the critical number consistent with the exohedral-endohedral transition experimentally observed. At this critical size the non-crystalline icosahedral growth pattern, related to the pure aluminium clusters, with the Ti atom in the surface, changes into a endohedral fcc-like pattern. The map of structural isomers, relative energy differences, second energy differences, and structural parameters were determined and analyzed. Moreover, we show the critical size depends on the net charge of the cluster, being different for the cationic clusters (nc = 21) and their neutral counterparts (nc = 20). For the Al(n)Ti(+) · Ar complexes, and for n < 21, the preferred Ar adsorption site is on top of the exohedral Ti atom, with adsorption energy in very good agreement with the experimental value. Instead, for n = 21, the Ar adsorption occurs on the top an Al atom with very low absorption energy. For all sizes the geometry of the Al(n)Ti(+) clusters keeps unaltered in the Ar-cluster complexes. This fact indicates that Ar adsorption does not influence the cluster structure, providing support to the experimental technique used. For nc = 21, the smallest size of endohedral Ti doped cationic clusters, the Ar binding energy decreases drastically, whereas the Ar-cluster distance increases substantially, point to Ar physisorption, as assumed by the experimentalists. Calculated Ar adsorption energies agree well with available experimental binding energies.

Structural and electronic properties of TM(n)[(BN)(3)H(6)](m) complexes with TM = Co (n, m = 1-3) and with TM = Fe, Ni, Ru, Rh, Pd (n = m = 1-3).

Using the density functional method with the generalized gradient approximation for the exchange and correlation, we investigated the geometrical and electronic properties of free-standing complexes of Con clusters combined with hydrogen-saturated boron-nitrogen (BN) rings [(BN)3H6]m. The Co atoms tend to form a subcluster capped by BN rings that preserve the Co subcluster against the environment and with which they weakly interact. Thus, the Co subcluster is capable of sustaining a noticeable magnetic moment. These facts are relevant for designing grains with localized magnetic moments. We also optimized those TMn[(BN)3H6]n complexes with n = 1-3 and TM = Fe, Ni, Ru, Rh, and Pd, starting with the ground-state geometry obtained previously for TM = Co, in order to analyze the dependence of the electronic properties with the number of d electrons in the transition-metal atoms.

Antiferromagnetic-like coupling in the cationic iron cluster of thirteen atoms.

We explore, within the density functional theory in the generalized gradient approximation to exchange and correlation, the map of spin isomers of the cationic Fe13(+) cluster in connection with recent X-ray magnetic circular dichroism spectroscopy experiments [M. Niemeyer et al., Phys. Rev. Lett. 2012, 108, 057201] which showed an anomalous low magnetic moment per number of 3d holes in this cluster. We systematically explore the low-lying magnetic excitations and correlate them with structural rearrangements and stability indicators. We obtain the observed low magnetic moment per 3d hole as the ground state of Fe13(+) and we demonstrate that, as supposed by the experimentalists, the cluster undergoes a magnetic transition from a ferromagnetic-like configuration to an antiferromagnetic-like one upon ionization. We unravel this unexpected magnetic behavior showing that it is concomitant with a Th-deformation of the icosahedral structure together with the electronic filling of this particular iron cluster. The spin-orbit interaction preserves this magnetic configuration which is essentially due to the spin. Our computed magnetic anisotropy energy supports the experimental interpretation of the cluster as fluxional due to the very weak coupling of the magnetic moment to an easy axis.

Structural, electronic, and magnetic properties Of Co(n)Cu(m) nanoalloys (m + n = 12) from first principles calculations.

Using the generalized gradient approximation (GGA) to density functional theory (DFT), we compute the electronic structure and related magnetic properties of free-standing Co(12-x)Cu(x) clusters (x = 0-12) with structures resulting from the optimization of those of the low-lying energy isomers of pure Co(12) and Cu(12) in which Co(Cu) were replaced by Cu(Co) atoms. Structural transitions for the lowest energy homotop are obtained as a function of the concentration, but in all cases, a clear surface segregation of Cu is found in the low concentration regime x < 5. The binding energy decreases monotonically when x increases. The dipole moment changes abruptly from 0.06 D for x = 2 to 0.59 D for x = 3 in coincidence with a structural change. The electronegativity of the lowest energy homotop exhibits minimum (maximum) value for x = 11 (x = 9). The x = 5, 9 clusters show local maxima of the hardness, of the excess energy, and of the second difference in energy, clear indicators of specially stable stoichiometries. The magnetic behavior of Co(12-x)Cu(x) is a monotonous function of the Co concentration, decreasing by steps of ≥2 µ(B) for every Co atom that is replaced by Cu, although for certain concentrations, different spin isomers, sometimes accompanied by structural transitions, are found close to the ground state. Ferromagnetic-like order is obtained as the ground state in all cases, contrary with the trend found in binary clusters of the same elements by other authors who predicted antiferromagnetic order. We analyze in detail the possible spin excitations in Co(12)Cu to demonstrate that local antiferromagnetic couplings can only exist as metastable spin states.

Ab initio study of the adsorption of NO on the Rh6(+) cluster.

The process of NO adsorption on the cationic cluster Rh(6)(+) is investigated using the density-functional theory (DFT) with the generalized gradient approximation (GGA) to exchange and correlation. We determine the geometries, electronic structure, and relevant energies for different structural and spin isomers of Rh(6)(0,±), and we study the consecutive adsorption of two NO molecules on the cationic cluster Rh(6)(+). With regard to the first NO molecule, different adsorption energies are found for the ground state octahedral structure of the bare cationic cluster and for the first isomer, which, having a prism-type structure, undergoes a structural transition to an octahedral symmetry upon dissociative adsorption of NO. Several dissociative NO adsorption processes are analyzed in comparison with molecular adsorption of NO to give support to the first step of the reaction inferred from experiments. With regard to the adsorption of a second NO molecule, the intermediate with lowest energy contains a preformed N(2) molecule. The energy of that complex is about 0.7 eV smaller than the sum of the free N(2) energy plus the lowest energy of the Rh(6)(+)-O(2) complex. This complex is composed of two separated O atoms occupying adjacent 2-fold bridging positions of the nearly undistorted Rh(6)(+) octahedral cluster. These findings are in qualitative agreement with experiments.

Study of the structural and electronic properties of Rh(N) and Ru(N) clusters (N < 20) within the density functional theory.

Using the density-functional theory (DFT) with the generalized gradient approximation to exchange and correlation, we compute the geometries, electronic structure, and related properties of free-standing rhodium and ruthenium atomic clusters with sizes below 20 atoms. We explore different structural and spin isomers per size, for which we determine the interatomic distances, binding energy, magnetic moment, HOMO-LUMO gap, and electric dipole moment. For many sizes, different implementations of DFT predict different properties for the lowest-energy isomers, thus illustrating the complex nature of these 4d transition metal elements at the nanoscale. We discuss our results for rhodium clusters in the context of recent electric deflection measurements.