RESUMO
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.
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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.
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We report a theoretical investigation of free-standing Fe(x)Co(y)Ni(z) ternary clusters with x + y + z = 5 and 6. Our study is performed within density functional theory as implemented in the GAUSSIAN 03 set of programs and with the BPW91/SDD level of theory. We analyze the geometries, chemical order, local and total magnetic moments, binding energies, excess energies, and second difference in the energy in the whole range of composition, from which structural, magnetic, and stability phase diagrams are predicted for these cluster sizes. We determine the optimal stoichiometries for these clusters as regards the maximum total magnetic moment and stability.
RESUMO
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.
RESUMO
We report the results of density-functional calculations of the structures, binding energies and magnetic moments of the clusters Mo(N) (N = 2-13), Mo(12)Fe, Mo(12)Co and Mo(12)Ni that were performed using the SIESTA method within the generalized gradient approximation for exchange and correlation. For pure Mo(N) clusters, we obtain collinear magnetic structures in all cases, even when the self-consistent calculations were started from non-collinear inputs. Our results for these clusters show that both linear, planar and three-dimensional clusters have a strong tendency to form dimers. In general, even-numbered clusters are more stable than their neighbouring odd-numbered clusters because they can accommodate an integer number of tightly bound dimers. As a consequence, the binding energies of pure Mo(N) clusters, in their lowest-energy states, exhibit an odd-even effect in all dimensionalities. Odd-even effects are less noticeable in the magnetic moments than in the binding energies. When comparing our results for pure Mo clusters with those obtained recently by other authors, we observe similarities in some cases, but striking differences in others. In particular, the odd-even effect in three-dimensional Mo clusters was not observed before, and our results for some clusters (e.g. for planar Mo(3) and Mo(7) and for three-dimensional Mo(7) and Mo(13)) differ from those reported by other authors. For Mo(12)Fe and Mo(12)Ni, we obtain that the icosahedral configuration with the impurity atom at the cluster surface is more stable than the configuration with the impurity at the central site, while the opposite occurs in the case of Mo(12)Co. In Mo(12)Co and Mo(12)Ni, the impurities exhibit a weak magnetic moment parallely coupled to the total magnetic moment of the Mo atoms, whereas in Mo(12)Fe the impurity shows a high moment with antiparallel coupling.