Communication: Structure of magnetic lanthanide clusters from far-IR spectroscopy: Tb n+ (n = 5−9)

Spectroscopic properties and activated mechanism of NO on isolated cationic tantalum clusters: A first-principles study

The adsorption and dissociation of NO on the cationic Ta₁₅⁺ cluster were investigated using the density-functional theory (DFT) calculations, and the Ta-centered bicapped hexagonal antiprism (BHA) structure of cationic Ta₁₅⁺ cluster can be identified as the global minimum, which reproduces well the infrared multiple photo dissociation (IR-MPD) spectrum. Our results show that the cationic BHATa₁₅⁺ cluster provides the hollow region for NO to interact effectively, and possess larger adsorption strength on the region than other sites. The density of states, charge density differences and frontier molecular orbitals were analyzed to understand the electronic properties of the stable NO-adsorbed isomers. The characteristic IR peaks of the firstly two low-lying isomers are properly assigned, in which the strongest IR peak originates from the N - O stretching vibration. For the dissociation of NO on the BHATa₁₅⁺ cluster, it is found that the reaction path II easily occurs rather than path I due to small reaction barrier, and the cluster may possess the great catalytic behavior to dissociate NO molecule. The present results will inevitably stimulate future theoretical and experimental studies for the design of novel Ta-based catalytic materials for the NO dissociation.

First-Principles Study of Structural, Electronic and Magnetic Properties of Metal-Centered Tetrahexahedral V15⁺ Cluster

The V-centered bicapped hexagonal antiprism structure (A), as the most stable geometry of the cationic V₁₅⁺ cluster, is determined by using infrared multiple photo dissociation (IR-MPD) in combination with density functional theory computations. It is found that the A structure can be stabilized by 18 delocalized 3c-2e σ-bonds on outer V₃ triangles of the bicapped hexagonal antiprism surface and 12 delocalized 4c-2e σ-bonds on inner trigonal pyramidal V₄ moiety, and the features are related to the strong p-d hybridization of the cluster. The total magnetic moments on the cluster are predicted to be 2.0 µ_B, which come mainly from the central vanadium atom.

Electronic structure and spectral properties of terbium(III) nitrate complex with hexamethylphosphoramide

Spectral properties of terbium(III) nitrate complex with hexamethylphosphoramide have been studied by quantum-chemical methods within the density functional theory and methods of luminescent and X-ray photoelectron spectroscopy. Analysis of the luminescence excitation spectrum of the complex has indicated the absence of intramolecular transfer of electronic excitation energy from the ligand levels to the resonance levels of the rare earth central ion, so luminescence of the complex is associated with the electronic f-f-transitions of Tb³⁺ ion (transitions ⁵D₄→⁷F_J, J=3-6). According to quantum-chemical modeling of the excited singlet and triplet levels of the complex, the excitation energy transfer from the ligands onto the central ion does not occur because of the significant difference of energies of their excited states.

Magnetism and exchange interaction of small rare-earth clusters; Tb as a representative

Here we follow, both experimentally and theoretically, the development of magnetism in Tb clusters from the atomic limit, adding one atom at a time. The exchange interaction is, surprisingly, observed to drastically increase compared to that of bulk, and to exhibit irregular oscillations as a function of the interatomic distance. From electronic structure theory we find that the theoretical magnetic moments oscillate with cluster size in exact agreement with experimental data. Unlike the bulk, the oscillation is not caused by the RKKY mechanism. Instead, the inter-atomic exchange is shown to be driven by a competition between wave-function overlap of the 5d shell and the on-site exchange interaction, which leads to a competition between ferromagnetic double-exchange and antiferromagnetic super-exchange. This understanding opens up new ways to tune the magnetic properties of rare-earth based magnets with nano-sized building blocks.

Unusual Temperature Dependence of Magnetization and Possible Magnetic Noncollinearity in Tm and Pr Clusters

Rare-earth metals in their bulk form possess rather similar crystallographic structures, which is due to the very similar features of their outer electronic states. On the other hand, their magnetic properties are of rich variety, which is related to the specific form of the indirect magnetic exchange interaction between the inner electronic shells. In cluster form, this interplay may lead to very unusual magnetic structures. Here we show how the magnetic moments vary with size and temperature in Tm and Pr clusters. While in Pr clusters clear atom-by-atom oscillations indicate antiferromagnetic ordering, smooth variation and anomalous temperature behavior in Tm is representative for an essentially non-collinear spin arrangement. Their electric behavior is also very different, with a metallic-like behavior of Pr and localized electronic states in Tm.

Can far-IR action spectroscopy combined with BOMD simulations be conformation selective?

The combination of conformation selective far-IR/UV double resonance spectroscopy with Born–Oppenheimer molecular dynamics (BOMD) simulations is presented here for the structural characterization of the Ac-Phe-Pro-NH₂ peptide in the far-infrared spectral domain, i.e. for radiation below 800 cm⁻¹.

Monoxides of small terbium clusters: a density functional theory investigation

To investigate the effect of oxygen atom on the geometrical structures, electronic, and magnetic properties of small terbium clusters, we carried out the first-principles calculations on TbnO (n = 1-14) clusters. The capping of an oxygen atom on one trigonal-facet of Tbn structures is always favored energetically, which can significantly improve the structural stability. The far-infrared vibrational spectroscopies are found to be different from those of corresponding bare clusters, providing a distinct signal to detect the characteristic structures of TbnO clusters. The primary effect of oxygen atom on magnetic properties is to change the magnetic orderings among Tb atoms and to reduce small of local magnetic moments of the O-coordinated Tb atoms, both of which serve as the key reasons for the experimental magnetic evolution of an oscillating behavior. These calculations are consistent with, and help to account for, the experimentally observed magnetic properties of monoxide TbnO clusters [C. N. Van Dijk et al., J. Appl. Phys. 107, 09B526 (2010)].

Emergence of antiferromagnetic ordering in Tbn (n = 2-33) clusters

Using the density functional theory (DFT) under both frameworks of the local density approximation (LDA) and the generalized gradient approximation (GGA), we show that the anomalous magnetic moment of Tbn (n = 2-20, 22, 33) clusters found in recent experiment [J. Appl. Phys. 2010, 107, 09B526] is due to the antiferromagnetic or ferromagnetic couplings between Tb atoms in clusters. The local spin magnetic moment of Tb atoms is in the range 5.1-5.7 μ(B), which is faintly influenced by the cluster sizes, geometrical structures, and spin arrangements. Emphasis is made on the Tb13 cluster to eliminate the large magnetic discrepancy between the experiment and calculation. Geometrical evolution shows that the square-antiprism motif with one centered atom is competitive with the icosahedral motif for small Tb clusters n = 9-12, whereas the close packed icosahedral growth is clearly favored for large clusters n = 13-20, 22, 33. The clusters containing 4, 7, 10, 13, and 19 atoms are found to be more stable than their neighboring sizes, in agreement with the early mass spectral measurements. The electronic properties including the HOMO-LUMO energy gaps, ionization potentials, electron affinities, and electric dipole moments are calculated, and more importantly, the variational trends of the magnetic moment and electric dipole moment qualitatively fit well with the experimental observations.