Cheng Lu

Ab Initio Search for Global Minimum Structures of Pure and Boron Doped Silver Clusters.

The global minimum structures of pure and boron doped silver clusters up to 16 atoms are determined through ab initio calculations and unbiased structure searching methods. The structural and electronic properties of neutral, anionic, and cationic Ag(n)B (n ≤ 15) and Ag(n)B2 (n ≤ 14) clusters are much distinct from those of the corresponding pure silver. Considering that Ag and B possess one and three valence electrons, respectively, both the single and the double boron-atom doped silver clusters with even number of valence electrons are more stable than those with odd number of electrons, a feature also observed in the pure silver clusters. We demonstrate that the species with a valence count of 8 and 14 appear to be magic numbers with enhanced stability irrespective of component or the charged state. A new putative global minimum structure of Ag13(-) cluster, with high symmetry of C(2v), is unexpectedly observed as the ground state, which is lower in energy than the previous suggested bilayer structure.

Insights into the geometries, electronic and magnetic properties of neutral and charged palladium clusters.

We performed an unbiased structure search for low-lying energetic minima of neutral and charged palladium PdnQ (nu2009=u20092–20, Qu2009=u20090,u2009+u20091 and –1) clusters using CALYPSO method in combination with density functional theory (DFT) calculations. The main candidates for the lowest energy neutral, cationic and anionic clusters are identified, and several new candidate structures for the cationic and anionic ground states are obtained. It is found that the ground state structures of small palladium clusters are more sensitive to the charge states. For the medium size Pdn0/+/– (nu2009=u200916–20) clusters, a fcc-like growth behavior is found. The structural transition from bilayer-like structures to cage-like structures is likely to occur at nu2009=u200914 for the neutral and cationic clusters. In contrast, for the anionic counterparts, the structural transition occurs at Pd13–. The photoelectron spectra (PES) of palladium clusters are simulated based on the time-dependent density functional theory (TD-DFT) method and compared with the experimental data. The good agreement between the experimental PES and simulated spectra provides us unequivocal structural information to fully solve the global minimum structures, allowing for new molecular insights into the chemical interactions in the Pd cages.

Systematic theoretical investigation of geometries, stabilities and magnetic properties of iron oxide clusters (FeO)nμ (n = 1–8, μ = 0, ±1): insights and perspectives

The structural properties of neutral and charged (FeO)nμ (n = 1–8, μ = 0, ±1) clusters have been studied using an unbiased CALYPSO structure searching method. As a first step, an unbiased search relying on several structurally different initial clusters has been undertaken. Subsequently, geometry optimization by means of density-functional theory with the Perdew and Wang (PW91) exchange–correlation functional is carried out to determine the relative stability of various candidates for low-lying neutral, anionic and cationic iron oxide clusters obtained from the unconstrained search. It is shown that the mostly equilibrium geometries of iron oxide clusters represent near planar structures for n ≤ 3. No significant structural differences are observed between the neutral and charged iron oxide clusters beyond sizes with n = 6. The relative stabilities of (FeO)nμ clusters for the ground-state structures are analyzed on the basis of binding energies and HOMO–LUMO gaps. Our theoretical results confirm that the binding energies of neutral and anionic (FeO)n0/− tend to increase with cluster size. Cationic (FeO)n+ exhibits a slight downward trend. It is worth noticing that (FeO)5 and (FeO)4−/+ are the most stable geometries for (FeO)nμ (n = 1–8, μ = 0, ±1) clusters. Lastly, an evident local oscillation of magnetic behavior is present in the most stable (FeO)nμ (n = 1–8, μ = 0, ±1) clusters, and the origin of this magnetic phenomenon is analyzed in detail.

Dynamical behavior of boron clusters

Several of the lowest energy structures of small and medium sized boron clusters are two-dimensional systems made up of a pair of concentric rings. In some cases, the barriers to the rotation of one of those rings relative to the other are remarkably low. We find that a combination of electronic and geometrical factors, including apparently the relative sizes and symmetries of the inner and outer rings, are decisive for the diminished barriers to in-plane rotation in these two dimensional clusters. A sufficiently large outer ring is important; for instance, expansion of the outer ring by a single atom may reduce the barrier significantly. A crucial factor for an apparent rotation is that the σ-skeleton of the individual rings remains essentially intact during the rotation. Finally, the transition state for the rotation of the inner ring comprises the transformation of a square into a diamond, which may be linked to a mechanism suggested decades ago for the isomerization of carboranes and boranes.

Prediction of Stable Ruthenium Silicides from First-Principles Calculations: Stoichiometries, Crystal Structures, and Physical Properties

We present results of an unbiased structure search for stable ruthenium silicide compounds with various stoichiometries, using a recently developed technique that combines particle swarm optimization algorithms with first-principles calculations. Two experimentally observed structures of ruthenium silicides, RuSi (space group P2(1)3) and Ru2Si3 (space group Pbcn), are successfully reproduced under ambient pressure conditions. In addition, a stable RuSi2 compound with β-FeSi2 structure type (space group Cmca) was found. The calculations of the formation enthalpy, elastic constants, and phonon dispersions demonstrate the Cmca-RuSi2 compound is energetically, mechanically, and dynamically stable. The analysis of electronic band structures and densities of state reveals that the Cmca-RuSi2 phase is a semiconductor with a direct band gap of 0.480 eV and is stabilized by strong covalent bonding between Ru and neighboring Si atoms. On the basis of the Mulliken overlap population analysis, the Vickers hardness of the Cmca structure RuSi2 is estimated to be 28.0 GPa, indicating its ultra-incompressible nature.

Evolution of the Structural and Electronic Properties of Medium-Sized Sodium Clusters: A Honeycomb-Like Na20 Cluster.

Sodium is one of the best examples of a free-electron-like metal and of a certain technological interest. However, an unambiguous determination of the structural evolution of sodium clusters is challenging. Here, we performed an unbiased structure search among neutral and anionic sodium clusters in the medium size range of 10-25 atoms, using the Crystal structure AnaLYsis by Particle Swarm Optimization (CALYPSO) method. Geometries are determined by CALYPSO structure searches, followed by reoptimization of a large number of candidate structures. For most cluster sizes the simulated photoelectron spectra of the lowest-energy structures are in excellent agreement with the experimental data, indicating that the current ground-state structures are the true minima. The equilibrium geometries show that, for both neutral and anionic species, the structural evolution from bilayer structures to layered outsides with interior atoms occurs at n = 16. A novel unprecedented honeycomb-like structure of Na20 cluster with C3 symmetry is uncovered, which is more stable than the prior suggested structure based on pentagonal structural motifs.

Crystal Structures, Stabilities, Electronic Properties, and Hardness of MoB2: First-Principles Calculations

On the basis of the first-principles techniques, we perform the structure prediction for MoB2. Accordingly, a new ground-state crystal structure WB2 (P63/mmc, 2 fu/cell) is uncovered. The experimental synthesized rhombohedral R3̅m and hexagonal AlB2, as well as theoretical predicted RuB2 structures, are no longer the most favorite structures. By analyzing the elastic constants, formation enthalpies, and phonon dispersion, we find that the WB2 phase is thermodynamically and mechanically stable. The high bulk modulus B, shear modulus G, low Poissons ratio ν, and small B/G ratio are benefit to its low compressibility. When the pressure is 10 GPa, a phase transition is observed between the WB2-MoB2 and the rhombohedral R3̅m MoB2 phases. By analyzing the density of states and electron density, we find that the strong covalent is formed in MoB2 compounds, which contributes a great deal to its low compressibility. Furthermore, the low compressibility is also correlated with the local buckled structure.

Probing the low-energy structures of aluminum-magnesium alloy clusters: a detailed study.

The effect of Mg doping on the growth behavior and the electronic properties of aluminum clusters has been investigated theoretically using the CALYPSO (Crystal structure AnaLYsis by Particle Swarm Optimization) method in combination with density functional theory calculations. Compared to pure aluminum clusters, the structure of Mg-doped clusters shows the charming transformation with increasing atomic number. The photoelectron spectra (PES) of the global minima of anionic Aln and AlnMg (n = 3-20) clusters have been calculated based on the time-dependent density functional theory (TD-DFT) method. The reliability of our theoretical methodology is easily corroborated by the good agreement between the experimental PES and the simulated spectra. Our findings bring forth an ionic bonding with enhanced stability for the Al6Mg cluster, paired with a surprisingly large HOMO-LUMO gap, as would be expected from the magic number of 20 valence electrons.

Determination of the microstructure, energy levels and magnetic dipole transition mechanism for Tm3+ doped yttrium aluminum borate

Yttrium aluminum borate (YAB) doped with rare-earth ions are promising materials for infrared lasers and self-frequency summing laser systems. The stable crystal forms of YAB doped with rare-earth ions are of great scientific interest. Here, we systematically study the structural evolution of Tm3+ doped YAB, by using an unbiased CALYPSO structure search method in conjunction with first principles calculations. We are able to identify a unique semiconducting phase with P321 space group that reveal Tm3+ ions occupy Y ion sites of octahedral symmetry. The electronic band structure shows an extremely narrow conduction band above the Fermi level of Tm3+ in YAl3(BO3)4, indicating that the impurity Tm3+ ions lead to an insulator to semiconductor transition. The atomic energy structure of the 4f12 configuration of Tm3+ in the YAB has been calculated by a crystal-field theory method, which includes the major electrostatic and spin–orbit interactions as well as various minor contributions, for 4fN tripositive lanthanide ions. Electric dipole induced transitions are calculated to be much stronger than magnetic dipole induced ones in most situations. However, detailed magnetic dipole calculations for individual crystal field levels indicate a large number of strong absorption lines and spontaneous emissions, including many in the visible spectrum and at longer wavelengths that would be an attractive medium for investigating the magnetic portion in the light-matter interactions.

Understanding the structural transformation, stability of medium-sized neutral and charged silicon clusters

The structural and electronic properties for the global minimum structures of medium-sized neutral, anionic and cationic Sinμ (nu2009=u200920–30, μu2009=u20090, −1 and +1) clusters have been studied using an unbiased CALYPSO structure searching method in conjunction with first-principles calculations. A large number of low-lying isomers are optimized at the B3PW91/6-311u2009+u2009G* level of theory. Harmonic vibrational analysis has been performed to assure that the optimized geometries are stable. The growth behaviors clearly indicate that a structural transition from the prolate to spherical-like geometries occurs at nu2009=u200926 for neutral silicon clusters, nu2009=u200927 for anions and nu2009=u200925 for cations. These results are in good agreement with the available experimental and theoretical predicted findings. In addition, no significant structural differences are observed between the neutral and cation charged silicon clusters with nu2009=u200920–24, both of them favor prolate structures. The HOMO-LUMO gaps and vertical ionization potential patterns indicate that Si22 is the most chemical stable cluster, and its dynamical stability is deeply discussed by the vibrational spectra calculations.