Yuanyuan Jin

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 (n = 2–20, Q = 0, + 1 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/+/– (n = 16–20) clusters, a fcc-like growth behavior is found. The structural transition from bilayer-like structures to cage-like structures is likely to occur at n = 14 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.

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.

Probing the structural evolution of ruthenium doped germanium clusters: Photoelectron spectroscopy and density functional theory calculations.

We present a combined experimental and theoretical study of ruthenium doped germanium clusters, RuGen− (n = 3–12), and their corresponding neutral species. Photoelectron spectra of RuGen− clusters are measured at 266 nm. The vertical detachment energies (VDEs) and adiabatic detachment energies (ADEs) are obtained. Unbiased CALYPSO structure searches confirm the low-lying structures of anionic and neutral ruthenium doped germanium clusters in the size range of 3 ≤ n ≤ 12. Subsequent geometry optimizations using density functional theory (DFT) at PW91/LANL2DZ level are carried out to determine the relative stability and electronic properties of ruthenium doped germanium clusters. It is found that most of the anionic and neutral clusters have very similar global features. Although the global minimum structures of the anionic and neutral clusters are different, their respective geometries are observed as the low-lying isomers in either case. In addition, for n > 8, the Ru atom in RuGen−/0 clusters is absorbed endohedrally in the Ge cage. The theoretically predicted vertical and adiabatic detachment energies are in good agreement with the experimental measurements. The excellent agreement between DFT calculations and experiment enables a comprehensive evaluation of the geometrical and electronic structures of ruthenium doped germanium clusters.

Exploration of stable stoichiometries, physical properties and hardness in the Rh-Si system: a first-principles study†

To understand the structural stability, physical properties, and hardness of the Rh–Si system, we have performed systematic first-principles crystal structure searches for various stoichiometries of rhodium silicides, utilizing the particle swarm optimization method. A new stable stoichiometry, Rh4Si5 with space group P21/m, has been found at atmospheric pressure, complementing the three well-known rhodium silicides of Rh2Si (Pnma), Rh5Si3 (Pbam), and RhSi (Pnma). Our calculations of the structural and mechanical properties of the known phases are in excellent agreement with the available experimental data and similar theoretical calculations. The elastic, electronic, and hardness properties of the Rh–Si stoichiometries are discussed. Our results suggest that the new rhodium silicide Rh4Si5 (P21/m) is a potentially hard material with the hardness of 20.1 GPa.

Prediction of Novel High-Pressure Structures of Magnesium Niobium Dihydride

On the basis of a combination of the particle-swarm optimization technique and density functional theory (DFT), we explore the crystal structures of MgH2, NbH2, and MgNbH2 under high pressure. The enthalpy-pressure (H-P) diagrams indicate that the structural transition sequence of MgH2 is α → γ → δ → ε → ζ and that NbH2 transforms from the Fm3̅m phase to the Pnma phase at 47.80 GPa. However, MgNbH2 is unstable when the pressure is too low or too high. Two novel MgNbH2 structures, the hexagonal P6̅m2 phase and the orthorhombic Cmcm phase, are discovered, which are stable in the pressure ranges of 13.24-128.27 GPa and 128.27-186.77 GPa, respectively. The P6̅m2 phase of MgNbH2 consists of alternate layers of polymetric NbH6 and MgH6 triangular prisms, while the Cmcm phase contains distorted MgH6 trigonal prisms. The calculated elastic constants and phonon dispersions confirm that both phases are mechanically and dynamically stable. The analyses of density of states (DOS), electron localization function (ELF), and Bader charge demonstrate that a combination of ionic and metallic bonds exist in both P6̅m2 and Cmcm phases. We hope the newly predicted magnesium niobium dihydrides with desirable electronic properties will promote future experimental and theoretical studies on mixed main group-transition metal hydrides.