Peng Shao

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.

Structural and Relative Stabilities, Electronic Properties, and Hardness of Iron Tetraborides from First Prinicples

First-principles calculations were carried out to investigate the structure, phase stability, electronic property, and roles of metallicity in the hardness for recently synthesized FeB4 with various different structures. Our calculation indicates that the orthorhombic phase with Pnnm symmetry is the most energetically stable one. The other four new dynamically stable phases belong to space groups monoclinic C2/m, orthorhombic Pmmn, trigonal R3̅m, and hexagonal P63/mmc. Their mechanical and thermodynamic stabilities are verified by calculating elastic constants, formation enthalpies, and phonon dispersions. We found that all phases are stabilized further under pressure. Above the pressure of about 50 GPa, the formation enthalpy of Pmmn is almost equal to that of P63/mmc phase. The analysis on density of states not only demonstrates that formation of strong covalent bonding in these compounds contributes greatly to their stabilities but also that they all exhibit metallic behavior which does not relate to the approach used. By considering metallic contributions, the estimated Vickers hardness values based on the semiempirical model show that the OsB4-structured FeB4, with a hardness of 48.1 GPa, well exceeding the limitation of superhardness (40 GPa), is more hard than the most stable phase. The others are predicted to be potential hard materials. Moreover, the atomic configuration and strong B-B covalent bonds are found to play important roles in the hardness of materials.

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 (n = 20–30, μ = 0, −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 B3PW91/6-311 + G* 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 n = 26 for neutral silicon clusters, n = 27 for anions and n = 25 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 n = 20–24, 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.

Iron-based magnetic superhalogens with pseudohalogens as ligands: An unbiased structure search

We have performed an unbiased structure search for a series of neutral and anionic FeL4 (L = BO2, CN, NO2, NO3, OH, CH3, NH2, BH4 and Li2H3) clusters using the CALYPSO (Crystal structure Analysis by Particle Swarm Optimization) structure search method. To probe the superhalogen properties of neutral and anionic FeL4 clusters, we used density-functional theory with the B3LYP functional to examine three factors, including distribution of extra electron, pattern of bonding and the nature of the ligands. Theoretical results show that Fe(BO2)4, Fe(NO3)4 and Fe(NO2)4 can be classified as magnetic superhalogen due to that their electron affinities even exceed those of the constituent ligands. The magnetic moment of Fe atom is almost entirly maintained when it is decorated with various ligands except for neutral and anionic (Li2H3)4. Moreover, the current work is also extended to the salt moieties formed by hyperhalogen/superhalogen anion and Na+ ion. It is found that these salts against dissociation into Na + FeL4 are thermodynamic stable except for Na[Fe(OH)4]. These results provides a wealth of electronic structure information about FeL4 magnetic superhalogens and offer insights into the synthesis mechanisms.

Prediction of the Iron-Based Polynuclear Magnetic Superhalogens with Pseudohalogen CN as Ligands

To explore stable polynuclear magnetic superhalogens, we perform an unbiased structure search for polynuclear iron-based systems based on pseudohalogen ligand CN using the CALYPSO method in conjunction with density functional theory. The superhalogen properties, magnetic properties, and thermodynamic stabilities of neutral and anionic Fe2(CN)5 and Fe3(CN)7 clusters are investigated. The results show that both of the clusters have superhalogen properties due to their electron affinities (EAs) and that vertical detachment energies (VDEs) are significantly larger than those of the chlorine element and their ligand CN. The distribution of the extra electron analysis indicates that the extra electron is aggregated mainly into pseudohalogen ligand CN units in Fe2(CN)5¯ and Fe3(CN)7¯ cluster. These features contribute significantly to their high EA and VDE. Besides superhalogen properties, these two anionic clusters carry a large magnetic moment just like the Fe2F5¯ cluster. Additionally, the thermodynamic stabilities are also discussed by calculating the energy required to fragment the cluster into various smaller stable clusters. It is found that Fe(CN)2 is the most favorable fragmentation product for anionic Fe2(CN)5¯ and Fe3(CN)7¯ clusters, and both of the anions are less stable against ejection of Fe atoms than Fe(CN)n-x.

Microhydration effects on the structures and electrophilic properties of cytidine

Microhydration effects on the geometrical structures, electron affinities and charge distributions of cytidine and their anions have been investigated systematically using density functional theory (DFT), by explicitly considering cytidine complexes with up to four water molecules. Various structures of neutral and anionic cytidine(H2O)n (n = 2–4) have been predicted, and N3, H–N4 and O2 are found to be the most favorable water-binding sites of cytidine. The adiabatic electron affinities of cytidine(H2O)n increase linearly with the number of hydrating water molecules, indicating that they would obtain a stronger ability to attract electrons with the hydration number increasing. By examining the SOMO and natural population analysis, we found the excess electron density is localized on the cytidine moiety, especially on the cytosine base unit. This may help explain why the hydrogen bond changes upon the extra electron attachment. In addition, the maps of the reduced density gradient isosurfaces show a rich visualization of the hydrogen bond, van der Waals interaction and steric effect.

Structures and bonding of auropolyboroenes [Au2(B4)xB3]−, [Au2(B4)xB2]2− and [Au2(B4)xB]+ (x = 2, 3): comparison with dihydride polyboroenes

Equilibrium structures of auropolyboroenes [Au2(B4)xB3]−, [Au2(B4)xB2]2− and [Au2(B4)xB]+ (x = 2, 3) are obtained from density functional theory-based calculations. Results show that the ground states of Au2B9+, Au2B13+ and Au2B142− can be obtained by adding two Au atoms to the corresponding ground-state pure boron clusters. For Au2B102−, Au2B11−, Au2B142− and Au2B15−, the ladder structures are proven to be the ground states at TPSS, OVGF, and CCSD(T) levels, which is similar to that of dihydride polyboroenes. AdNDP analysis indicates that the two rows of boron atoms in these auropolyboroenes are bonded by delocalized three-, four-, or five-center σ and π bonds. Especially, the dominant bonding patterns in Au2B11− and Au2B142− bear similarities to those of dihydride polyboroenes. The photoelectron spectroscopy (PES) spectra for anionic clusters were simulated to facilitate the experimental PES spectra. In addition, the fragmentation energies and products against different decay channels are estimated and discussed.

Density function study transition metal chromium-doped alkali clusters: the finding of magnetic superatom

The structures, stabilities and magnetic properties of CrXn (X = Na, Rb and Cs; n up to 9) clusters are studied using density functional theory to search for the stable magnetic superatoms. The geometrical optimisations indicate the ground-state structures of CrXn evolve toward a close packed structure with an interior Cr atom surrounded by X atoms as the cluster size increase. Their stabilities are analysed by the relative energy, gain in energy (ΔE(n)) and the highest unoccupied molecular orbital and lowest unoccupied molecular orbital gaps. Furthermore, the magnetic moments of CrXn clusters show an odd–even oscillation. Here, we mainly focus on the CrX7 (X = Na, Rb and Cs) clusters due to the same valence count as the known stable magnetic superatoms VNa8, VCs8 and TiNa9. Although these clusters all have a filled electronic configuration 1S21P6 and large magnetic moment 5 μB, our studies indicate that only CrNa7 is highly stable compared to its nearest neighbours, while CrRb7 and CrCs7 clusters are less stable. This suggests that Cr-doped Na7 is most appropriate for filled electronic configuration and CrNa7 is shown to be a stable magnetic superatom. More interesting, we find CrRb8 and CrCs8 with the filled electronic configuration 1S21P6 have higher stability and large magnetic moment 6 μB in their respective series.