Ai-Jie Mao

Density functional study on size-dependent structures, stabilities, electronic and magnetic properties of AunM (M = Al and Si, n = 1–9) clusters: comparison with pure gold clusters

The density functional theory (DFT) method has been employed to systematically investigate the geometrical structures, relative stabilities, and electronic and magnetic properties of AunM (M = Al and Si, n = 1–9) clusters for clarifying the effect of Al (Si) modulation on the gold nanostructures. Of all the clusters studied, the most stable configurations adopt a three-dimensional structure for AunAl at n = 4–8 and AunSi at n = 3–9, while for pure gold systems, no three-dimensional lowest energy structures are obtained. Through a careful analysis of the fragmentation energy, second-order difference of energy, HOMO-LUMO energy gap, and magnetic moment as a function of cluster size, an odd-even alternative phenomenon has been observed. The results show that the clusters with even-number valence electrons have a higher relative stability, but lower magnetic moments. Furthermore, Al (Si) doping is found to enhance the stabilities of gold frameworks. In addition, the charge analysis has been given to understand the different effects of individual doped atom on electronic properties and compared further.

Pressure-induced structural transition and thermodynamic properties of NbN and effect of metallic bonding on its hardness

Using first- principles calculations, the elastic constants, thermodynamic properties and structural phase transition of NbN under high pressure are investigated by means of the pseudopotential plane- waves method, in addition to the effect of metallic bonding on its hardness. Three candidate structures are chosen to investigate NbN, namely, rocksalt (NaCl), NiAs and WC types. On the basis of the third- order Birch- Murnaghan equation of states, the transition pressure Pt (Pt = 200.64GPa) between the WC phase and the NaCl phase of NbN is predicted for the first time. Elastic constants, formation enthalpies, shear modulus, Youngs modulus, and Poissons ratio of NbN are derived. The calculated results are found to be in good agreement with the available experimental data and theoretical values. According to the quasi-harmonic Debye model, the Debye temperature under high pressure is derived from the average sound velocity. Moreover, the effect of metallic bonding on the hardness of NbN is investigated and the hardness shows a gradual decrease rather than increase under compression. This is a quantitative investigation on the structural and thermodynamic properties of NbN, and it still awaits experimental confirmation. Copyright (C) EPLA, 2010

Pressure-induced structural transition of OsN2 and effect of metallic bonding on its hardness

Using first-principles calculations, the elastic constant, structural phase transition and effect of metallic bonding on the hardness of OsN2 under high pressure are investigated by means of the pseudopotential plane-waves method. Five candidate structures are chosen to investigate for OsN2, namely, the pyrite, CoSb2-type, marcasite, simple hexagonal and tetragonal structures. A comparison among the formation energies of OsN2 explains the synthesis of OsN2 marcasite under high pressure. On the basis of the third-order Birch-Murnaghan equation of states, the transition pressure Pt (Pt=223?GPa) between the marcasite and simple tetragonal phase is determinated. Elastic constants, shear modulus, Youngs modulus, Poissons ratio and Debye temperature are derived. The calculated values are, generally speaking, in good agreement with experiments and other theoretical calculations. Our calculation indicates that the N-N bond length is one determinative factor for the ultrahigh bulk moduli of the heavy-transition-metal dinitrides. Moreover, based on Mulliken overlap population analysis in first-principles technique, a semiempirical method to evaluate the hardness of multicomponent crystals with partial metallic bonding is presented. The effect of metallic bonding on the hardness of OsN2 is investigated and the hardness shows a gradual decrease rather than increase under compression, which is different from diamond. This is a quantitative investigation on the structural properties of OsN2, and it still awaits experimental confirmation.

Structural and electronic properties of silver-doped gold clusters Au n Ag v (2 ≤ n ≤ 10; v = 0, ±1): comparison with pure gold clusters

The structural and electronic properties of silver-doped gold clusters Au n Ag v (2 ≤ n ≤ 10; v = 0, ±1) have been systematically investigated using density functional theory. The results show that the ground state optimal structures of the cationic and neutral clusters are found to be planar up to n = 3 and 9, respectively. However, for the anionic clusters, no three-dimensional lowest-energy structures are obtained according to DFT calculations. The calculated binding energy and dissociation energy as a function of cluster size exhibit odd–even alternations. The natural population analysis indicates that in Au n Ag v clusters charges transfer from the Ag atom to the Au frames. The trends for the vertical detachment energies, adiabatic electron affinities, adiabatic ionization potentials, and chemical hardness of Au n Ag v clusters, as the cluster size increases, are studied in detail and compared with the available experimental data.

A DFT study on equilibrium geometries, stabilities, and electronic properties of small bimetallic Na-doped Au(n) (n = 1-9) clusters: comparison with pure gold clusters.

A systematic study on the geometric structures, relative stabilities, and electronic properties of small bimetallic AunNa (n = 1-9) clusters has been performed by means of first-principle density functional theory calculations at the PW91PW91 level. The results show that the optimized ground-state isomers adopt planar structures up to n = 5, and the Na-capped geometries are dominant growth patterns for n = 6-9. Dramatic odd-even alternative behaviors are obtained in the second-order difference of energies, fragmentation energies, highest occupied-lowest unoccupied molecular orbital energy gaps, and chemical hardness for both AunNa and Aun+1 clusters. It is found that Au5Na and Au6 have the most enhanced stability. Here, the size evolutions of the theoretical ionization potentials are in agreement with available experimental data, suggesting a good prediction of the lowest energy structures in the present study. In addition, the charge transfer has been analyzed on the basis of natural population analysis.

Pressure effect on the properties of magnetic moments and phase transitions in YMnO3 from first principles

We report the properties of magnetic, electronic, phonon frequencies and magnetic phase transitions in orthorhombic perovskite YMnO3 by means of first-principles calculations. Our results argue that the magnetic ground state is indeed E-type antiferromagnetic ordering at zero temperature and zero pressure. In the range of 0 to 150 GPa, the magnetic phase undergoes two evolutions: first is E(Pmn21)-to-A(Pnma)-AFM phase at around 48 GPa, corresponding to the ferroelectric to paraelectric phase transition; second is A-to-G-AFM phase at around 131.7 GPa. The Mn3+ magnetic moments of the considered magnetic configurations exhibit a mutation (i.e., they decrease nearly by 50%) between 40 and 50 GPa. In parallel, to further investigate the magnetic ground states at different pressures, we also analyze the phonon spectra and electronic properties.

Structural phase transition and spin reorientation of LaFeO3 films under epitaxial strain

Structural phase transition and spin reorientation of orthoferrites LaFeO3 epitaxially grown along the pseudocubic (001) direction are investigated based on first-principles calculation. In the process of the LaFeO3 film growth, two phases Pmc21(3) and P4mm with the larger ferroelectric are obtained. And spin reorientation transition happened which induced by strain from Γ4 to Γ2 about misfit strain 2.28% in c-ePbnm phase. The critical strain for observation of the Γ4–Γ2 transition in LaFeO3 is very small, which will be a great surprise for experimental worker.

Theoretical study on local defect structure of (FeO4)5― clusters in YGG and LGG crystals

The optical spectrum and the local defect structure of tetrahedral (FeO4)(5-) clusters are investigated in yttrium gallium garnet (YGG) and lutetium gallium garnet (LGG) crystals by means of the 252x252 complete energy matrices for d(5) configuration ions in tetragonal ligand field. The results show that the local defect structures around tetrahedral Fe3+ centers display an expansion effect. Simultaneously, the (FeO4(5-) clusters in the two different crystals have very similar local structures, which are close to those in YIG garnet crystal. Finally the relative curves of the zero-field splitting energies DeltaE1 and DeltaE2 in the ground state (6)A(1) varying with the structure parameter theta are plotted.

Investigation of the Local Lattice Structure and the Effects of the Orbital Reduction Factor on the g Factors of a Trigonal [Ni(H2O)6]2+ Cluster in NiTiF6 · 6H2O and ZnSiF6 · 6H2O Crystals at Different Temperatures

The local octahedral environment of Ni2+ in NiTiF6 ・ 6H2O and ZnSiF6 ・ 6H2O crystals with a trigonal distortion has been studied at different temperatures, based on the complete energy matrices. The calculated results showed that the local lattice structure around an octahedral Ni2+ centre in NiTiF6 ・ 6H2O and ZnSiF6 ・ 6H2O exhibits a compression distortion. Simultaneously, the orbital reduction effect on the g factors has been studied. The relationship between Δg = g||−g⊥ and orbital reduction factor k at 4.2, 77 and 298 (302) K has been discussed, suggesting that there is an almost linear relation between k and Δg for the Ni2+ ion in NiTiF6 ・ 6H2O and ZnSiF6 ・ 6H2O at each temperature.