Bi-Yu Tang

Structural and mechanical properties of Mg17Al12 and Mg24Y5 from first-principles calculations

Using density functional theory within GGA approximation, the structural and elastic properties of two important phases (Mg17Al12 and Mg24Y5) in Mg-based alloys have been studied. The obtained equilibrium structural parameters for both phases agree very well with experimental data. The calculated negative cohesive energy and formation energy show that both cubic precipitates have strong structural stability as well as good alloying ability. Three independent single-crystal elastic constants (C11, C12 and C44) at zero pressure as well as polycrystalline mechanical parameters such as bulk modulus B, shear modulus G, Youngs modulus E, Poissons ratio ν and anisotropy value A for both phases have been calculated. The mechanical properties of the cubic phases such as ductility and tenacity are further analysed and discussed.

Electronic structure and Fermi surface character of LaNiPO from first principles

Based on first-principles calculation, we have investigated electronic structure of a ZrCuSiAs structured superconductor LaNiPO. The density of states, band structures, and Fermi surfaces have been given in detail. Our results indicate that the bonding of the La\char21{}O and Ni\char21{}P is strongly covalent whereas binding property between the LaO and NiP blocks is mostly ionic. It is also found that four bands are across the Fermi level and the corresponding Fermi surfaces all have a two-dimensional character. In addition, we also give the band decomposed charge density, which suggests that orbital components of Fermi surfaces are more complicated than cuprate superconductors.

First-principles study of a double-cation alkali metal borohydride LiK(BH4)2

Metal borohydrides have been attracting great interest as potential candidates for use as advanced hydrogen storage materials because of their high gravimetric hydrogen densities. In the present study, first-principles calculations have been performed for the newly reported dual-cation alkali metal borohydride LiK(BH4)2, using density functional theory (DFT) within the generalized gradient approximation and the projected augmented wave method. LiK(BH4)2 is found to have an orthorhombic structure in the space group Pnma (No 62) with nearly ideal tetrahedral shape. It is an insulating material having a DFT-calculated wide band gap of 6.08 eV. Analysis of the electronic structure shows an ionic interaction between metal cations and (BH4)− and the covalent B–H interaction within the (BH4)− tetrahedron. The enthalpy of the formation reaction from primary elements is calculated and found to be −449.8 kJ mol−1. The decomposition temperature (Tdec) of LiK(BH4)2 lies between those of LiBH4 and KBH4, which suggests that the hydrogen decomposition temperature of metal borohydrides can be precisely adjusted by the appropriate combination of cations.

Ab initio study on the thermal properties of the fcc Al3Mg and Al3Sc alloys

Ab initio density functional theory (DFT) and density function perturbation theory (DFPT) have been used to investigate the thermal properties of the fcc Al3Mg and Al3Sc alloys over a wide range of pressure and temperature, in comparison with fcc Al. Phonon dispersions were obtained at equilibrium and strained configurations by density functional perturbation theory. Using the quasiharmonic approximation for the free energy, several thermal quantities of interest, such as the thermal Gr¨ uneisen parameter, heat capacity, thermal expansion coefficient and entropy, were calculated as a function of temperature and pressure, and the variation features of these quantities were discussed in detail. This investigation provides useful information for design and applications of technologically relevant Al-based alloys.

Bending rigidity of transition metal dichalcogenide monolayers from first-principles

Due to the presence of a sizeable direct band gap, three-atom-thick transition metal dichalcogenide (TMDC) monolayers have been suggested as important candidates for flexible electronic and optoelectronic devices recently. The in-plane elasticity of TMDC monolayers has been investigated extensively, however, little is known about their bending rigidity. Here, we have determined bending rigidities of single-layer MX2 (M = Mo, W; X = S, Se) by fitting the energetics of single wall nanotubes from first-principles to the Helfrich Hamiltonian for the configurational energy of membranes. This parameter-free approach can avoid the controversy induced by ambiguous definition of the thickness of monolayers, which are required in the empirical determination of bending rigidity using classical shell theory. The obtained direction-dependent bending rigidities of single-layer MoS2 are 9.10 and 9.61 eV along the armchair and zigzag directions, which are larger than that estimated using shell theory but similar to the previous analytic formula based on an empirical potential. Moreover, the relative magnitude of bending rigidities for different TMDCs are found to be MoS2 < MoSe2 < WS2 < WSe2, which is in conflict with shell theory. Further analysis indicates that this inconsistence can be understood by a competition mechanism between two-dimensional elastic modulus of monolayers and the structural relaxation of nanotubes.

Role of electronic correlation in high-low temperature phase transition of hexagonal nickel sulfide: a comparative density functional theory study with and without correction for on-site Coulomb interaction.

The structural, electronic, magnetic, and elastic properties of hexagonal nickel sulfide (NiS) have been investigated comparatively by Density Functional theory (DFT) and DFT plus correction for on-site Coulomb interaction (DFT+U), in which two different exchange correlation functionals local density approximations (LDA) and general gradient approximations (GGA) in the form of Perdew-Burke-Ernzerhof (PBE) are used. Our results indicate LDA and PBE methods predict hexagonal NiS to be a paramagnetic metal whereas LDA(PBE)+U calculations with reasonable on-site Coulomb interaction energy give the antiferromagnetic insulating state of low temperature hexagonal NiS successfully. Meanwhile, compared with LDA(PBE) results, LDA(PBE)+U methods give larger lattice parameters, crystal volume, and shear constant c44, consistent with the experimental picture during high-low temperature phase transition of hexagonal NiS, in which an increase of the shear constant c44 and lattice parameters were found in the low-temperature antiferromagnetic phase. The present DFT and DFT+U calculations provide a reasonable description for the properties of high temperature and low temperature hexagonal NiS respectively, which indicates that electronic correlation is responsible for this high-low temperature phase transition.

Crystal feature and electronic structure of novel mixed alanate LiCa(AlH4)3: a density functional theory investigation

The crystal structure of LiCa(AlH4)3 was investigated via first principle calculations, especially the positions of hydrogen atoms undetected in XRD experiments were predicted, then the thermodynamic favourability of the experimentally reported structure with respect to several candidates from the inorganic crystal structure database (ICSD) was confirmed. It is found that hexagonal packing of AlH4 layers along the c axis is present in LiCa(AlH4)3, and the detailed geometrical feature is further revealed. The electronic structures show that in LiCa(AlH4)3 the Li–AlH4 interaction is more covalent than in LiAlH4, while the Ca–AlH4 covalence is less than in Ca(AlH4)2. The overall stronger covalence in LiCa(AlH4)3 leads to weakened Al–H bonds. The Li–H interaction in LiCa(AlH4)3 dramatically turns out to be strong bonding, opposite to the Li–H anti-bonding in LiAlH4. The Ca–H bonds are more anti-bonding in LiCa(AlH4)3.

Microstructure of 18R-type long period ordered structure phase in Mg97Y2Zn1 alloy

The microstructure of the 18R-type long period stacking ordered (LPSO) phase in Mg97Y2Zn1 alloy was investigated by the first principles calculation. The arrangement rule of Zn and Y atoms in the LPSO structure is determined theoretically. The calculation results reveal that the additive atoms are firstly located in the fault layers at the two ends of the 18R-type LPSO structure, and then extend to fault layers in the interior, which is in good agreement with the experimental observations. This feature also implies the microstructural relationship between 18R and other LPSO structures. The cohesive energy and the formation heat indicate the dependence of the stability of 18R LPSO structure on contents of Y and Zn atoms. The calculated electronic structures reveal the underlying mechanism of microstructure and the stability of 18R LPSO structure.

Study of the structural, elastic and electronic properties of ordered Ca(Mg1−xLix)2 alloys from first-principles calculations

Theoretical investigations were performed to study the structural, elastic and electronic properties of ordered ternary Ca(Mg1−xLix)2 (x=0, 0.25, 0.75, 1) Laves phases with C14 structure from density functional theory. The optimized lattice constants of the Ca(Mg1−xLix)2 phase agreed well with the experimental data, and stabilities were lowered with increase in Li content. Elastic properties, including elastic constants, elastic moduli and elastic anisotropies, were studied in detail. The obtained results indicated that the ductilities of ternary Ca(Mg1−xLix)2 (x=0.25, 0.75) phases were better, while the anisotropies were more significant in comparison with binary Ca(Mg1−xLix)2 (x=0, 1) phase. The electronic structures were further investigated to uncover the underlying mechanism for structural stability and elastic properties.

ROTATIONAL ALIGNMENT OF PRODUCT MOLECULES FROM THE REACTION Ca + HCl→CaCl + H

The product rotational polarization in the Ca + HCl→CaCl + H reaction at collision energy of 20 kcal/mol has been studied via the quasiclassical trajectory method on a new ab initio potential surface. The P(θr) distribution of angle between k and j′, and the dihedral angular distribution P(Φr) characterizing k - k′ - j′ correlation are discussed, the angle distribution P(θr, Φr) of product rotational vectors in the form of polar plot in θr and Φr are shown. Furthermore, four PDDCSs (2π/σ)(dσ00/dωt), (2π/σ)(dσ20/dωt), (2π/σ)(dσ22+/dωt) and (2π/σ)(dσ21-/dωt) are also presented. The present calculations reveal that the product rotational alignment is very strong. Finally, the state distributions of the product CaCl are investigated. The results showed that the CaCl product was formed with high vibrational and rotational excitation.