Papers by Author: Dong Chen

Abstract: Using quantum mechanics plane-wave approach based on the density functional theory, the lattice constants of Ni_xAl at different Ni concentrations (x=1, 3) are predicted. Optical properties such as dielectric function, energy loss function and reflectivity are also investigated. Results show that with the increase of Ni constituent, the location of the peak in loss function moves to the lower energy region, but the peak height increases. At 0eV, the reflectivity increases rapidly with the Ni concentration. The reflectivity of NiAl and Ni₃Al are pronounced in the UV region (not in the visible light region). The dielectric properties, namely the real and imaginary parts of the dielectric function, changed significantly with Ni constituent.

Abstract: The crystal structures, lattice parameters, volumes, elastic constants, bulk moduli and shear moduli of the binary NiAl and Ni₃Al alloys have been predicted by taking the first-principles plane-wave method in combination with ultra-soft pseudo-potentials. Also the pressure dependence of C_ij, B and G are described and quantitatively discussed. The calculated results agree with the experimental data. The elastic constants obtained from our calculations meet their mechanical stability criteria. The DOS results show that the strong Ni-Al interaction plays an important role in the chemical bond of the Ni-Al alloys. Our predictions should be testified by the experimental investigations.

Abstract: The state-of-the-art plane-wave methods combined with ultra-soft pseudo-potentials were employed to study the crystal and electronic structures (density of state, band structure) of aluminum in its hcp and bcc structures. In our computation we used the PBE functional, which predicts lattice constants very close to the experimental data. The calculations reveal that the whole valence band of Al is dominated by the 3s and 3p states while the conduction band is mainly contributed by the 3p band. The band structure shows that bcc-Al has a 0eV gap, which reflects its metallic character. The dispersion curves near the valence band maximum and conduction band minimum are quite flat. Generally speaking, our work is an attempt to study the high pressure electronic structures of Al, which needs to be verified by experiments.

Abstract: Based on the first-principles scheme, we have built several hcp-Al crystals and have calculated the lattice constants, cell volumes and elastic constants at high pressures using the plane-wave pseudo-potential method within ultrasoft pseudo-potentials. The basic thermal properties of Al are investigated by using the quasi-harmonic Debye model. The results show that the bulk moduli are still very large at high temperatures; hence hcp-Al can retain its stability at high pressure and high temperature. On the other hand, the temperature effects on the heat capacity and thermal expansion are significant. Several interesting phenomena have been observed in these quantities at T> 300K. More importantly, the negative shear moduli of hcp-Al indicate that our calculation results need to be verified by experiments in the near future.

Abstract: The structural and elastic properties of the cubic spinel Ge₃N₄ semiconductor have been investigated using the ab initio scheme within the generalized gradient approximation in the framework of density functional theory. Quantities such as lattice constants and elastic constants of interest are calculated. A good agreement is found between our results and the other data. Through the lattice dynamics, in which the finite displacement method is used, we have obtained successfully the thermal properties such as the phonon curve, free energy, heat capacity and Debye temperature in the whole temperature range from 0 to 1000K. It is the authors ambition that these results will inspire further experimental study on the Ge-based semiconductors. * Corresponding author: CHEN Dong

Abstract: Ab initio electronic structures for β-Ge₃N₄ are investigated using ultrasoft pseudo-potential method within the generalized gradient approximation functional. The lattice parameters are predicted theoretically, and are compared with available experimental data and the other theoretical results. The elastic constants calculations reveal that the phenacite structure is a stable phase in the pressure range of 020GPa. The high bulk modulus (B=180.2GPa) indicates that β-Ge₃N₄ is a relatively hard material. β-Ge₃N₄ has a direct band gap of 2.94eV, indicating its promising applications as a semiconductor in electronics and optical fibers. On the other hand, the density of states analysis is also included in this paper.* Corresponding author: CHEN Dong

Abstract: The lattice parameters (lattice constants a, c), elastic properties (elastic constants, bulk modulus, shear modulus) and optical parameter (dielectric function) are investigated from a theoretical perspective using computer simulation in the frame of density functional theory. The calculated lattice constants and elastic moduli are in agreement with the theoretical results. We found that anatase can retain its stability in the pressure interval 020Gpa. The anisotropy of this compound is found to increase with applied pressure. Moreover, the dielectric functions are also discussed. The plasma frequency and static dielectric constant of TiO₂ are 16eV and 6.1, respectively. * Corresponding author: Dong CHEN

Abstract: In this paper, we investigate the structural, electronic and elastic properties of rutile using the ultra-soft pseudo-potential scheme in the framework of density functional theory, together with the generalized gradient approximation. The calculated lattice constants and elastic constants are generally consistent with the other results. Our aim is twofold. First, density functional theory is a fine theory that can obtain reliable results. Second, rutile can be used in the modern industry, thus it should be carefully investigated. The elastic constants dependences on pressure are calculated. It is found that rutile is stable in the pressure range of 020Gpa. The anisotropy of this compound increases with applied pressure. Besides, the analysis of band structure is also given. The calculated band structure shows that rutile belongs to direct-forbidden-gap semiconductors.

Abstract: The reflectivity, loss function, refractive index, extinction coefficient and dielectric function of the LaNi₅ and LaNi_4.5Sn_0.5 intermetallic compounds are investigated through the plane-wave pseudo-potential method based on the density functional theory. The effects of Sn impurity are discussed and some interesting features are found in the low frequency region. Some important optical properties such as static dielectric constant and static refractive index are obtained. The equation [n (0)]²=ε₁(0) is satisfied according to our calculation, which indicates that our results are correct and reasonable. Nevertheless, the calculated results need to be testified in the future due to the lack of experimental data.

Abstract: The aim of this study is to determine the optical parameters of the ternary alloys LaNi_4.5M_0.5 (M=Al, Mn) by using first-principles calculations. Through the computer simulation, we have successfully obtained the absorption coefficient, optical conductivity and dielectric function of these alloys. These alloys are transparent for the electromagnetic waves with frequencies of 12eV~15eV. The calculated dielectric constants are 188.3 and 257.6 for LaNi_4.5Al_0.5 and LaNi_4.5Mn_0.5, respectively. Our result is compared with the previous study and a good agreement is found.