Papers by Keyword: First-Principles Calculations

Abstract: The effects of Cu addition on the crystal structure, phase stability and magnetic properties of Ni₈Mn_4-xGa₄Cu_x (x=0, 0.5, 1, 1.5 and 2) ferromagnetic shape memory alloys are systematically investigated by first-principles calculations. The formation energy results indicate that the added Cu preferentially occupies the Mn sites in Ni₂MnGa alloy. The formation energy results indicate that ferromagnetic austenite is more stable than the paramagnetic one. The ferromagnetic state becomes instable and paramagnetic state becomes more stable when Mn is gradual substituted by Cu. Furthermore, the electronic density of states gives rise to the difference in the magnetic properties.

Abstract: Ni-Mn-In is a novel type of magnetic shape memory alloy, its shape memory effect has been realized through magnetic field induced reverse martensitic transformation. A variety of point defects would be generated during composition adjustment process, such as antisite defect, vacancy and exchange. The first–principles calculations within the framework of the density functional theory using the Vienna ab initio software package (VASP) have been used in this paper to investigate the defect formation energy and electronic configuration of the off-stoichiometric Ni-X-In (X= Mn, Fe and Co) alloys. The In antisite on the X sublattice (InX) and the Ni antisite on the X sublattice (NiX) have the lowest formation energies in the investigated series. The formation energy of the Ni vacancy is the lowest, while that of the in vacancy is the highest. It is confirmed that the in constituent plays a dominant role for stabilizing the austenitic phase.

Abstract: In this chapter, the modeling techniques of the thermodynamic and diffusion properties based on density functional theory in ionic materials, specifically oxide ceramic materials or ionic conductor materials are reviewed. Section 1 is the introduction of this book chapter. Section 2 is devoted to introduce the modeling methods of first-principles finite temperature thermodynamics, including quasi-harmonic phonon calculations and the Debye model. In the phonon model, the frozen phonon method, the linear response method, and the newly developed mixed-space method to model ionic polar materials are discussed. Section 3 introduces the general atomic diffusion theory, first-principles transition state calculations (double-well approach), and ab initio molecular dynamics simulations of the diffusion coefficients in ionic materials. Section 4 discusses some of the recent works of first-principles prediction of the thermodynamic and diffusion properties of ionic materials from our group and in the literature, with a focus on oxides for energy applications. Section 5 summarizes this book chapter.

Abstract: The un-passivated and passivated 6H-SiC(0001) surface and Si(-220)/6H–SiC (0001) interface are investigated by first-principles calculation based on density functional theory. It is demonstrated that the surface energy of 6H-SiC(0001) surface with seven atom-layers converges well. When the surface is passivated with sulfur(S), the density of states decreases obviously, implying that the passivated 6H-SiC(0001) surface are more stable. Four specific geometry models of Si(-220)/6H–SiC(0001) interface structures with different terminations are chosen. The calculated adhesion energies suggest that, for un-passivated interface, the atomic binding force and interface stability of C-termination interface are stronger than Si-termination interface, while for passivated interface, the tendency is opposite. The calculations about the density of states of un-passivated interfacial suggest that the Si-Si covalent bonds are formed at Si-terminated interface, and that C-Si covalent bonds are formed at C-terminated interface. After the interface is passivated with S atoms, the interaction between Si and S atoms is observed.

Abstract: We investigated the effect of SiC stacking on the 4H-SiC/SiO₂ interface via first principles calculations. Interlayer states are observed along the SiC conduction band edge, and are affected by the local structure at the interface. The location of these states changes depending on which of two lattice sites, h or k is at the interface. This difference is important for SiC based metal-oxide-semiconductor field-effect transistors which rely on the electronic structure of the conduction band.

Abstract: Grain refining is one of the most important issues in the applications of Mg alloys, which directly determines mechanical properties and deformability. Therefore the understanding of grain refining mechanism during solidification will be benefit to develop new grain refiners. Herein refining role was elucidated by the first principles calculations based on adsorption behavior of a Mg atom on the closest-packed planes of grain refiners (Zr (001), Al₂Y(311) and Al₄C₃(102)). Taking into account different sites, the site with the maximum adsorption energy value generally corresponded to the most possible location. The adsorption energy results show that the possible refining turn follows Al₄C₃(102)>Zr (001)>Al₂Y(311). Meanwhile, the structural optimization confirmed that the Mg atom connected with two C atoms on the top of zig-zag plane of Al₄C₃(102), three Zr atoms at the hcp position on Zr (001), and two Y atoms and one Al atom at the bottom of zig-zag plane of Al₂Y(311). The density of states revealed that the variation of d-orbital electrons of Mg atom became apparent during adsorption process. The values of Mulliken charges were 0.898 e in Al₄C₃(102), 0.410 e in Zr (001) and 0.245 e in Al₂Y(311), respectively. This tendency agrees well with the previous experimental results. It indicates the adsorption energy on the closest-packed planes can be regarded as a prerequisite to select new grain refiners for Mg alloys in future.

Abstract: The electronic structure and optical properties of wurtzite ZnO nanofilms with different thickness are investigated systematically by using the first-principles approach. The results indicate that the valence band properties of the ZnO nanofilms are mainly determined by the Zn: 3d state and O: 2p state. And its conduction band properties are determined by Zn: 4s state and Zn: 4p state. The band gap decreases with the thickness of nanofilms increasing in [0001] direction. It is also found that the interband transition absorption edge of ZnO nanofilms decreasing from 5.5 eV to 2.7 eV with the thickness of nanofilms increasing from single layer to five layers. The interband transition of reflection spectrum occurs mainly in the range of 10 eV to 18 eV, which is in line with the ionic bonding characteristic of wurtzite ZnO.

Abstract: The total energy, the electronic properties, phase transitions, and elastic properties of Cu₂ZnSnS₄ (CZTS) in the three structures are investigated by first-principles calculations based on density functional theory. Results show that the total energies of stannite (ST) and primitive-mixed CuAu (PMCA) structures are higher than that of kesterite-type (KS), and the KS is the ground state structure. Relationships between enthalpy and pressure of the KS, ST and PMCA structure of CZTS are also investigated at 0 K, since the pressure can have profound impacts on the electronic structure, possible phase transitions and structure stability. And results also show that KS structure is always the most stable; ST is the second; and the PMCA structure is the most unstable; phase transitions of three structures could not occur in high pressure. The high ratios of shear modulus to bulk modulus (G/B) indicate that CZTS compounds in three types have ductile behaviors. The Poisson ratios for the three structures are from 0.27 to 0.31, which again proves that all structures of CZTS have better plasticity. The results can increase more hints about further research directions, and these effects can play an important role in future experimental preparation technology and theoretical work of CZTS materials.

Abstract: The energetic study of B effects on the oxidation of γ-TiAl alloys are performed by using the first-principles method based on Density Functional Theory (DFT) in this paper. The surface and interface segregatation of B as well as of the surface adsorption of O are discussed. B is found to preferentially segregat to TiAl subsurface with respect to γ-TiAl bulk. The B segregation at surface decreases oxygen coverage in the initial oxidation process of γ –TiAl alloys, thereby it is beneficial to the decrease of the growth of γ–TiAl alloys oxide film. In the initial oxidation process, oxygen prefers to stay in the vicinity of surface Ti atoms, and B addition is beneficial for the decrease of the growth of A1₂O₃ and TiO₂. After the formation of Al₂O₃ oxide film, B is energetically favoured stayed at interstitial site of α-Al₂O₃ (0001)/γ-TiAl (111) interface, and enhances the adhesion of this interface.

Abstract: First-principles pseudopotential calculations have been performed to investigate the structural stability and electronic properties of magnesium considering three possible structures under high pressure. The results show that magnesium crystallizes in the hcp structure is to be the most stable structure at the ground state, because of the lowest total energy. Magnesium undergoes a pressure-induced phase transition from the hcp structure to bcc structure at 65 GPa. And no further transition is found up to 220 GPa. The electronic structure properties of three structures of magnesium are also calculated and discussed. The structural stability mechanism is also explained through the electronic structures of three phases.