First-principles Pseudopotential Plane Wave Research Articles

Study on the mechanism of co-catalyzing Mg–Al–Y hydrogen storage alloys with Co@C and multiple fluorides

In this study, the different kinds of fluorides (NiF2, CrF2, and ZrF4) are combined with the carbonaceous catalyst Co@C to explore their co-catalytic effect on the Mg–Al–Y-based hydrogen storage material. The obtained composites are named as Mg91Al5Y4-5 wt.%TmFx (TmFx = NiF2, CrF2 and ZrF4)-5 wt.%Co@C. The isothermal and non-isothermal hydrogen storage properties of the composites were measured by Sievert apparatus and differential scanning calorimeter, respectively. X-ray diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscope (TEM) were used for characterizing the phase component and observing microtopography. To make sense of the action mechanism of fluorides, the MgH2 supercell was established to study the occupation of F and Ni atoms and the first-principle pseudopotential plane wave method was utilized to calculate the heat of formation (ΔHsys) of these systems and the dissociation energy (ΔE) of single H atom. According to the results, the Mg91Al5Y4-5 wt.%NiF2-5 wt.%Co@C specimen has the minimal enthalpy change of hydrogen evolution (75.7 kJ/mol H2) and activation energy of dehydrogenation (81.4 ± 1.09 kJ/mol), showing the optimal hydrogen storage performances. The calculation results are consistent with the experiments that the introduction of Ni leads to the decline in ΔHsys and ΔE, suggesting the destabilization of MgH2 and the enhancement of dehydrogenation. The F atom occupying H or the gap position will lead to the increase of ΔHsys and induce the system to be more stable.

Effect of Doping on the Photoelectric Properties of Borophene

Borophene is a new type of two-dimensional material with a series of unique and diversified properties. However, most of the research is still in its infancy and has not been studied in depth. Especially in the field of semiconductor optoelectronics, there is no related research on the modulation of photoelectric properties of borophene. In this work, we focus on the effect of doping on the photoelectric properties of borophene by using the first-principles pseudopotential plane wave method. We calculate the geometric structure, electronic structure, Mulliken population analysis, and optical properties of impurity (X = Al, Ga) doped α-sheet borophene. The results show that α-sheet borophene is an indirect band gap semiconductor with 1.396 eV. The band gap becomes wider after Al and Ga doping, and the band gap values are 1.437 eV and 1.422 eV, respectively. Due to the orbital hybridization between a small number of Al-3p electrons and Ga-4p state electrons and a large number of B 2p state electrons near the Fermi level, the band gap of borophene changes and the peak value of the electron density of states reduces after doping. Mulliken population analysis shows that the B0-B bond is mainly covalent bond, but there is also a small amount of ionic bond. However, when the impurity X is doped, the charge transfer between X and B atoms increases significantly, and the population of the corresponding X-B bonds decreases, indicating that the covalent bond strength of the chemical bonds in the doped system is weakened, and the chemical bonds have significant directionality. The calculation of optical properties shows that the static dielectric constant of the borophene material increases, and the appearance of a new dielectric peak indicates that the doping of Al and Ga can enhance the ability of borophene to store electromagnetic energy. After doping, the peak reflectivity decreases and the static refractive index n0 increases, which also fills the gap in the absorption of red light and infrared light by borophene materials. The research results provide a basis for the development of borophene materials in the field of infrared detection devices. The above results indicate that doping can modulate the photoelectric properties of α-sheet borophene.

A new approach to regulate the photoelectric properties of two-dimensional SiC materials: first-principles calculation on B-N co-doping

This paper describes a new approach to regulate the photoelectric properties of two-dimensional SiC materials. The first-principles pseudo-potential plane wave method is used to calculate the geometric structure, electronic structure and optical properties of two-dimensional (2D) SiC co-doped by the adjacent elements of C-Si (such as B and N). The results show that: after B-N co-doping, the supercell lattices of 2D SiC are observed obviously deformation near the doped atoms. Meanwhile, the band structures of 2D SiC co-doped by B-N become rich. As the impurity level enters the forbidden band, the band gap decreases, and the distribution of density of states near the Fermi level changes accordingly. The calculation of optical properties shows that the ability to absorb electromagnetic waves of 2D SiC has been enhanced obviously in the low energy range after B-N co-doping. The reason is originated from the transition of the 2p state of B and N. At the same time, the static dielectric constant increases and the peak of reflectivity decreases. The above results indicate that the optoelectronic properties of 2D SiC can be modulated by co-doping B-N.

First-principles calculation on the structure stability, elastic properties and electronic structure of P-doped Mg2Si

In this study, the effect on the structure stability, elastic properties and electronic structure of P-doped Mg2Si were studied by the first-principles pseudopotential plane wave method based on density functional theory. The lattice constants, formation enthalpy, cohesive energy, elastic constants, and elastic moduli of Mg2Si, Mg7Si4P, Mg8Si3P and Mg8Si4P were calculated, and the electronic structure analysis was also performed. The occupation tendency, structural stability, bonding characteristics, orbital hybridization and the change of conductivity of doping P atoms in the matrix were further investigated. Among them, the research results of formation enthalpy, cohesive energy and elastic constant show that Mg2Si, Mg8Si3P and Mg8Si4P can all exist stably in the system, and the crystal structure of Mg7Si4P can not exist stably. P atoms doping into the Mg2Si lattice tend to occupy Si atoms position preferentially. The results of elastic modulus study show that Mg2Si and Mg8Si4P are brittle phase and Mg8Si3P is ductile phase. The plasticity and toughness of Mg2Si alloy system are improved by doping P atoms. The electronic structure analysis shows that the method of doping P atoms changes the orbital hybridization and bonding characteristics of the system. The Mg-P and Si-P covalent bond formed by Mg8Si3P and Mg8Si4P increase the structure stability. The energy band structure analysis also show reduction of the band gap from 0.224 to 0.184 eV for Mg2Si with P dopants at the substitutional Si-sites and the band gap closure in the system with interstitial P-impurities. It enhances the metallic property of the material, and Mg8Si4P phase also transform from its semiconducting to metallic state. Consequently, this method both increases the carrier concentration and reduces the energy of free electron transition. The conductivity of the Mg2Si alloy system will be improve.

First-principles study of solid solution strengthening in Mg–X (X=Al, Er) alloys

To study the solid solution strengthening effect on magnesium (Mg)–X (X = Al, Er) alloys, supercell models of Mg, \(\hbox {Mg}_{35}\hbox {Er}\) and \(\hbox {Mg}_{35}\hbox {Al}\) are established to perform the first-principles pseudopotential plane wave calculations based on density functional theory. The calculated cohesive energy of \(\hbox {Mg}_{35}\hbox {Er}\) is lower than that of \(\hbox {Mg}_{35}\hbox {Al}\). This indicates that \(\hbox {Mg}_{35}\hbox {Er}\) has better structural stability than \(\hbox {Mg}_{35}\hbox {Al}\). The bulk modulus, Young’s modulus and shear modulus of the solid solutions increases simultaneously when Al and Er are doped into the Mg matrix. Moreover, the solid solution strengthening of Er is much higher than the Al containing alloy. The order of toughness of the three solutions from the highest to the lowest is Mg, \(\hbox {Mg}_{35}\hbox {Er}\) and \(\hbox {Mg}_{35}\hbox {Al}\), while the order of increasing elastic anisotropy is in the reverse order. The number of bonding electrons of \(\hbox {Mg}_{35}\hbox {Er}\) in the low-energy region of the Fermi level is much higher than that of \(\hbox {Mg}_{35}\hbox {Al}\), and the density of states of \(\hbox {Mg}_{35}\hbox {Er}\) at the Fermi level is higher than that of \(\hbox {Mg}_{35}\hbox {Al}\). Compared with Al atoms, Er atoms share more electric charges with Mg atoms, which leads to an increasingly uniform charge distribution around Er atoms.

Prediction on elastic and thermal properties of defective L12–Al3Li intermetallics

The effects of Al vacancy, Li vacancy, Al anti-site and Li anti-site point defects on the elastic and thermal properties of L12–Al3Li intermetallic compound were investigated by the first-principle pseudopotential plane-wave calculations. The results show that Li anti-site and Al anti-site can easily form in L12–Al3Li. Li vacancy slightly increases hardness and the Debye temperature, whereas Li anti-site, Al anti-site, and Al vacancy impose the opposite influence. All these point defects have no obvious impact on the brittleness of L12–Al3Li. Under the Debye temperature, the specific heat of L12–Al3Li weakly decreases with the content of Li vacancy, but increases with those of Li anti-site, Li anti-site, Al anti-site, and Al vacancy.

Insight into the structural, electronic and elastic properties of AInQ2 (A: K, Rb and Q: S, Se, Te) layered structures from first-principles calculations

First-principles calculations were performed to explore the structural, elastic and electronic properties of the ternary indium chalcogenides AInQ2 (A: K, Rb and Q: S, Se, Te) in both monoclinic and triclinic phases. This study is carried out by using the first-principles pseudopotential plane-wave (PP–PW) method as implemented in CASTEP code. Both the generalized gradient approximation of Perdew–Burke–Ernzerhof scheme (GGA–PBE) and the Heyd–Scuseria–Ernzerhof (HSE06) hybrid functional were used to treat the exchange-correlation interactions. In order to confirm the previous reports and to understand the effect of symmetry in determining the physical properties of these layered materials we have calculated the structural and the electronic properties at the equilibrium lattice constant for both the systems. The single-crystal elastic constants Ci j are calculated using the stress-strain approach. The elastic moduli of the polycrystalline aggregates and their related properties are obtained in the framework of Voigt–Reuss–Hill approximations. Electronic band structure indicates the semiconducting behaviour with a direct band gap at Γ–Γ. The results obtained from the (HSE06) hybrid functional are in excellent agreement with the available experimental data and computed results for the monoclinic and triclinic structures.

Mechanism of surface effect and selective catalytic performance of MnO2 nanorod: DFT+U study

The mechanism of surface effect and selective catalytic performance of MnO2 nanorods remain mysterious at present. Using first-principles pseudo-potential plane wave method, the surface energy, cohesive energy, geometrical and electronic structure for MnO2 in the evolution of crystal→bulk surface→nanorod morphologies have been systematically calculated and analyzed. The results show that the surface energy is increased along with the decrease of geometry configuration in crystal→bulk surface→nanorod as a whole. However in three nanorod morphologies, the surface energy is increased along with the additional geometry configuration, wherein the largest nano(III) has the largest surface energy and lowest cohesive energy. These characters are originated from their changes in geometry structure and lost in Mulliken charges of atoms along surface planes. Electronic structure shows that the selective catalytic activity of MnO2 nanorods is originated from their unique states of valence electrons, which occupying the highest occupied molecular orbital (HOMO) only appears on (110) Miller surface layers and the lowest unoccupied molecular orbital (LUMO) only on (100) Miller surface layers, respectively. Such attracting phenomenon has significantly difference with that of MnO2 bulk surface. Thereinto, a transitional model of [(100×110)] microfacet model is found to exhibit much more approaching surface performance to MnO2 nanometer structure somewhere. Thus, our findings open an avenue for detailed and comprehensive studies on the growth and catalysis of MnO2 nanomaterials.

Band structure, Fermi surface, elastic, thermodynamic, and optical properties of AlZr3, AlCu3, and AlCu2Zr: First-principles study

The electronic properties (Fermi surface, band structure, and density of states (DOS)) of Al-based alloys AlM3 (M = Zr and Cu) and AlCu2Zr are investigated using the first-principles pseudopotential plane wave method within the generalized gradient approximation (GGA). The structural parameters and elastic constants are evaluated and compared with other available data. Also, the pressure dependences of mechanical properties of the compounds are studied. The temperature dependence of adiabatic bulk modulus, Debye temperature, specific heat, thermal expansion coefficient, entropy, and internal energy are all obtained for the first time through quasi-harmonic Debye model with phononic effects for T = 0 K–100 K. The parameters of optical properties (dielectric functions, refractive index, extinction coefficient, absorption spectrum, conductivity, energy-loss spectrum, and reflectivity) of the compounds are calculated and discussed for the first time. The reflectivities of the materials are quite high in the IR–visible–UV region up to ∼ 15 eV, showing that they promise to be good coating materials to avoid solar heating. Some of the properties are also compared with those of the Al-based Ni3Al compound.

Electronic structures and magnetic properties of rare-earth-atom-doped BNNTs

Stable geometries, electronic structures, and magnetic properties of (8,0) and (4,4) single-walled BN nanotubes (BNNTs) doped with rare-earth (RE) atoms are investigated using the first-principles pseudopotential plane wave method with density functional theory (DFT). The results show that these RE atoms can be effectively doped in BNNTs with favorable energies. Because of the curvature effect, the values of binding energy for RE-atom–doped (4,4) BNNTs are larger than those of the same atoms on (8,0) BNNTs. Electron transfer between RE-5d, 6s, and B-2p, N-2p orbitals was also observed. Furthermore, electronic structures and magnetic properties of BNNTs can be modified by such doping. The results show that the adsorption of Ce, Pm, Sm, and Eu atoms can induce magnetization, while no magnetism is observed when BNNTs are doped with La. These results are useful for spintronics applications and for developing magnetic nanostructures.

Micromechanism of Cu and Fe alloying process on the martensitic phase transformation of NiTi-based alloys: First-principles calculation

Using first-principles pseudo-potential plane wave method, the formation enthalpy ΔH, binding energy ΔE, elastic constants, and electronic structure were calculated and analyzed carefully for NiTiX (X = Cu, Fe) shape memory alloy. The results show that the Cu or Fe element prefers to occupy the Ni site in the NiTi matrix phase respectively. Compared with the NiTi matrix phase, the ΔH, ΔE, c44 and c′ of NiTi (Cu) are similar to each other. However, the structural stability of the NiTi phase is improved obviously by the Fe alloying process. Simultaneously, the shear modulus c44 and c′ of NiTi (Fe) are larger than those of the NiTi matrix phase. Furthermore, Milliken population results indicate that QCu–Ti is smaller than QNi–Ti after the Cu alloying process, but QFe–Ti is larger than QNi–Ti. The electron density difference shows that some covalent bonding exists between Fe and Ti elements. Based on the upward analysis, the difference in the phase stability and elastic constants of NiTiX (X = Cu, Fe) is the substantial mechanism for the different Ms of NiTiX (X = Cu, Fe) although Cu or Fe substitutes for the same atom Ni elements in the NiTi matrix phase.

Nitrogen and vanadium Co-doped TiO 2 mesosponge layers for enhancement in visible photocatalytic activity

Abstract Novel N and V co-doped TiO 2 mesosponge (N-V-TMSW) layers were successfully prepared by one-step hydrothermal treatment of TiO 2 nanotube arrays, and the phase composition, morphology and optical property were characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS) and UV–vis diffusion reflectance spectroscopy (DRS). The crystal structure and density of states were studied by means of the first-principle pseudo-potential plane wave. The results indicated that titanium ions and oxygen atoms in TiO 2 were successfully substituted by vanadium ions and nitrogen atoms, respectively. The sample N-V0.1-TMSW showed a remarkable absorption in the visible light range of 400–600 nm and high visible photocatalytic activity

Nb solution influencing on phase transformation temperature of Ni47Ti44Nb9 alloy

The effect of annealing temperature on the phase transformation temperature and microstructure of as-rolled Ni47Ti44Nb9 alloy has been investigated, and the microstructural mechanism has been systematically calculated and analyzed. The results show that annealing treatment elevates phase transformation temperature. With the increase of annealing temperature, Ms temperature increases abruptly below 900°C, yet above 900°C, Ms shows no substantial change. Nb gradually dissolves into NiTi matrix, until complete solution at 950°C. Based on first-principles pseudo-potential plane wave method, a further investigation into micro-mechanism of effect of Nb solution into NiTi(Nb) on the phase transformation temperature is studied by analyzing elasticity modulus and population. Comparing with NiTi matrix, shear modulus c′ and c44 of NiTi(Nb) are rather lower. Following Nb solution into NiTi matrix, Ni–Ti bond is depressed, meanwhile weak Ni–Nb bond and Ti–Nb bond greatly influenced by temperature are formed, as a result of Ms increasing for NiTi(Nb), maybe this is the just micro-mechanism of Nb solution into NiTi alloy.

Effects of B–N co-doping into the ultra-small diameter zigzag single-walled carbon nanotubes: A density functional theory study

The boron(B)–nitrogen(N) co-doping ultra-small diameter (5,0) and (4,0) single-walled carbon nanotubes (SWCNTs) have been investigated by using the first-principles pseudopotential plane wave method. The results show that B–N pairs are easy to dope into ultra-small diameter zigzag nanotubes, which have numbers of differences with other large diameter nanotubes, and the B–N co-doping configurations are the energetically stable structures. Furthermore, by introducing B–N pairs, the (5,0) and (4,0) SWCNTs both transform from metal to semiconductor. These ultra-small diameter nanotubes have potential to be applied in semiconductor integrated circuit of extremely small size.

The Effect of Cr Addition on the Microstructure of TiNi: First-Principles Calculations

Based on the first-principles pseudo-potential plane wave method, the effect of Cr addition on the microstructure of NiTi alloys are characterized and assessed with the formation enthalpy (ΔH), binding energy (ΔE) and density of states (DOS) distribution of NiTi (Cr) crystals. The results show that the binding energy of NiTi (CrNi) is slightly smaller than that of NiTi (CrTi), but the formation enthalpy of NiTi (CrTi) is much smaller than that of NiTi (CrNi).So the Ms of NiTi alloy could be decreased by the addition of ratio for Ni/Ti. And the electronic microstucture of NiTi (CrNi) alloy are also illustrated clearly.

FIRST PRINCIPLE STUDY OF MC (M=AL, GA AND IN) AT EQUILIBRIUM AND UNDER NEGATIVE STRESS

The electronic and magnetic properties of the hypothetical compounds of MC (M=Al, Ga and In) are investigated by using first-principle calculations and pseudopotential plane wave self-consistent field method based on density functional theory. In order to find the most stable phase of MC (M=Al, Ga and In), we study them in zinc-blende (ZB), rocksalt (RS), wurtzite and NiAs crystal structures. We find that the most stable crystal structure is the RS structure by comparing the total energies of these compounds in different structures. Our calculations show that these compounds are metals in all of these structures. However, many of the nanospintronic devices are the heterojunctions of half-metals and semiconductors where the crystal structures of the layers that are grown on the semiconductors are the same as that of the semiconductors. Because most of the semiconductors have ZB structure, we investigate the electronic and magnetic properties of these compounds in this structure. It is shown that although MC (M=Al, Ga and In) compounds are metals in ZB structure, they become half-metal when relatively high negative stresses are applied over them. Therefore these compounds could be used in spintronic devices.

FIRST-PRINCIPLES STUDY ON β-SiC/BNNT CORE/SHELL NANOCABLE

In this paper, we studied the structural and electronic properties of core/shell nanocables composed of cubic silicon carbide nanowires (β- SiCNW ) and boron nitride nanotubes (BNNT) using first-principles pseudopotential plane wave method within density functional theory. Our results show that the β- SiC /BNNT heterojunction structures are metallic, which primarily originates from the contributions of the BNNTs and the surfaces of SiCNW s. The BNNTs exhibit metallic characters after the SiC nanowires are inserted. The transition of the BNNTs is attributed to the charge transfer between BNNTs and SiCNW s.

Microcosmic mechanism of carbon influencing on NiTiNb9 alloy

The microcosmic mechanism, by which the C impurity decreases the Ms and fracture toughness, remains mysterious at present. Using first-principles pseudo-potential plane wave method, the formation enthalpy ΔH, binding energy ΔE, electronic structure and diffusivity of the C element in NiTiNb9 shape memory alloy have been systematically calculated and analyzed in the thermodynamic and kinetic processes. The results show that the addition of C trends to compose carbonization “cluster” in NiTi matrix phase, which not only can decrease the Ms of alloy by enhancing the ratio Ni/Ti, but also it can slack down the ductility by its special “jujube nut ” system. Otherwise, because of the unfilled s orbits, the Nb element can enhance the formation ability and diffusivity of carbonization “cluster” structure, and promote the impact of the C element to the shape memory effect and mechanics performance in NiTiNb alloy. Thus, our findings open an avenue for detailed and comprehensive studies of alloying shape memory alloy.

First-principles study on stability and electronic properties of W(001), W(110) and W(111) surfaces

Based on Density Functional Theory (DFT), using first-principles pseudopotential plane wave method, surface relaxation, surface energy and Density of states (DOS) of metal W on low index surfaces (001), (110) and (111) were calculated. The results showed that: the relaxation and surface energy were |Δ14(110)|<|Δ14(111)|<|Δ14(001)| and Esurf(110)<Esurf (111)<Esurf(001), respectively. The surface relaxation and surface energy of W(110) were both minimum, which proved that W(110) was the most stable surface. This result was in good agreement with experimental XRD pattern. According to Density of states, the essence of the distinct stability of W(001), W(110) and W(111) was discussed further.

First-principles LDA + U calculations investigating the lattice contraction of face-centered cubic Pu hydrides

Plutonium metal can be loaded with hydrogen, which forms complicated solid solutions and compounds, and leads to significant changes in electronic structure. A first-principles pseudopotential plane wave method with added Hubbard parameter U was employed to investigate the electronic and structural properties of face-centered cubic Pu hydrides (PuHx, x=2, 2.25, and 3). The decrease in calculated lattice parameters with increasing x is in reasonable agreement with experimental findings. Comparative analysis of the electronic-structure results for a series of PuHx compositions reveals that lattice contraction occurs due to enhanced chemical bonding and the size effects involving interstitial atoms. We find that the size effects are the driving force for the abnormal lattice contraction.