Thermoelectric properties and electronic structure of Te-doped CoSb3 compounds

Co₄Sb_12−xTe_x compounds were prepared by mechanical alloying combined with cold isostatic pressing, and the effects of Te doping on the thermoelectric properties were studied. The electronic structure of Te-doped and undoped CoSb₃ compounds has been calculated using the first-principles plane-wave pseudo-potential based on density functional theory. The experimental and calculated results show that the value of the solution limit x of Te in Co₄Sb_12−xTe_x compounds is between 0.5 and 0.7. The Fermi surface of CoSb₃ is located between the conduction band and the valence band, and its electrical resistivity decreases with increasing temperature. The density of states is mainly composed of Co 3d and Sb 5p electrons for intrinsic CoSb₃.The Fermi surface of Te-doped compounds moves to the conduction band and its electrical resistivity increases with increasing temperature, exhibiting n-type degenerated semiconductor character. Under the conditions of the experiment, the maximum value 2.67 mW/m K² of the power factor for Co₄Sb_11.7Te_0.3 is obtained at 600 K; this is about 14 times higher than that of CoSb₃.     

First-principles study on electronic structure and elastic properties of Fe16N2

We have performed first-principles study on structural stability, elastic properties and electronic structure of Fe₁₆N₂ by applying LSDA+U method. The calculated values of formation energy and reaction enthalpy for decomposition reaction indicate that Fe₁₆N₂ is a thermodynamically stable phase at the ground state. The six independent elastic constants are derived and the bulk modulus, Young's modulus, shear modulus, and Poisson's ratio are determined as 180 GPa, 199 GPa, 76 GPa and 0.32, respectively. The elastic constants meet all the mechanical stability criteria. The ductility of Fe₁₆N₂ is predicted by Pugh's criterion. The strong bonding between Fe and N atoms results in high values of elastic constants C₁₁ and C₃₃, and contributes to the strengthening of the Fe₁₆N₂ structural stability. The total and partial densities of states (DOS) suggest the existence of hybridization between N-p and Fe-d bands. The position of the Fermi level in DOS curve implies that Fe₁₆N₂ is a metastable phase.     

The mechanical,thermodynamic and electronic properties of Al3Nb with DO22 structure: A first-principles study

The lattice constants, elastic properties, electronic structure and thermodynamic properties of Al₃Nb with DO₂₂ structure have been investigated by the first-principles calculation. The calculated lattice constants were consistent with the experimental values, and the structural stability was also studied from the energetic point of view. The single-crystal elastic constants (C_ij) as well as polycrystalline elastic parameters (bulk modulus B, shear modulus G, Young's modulus E, Poisson's ratio υ and anisotropy value A) were calculated, and brittleness of Al₃Nb was discussed in detail. Besides, the electronic structure of tetragonal Al₃Nb was studied, which indicates a mixture of metallic bond and covalent bond in Al₃Nb and reveals the underlying mechanism of the stability and elastic properties of Al₃Nb. Finally, the thermodynamic properties of Al₃Nb were calculated and the physical properties such as heat capacity and Debye temperature were predicted within the quasi-harmonic approximation.     

第一性原理研究3C-SiC/Mg复合材料界面的电子结构

基于密度泛函理论的第一性原理平面波赝势方法对4种3C-SiC(111)/Mg(0001)界面模型进行研究.界面间距和粘附功的计算表明,结构优化之后的界面模型只在z轴方向发生了移动,界面间距发生了不同程度的缩短;中心型模型的稳定性强于顶位型模型,C终端结构的稳定性强于Si终端结构,中心型C终端的界面模型具有最大的粘附功(2.5834 J/m~2)和最小的界面间距(1.7193?),是4种模型中最稳定的结构. Mulliken电荷、电荷密度分布、差分电荷密度和态密度的计算表明,中心性结构的Si终端和C终端模型界面处存在共价键、离子键和金属键.     

First-principles study of the structural, magnetic, and electronic properties of LiMBO3 (M = Mn, Fe, Co)

We present a first-principles calculation for the structural, magnetic, and electronic properties of LiMBO₃ (M = Mn, Fe, Co). Along the [0 0 1] direction, transition metals shows antiferromagnetic coupling in LiMBO₃ of both hexagonal and monoclinic lattices. The calculated magnetic moment of 5μB per formula unit is close to the experimental value. These compounds are semiconductors with band gap of 0.4-2 eV, and with average intercalation voltages of 2-4.8 V.     

晶粒尺寸对CoSb3化合物热电性能的影响

系统地研究了晶粒尺寸对CoSb₃化合物热电性能的影响规律，结果表明晶粒尺寸对CoSb₃化合物的晶格热导率κ_p、电导率σ、能隙宽度E_g和Seebeck系数α有显著影响.当晶粒尺寸由微米尺度减小到纳米尺度时，晶格热导率κ_p显著降低，Seebeck系数α有较大幅度的增加，能隙宽度E_g变宽，电导率σ有一定程度的下降.平均晶粒尺寸为200nm的CoSb₃化合物在温度为700K时，ZT值达到0.43，比平均晶粒尺寸为5000nm的试样增加了4倍.     

First-principles study on electronic structure of Sr2Bi2O5 crystal

The electronic structure of Sr₂Bi₂O₅ is calculated by the GGA approach. Both of the valence band maximum and the conduction band minimum are located at Γ-point. This means that Sr₂Bi₂O₅ is a direct band-gap material. The wide energy-band dispersions near the valence band maximum and the conduction band minimum predict that holes and electrons generated by band gap excitation have a high mobility. The conduction band is composed of Bi 6p, Sr 4d and O 2p energy states. On the other hand, the valence band can be divided into two energy regions ranging from −9.5 to −7.9 eV (lower valence band) and from −4.13 to 0 eV (upper valence band). The former mainly consists of Bi 6s states hybridizing with O 2s and O 2p states, and the latter is mainly constructed from O 2p states strongly interacting with Bi 6s and Bi 6p states.     

A comparative study of electronic structure and magnetic properties of SrCrO3 and SrMoO3

A comparative study of electronic structure and magnetic properties of SrCrO₃ and SrMoO₃ has been carried out using FPLAPW method with density-functional theory. The calculated results suggest that both compounds are nonmagnetic (NM) metal in cubic structures at room temperature, and they exhibit very similar band structure and electronic properties except more extend Mo 4d orbitals than Cr 3d electronic states. However, the electronic structure and magnetic properties exhibit remarkable differences between them in the low temperature phases. SrCrO₃ is with a C-AFM ground state with magnetic moment of 1.18μ_B/Cr in the tetragonal structure, while SrMoO₃ is with a NM ground state in the orthorhombic structure. It is assumed that the extend 4d orbitals may be the reason which results in NM solution at low temperature phase of SrMoO₃.     

Ab initio study of structural and electronic properties of BiAlO3 and BiGaO3

The first principles within the full potential linearized augmented plane wave (FP-LAPW) method was applied to study the structural and electronic properties of cubic perovskite-type compounds BiAlO₃ and BiGaO₃. The lattice constant, bulk modulus, its pressure derivative, band structure and density of states were obtained. The results show that BiGaO₃ should exhibit higher hardness and stiffness than BiAlO₃. The Al–O or Ga–O bonds are typically covalent with a strong hybridizations as well as Bi–O ones that have a significant ionic character. Both materials are weakly ionic and exhibit wide and indirect band gaps, which are typical of insulators.     

Magnetic and electronic structure calculations of antiferromagnetic Mn2As

Magnetic and electronic structure calculations are performed for Mn₂As with antiferromagnetic (AFM), ferromagnetic (FM), and ferrimagnetic (FIM) spin ordering, using the full-potential linearized augmented plane-wave (FLAPW) method based on the generalized gradient approximation (GGA). It is shown that AFM is the magnetic ground state of Mn₂As, which is in agreement with the experimental observations. At a low temperature (0 K), AFM-FIM transition is also predicted which is consistent with the previous predictions. The ground state stability of the magnetic structure of Mn₂As is attributed to the nearest Mn (I) and Mn (II) antiferromagnetic interaction. The calculated magnetic moment of Mn (II) is found to be in good agreement with the neutron diffraction experiment while there is a disagreement for the magnetic moment of Mn (I). The different magnetic moments are reflected in the electronic structures of Mn₂As and the exchange splitting between Mn atoms is shown to be an intra-atomic effect.     

Structural and Electronic Properties of Li2Mg(NH)2 for Hydrogen Storage: First-principles Study

应用第一原理方法研究了储氢材料α-Li₂Mg(NH)₂和β-Li₂Mg(NH)₂两种构型的结构性质和电子性质．计算优化得到的晶胞参数和N-H键长符合实验得到的数据．通过Murnaghan状态方程得到了体积模量和零压力下的能量,计算结果表明α-Li₂Mg(NH)₂为基态构型．通过Mulliken布居分析说明α构型的N-Li/Mg的离子特性和N-H间的交互作用都弱于β构型．态密度分析结果表明,价带轨道主要由N原子的s轨道和p轨道占据,并与H原子的s轨道杂化．     

Effect of polar distortion on electronic structures of (001) LaGaO3/SrTiO3 interface

First-principles calculations of electronic structures of (001) epitaxial LaGaO₃/SrTiO₃ heterostructures were performed in the framework of density functional theory. The effects of atomic relaxation on electronic characteristics of both n-type (LaO)⁺/(TiO₂)⁰ and p-type (GaO₂)⁻/(SrO)⁰ interfaces are investigated. It is found that the n-type interface remains metallic, whereas the p-type interface becomes insulating after atomic relaxation. Polar distortion in the LaGaO₃ layers associated with the atomic relaxation strongly screens the intrinsic electric field induced by periodically stacking (LaO)⁺ and (GaO₂)⁻ charged atomic layers on SrTiO₃ with charge neutral (001) atomic layers. This relieves the trend to a polar catastrophe and reduces the carrier charge density on the interface.     

Electronic structures,magnetic properties and band alignments of 3d transition metal atoms doped monolayer MoS2

Utilizing first-principles calculations, the electronic structures, magnetic properties and band alignments of monolayer MoS₂ doped by 3d transition metal atoms have been investigated. It is found that in V, Cr, Mn, Fe-doped monolayers, the nearest neighboring S atoms (S_NN) are antiferromagnetically polarized with the doped atoms. While in Co, Ni, Cu, Zn-doped systems, the S_NN are ferromagnetically coupled with the doped atoms. Moreover, the nearest neighboring Mo atoms also demonstrate spin polarization. Compared with pristine monolayer MoS₂, little change is found for the band edges' positions in the doped systems. The Fermi level is located in the spin-polarized impurity bands, implying a half-metallic state. These results provide fundamental insights for doped monolayer MoS₂ applying in spintronic, optoelectronic and electronic devices.     

First-principle study on the electronic and optical properties of Mn-doped SnO2

Using the full-potential linearized augmented plane wave method (FP-LAPW), we have investigated the electronic and optical properties of Sn_1−xMn_xO₂ (x=0, 0.0625, 0.125, 0.1875, 0.25). The doped Mn results in reduction of the band gap, which can be attributed to a series of impurity bands at the bottom of the conduction band caused by the strong hybridization between Mn 3d and O 2p. The results also show that the Mn-doped systems tend to convert into p-type semiconductor with direct band gaps. With the increase of Mn concentration, both the imaginary part of dielectric function and the absorption spectrum show red-shift corresponding to the change of band gaps.     

The electronic and optical properties of Cu doped CdI2

Based on the density functional pseudopotential method, the electronic structures and the optical properties of CdI₂ doped with Cu are investigated in detail. The calculation results indicate that the defect of Cu(Cd) exists steadily with a certain solubility. For the Cu doped CdI₂, the new highly localized impurity bands induced by Cu 3d states lie just across the Fermi energy at the top of the valence band. The doping of Cu induces reduction of band gap of CdI₂; red shifts are revealed in both the imaginary part of dielectric function and the absorption spectra corresponding to the change in band gaps. Moreover, the study of the reflection spectrum and the loss function shows that the doped Cu is responsible for the increased reflection peak intensity and the red shift of the plasma resonant frequency of CdI₂.     

Ab initio calculations of the stability and lattice dynamics of the MgSiO3 post-perovskite

Using the first-principles computations we have calculated phonon-dispersion relations and the phonon density of states for perovskite and post-perovskite phases of MgSiO₃. From them using the quasiharmonic approximation we have estimated the free energies, bulk modulus and volume expansion coefficients of both structures at various pressures and temperatures. Our calculations indicate that the thermal expansivity of MgSiO₃ changes very little across the phase transition. We have determined the P-T coexisting line of perovskite and post-perovskite phases.     

First-principles study of the electronic and magnetic properties of a Nickel-Zinc ferrite: ZnxNi1−xFe2O4

Using first-principles density functional theory within the generalized gradient approximation method, the effect of Zn doping on electronic and magnetic properties of NiFe₂O₄ ferrite spinel has been studied. The crystal structure of the compounds is assigned to a pseudocubic structure and the lattice constant increases as the Zn concentration increases. Our spin-polarized calculations give a half-metallic state for NiFe₂O₄ and a normal metal state for Zn_xNi_1−xFe₂O₄ (0<x≤0.5). Based on the magnetic properties calculations, it is found that the saturation magnetic moment enhances linearly with increase in the Zn content in NiFe₂O₄. The Zn doping in NiFe₂O₄ also induces strong ferrimagnetism since it decreases the magnetic moment of A-sites.     

Temperature and field dependent electronic structure and magnetic properties of LaCoO3 and GdCoO3

The transformation of the band structure of LaCoO₃ in the applied magnetic field has been theoretically studied. If the field is below its critical value B_C≈65 T, the dielectric band gap decreases with the field, thus giving rise to negative magnetoresistance that is highest at T≈300÷500 K. The critical field is related to the crossover between the low- and high-spin terms of Co³⁺ ions. The spin crossover results in an insulator–metal transition induced by an increase in the magnetic field. Similar calculations have been done for GdCoO₃ which is characterized by large spin gap∼2000 K.     

A first-principles study of the electronic structure of the sulvanite compounds

We have investigated by means of first-principles total energy calculations the electronic structure of the sulvanite compounds: Cu₃VS₄, Cu₃NbS₄ and Cu₃TaS₄; the later is a possible candidate as a p-type transparent conductor with potential applications in solar cells and electrochromic devices. The calculated electronic structure shows that these compounds are indirect band gap semiconductors, with the valence band maximum located at the R-point and the conduction band minimum located at the X-point. The character of the valence band maximum is dominated by Cu d-states and the character of the conduction band minimum is due to the d-states of the group five elements. From the calculated charge density and electron localisation function we can conclude that the sulvanite compounds are polar covalent semiconductors.     

Evaluation for the configurational and electronic state of SO3 adsorbed on Pt surface

We evaluate the adsorption of SO₃ molecule on the Pt (1 1 1) surface using the first-principles calculations by a slab model with a periodic boundary condition. We find that there are four stable adsorption configurations on the Pt surface, where SO₃ molecules are adsorbed above the three-fold fcc and hcp sites. In two of these configurations, S and two O atoms are bound to the Pt atoms, and in two other of them, all the three O atoms are bound to Pt surface atoms. Besides, it is found that molecular orbitals of SO₃ and those of Pt surface are hybridized in the active metal d-bands region, that the localized molecular orbitals in SO₃ are stabilized, and that the charge is transferred from Pt to S 3p by SO₃ adsorption on Pt surface though the other interaction of S and O (bound to Pt) component with Pt is little. In addition, the bond between S and O bound to Pt become weak by SO₃ adsorption on Pt surface because the charge polarization to O-Pt bond weakens the bond between S and O bound to Pt. This interaction is assumed to encourage the breakage of S-O bond.