Ab initio calculations of yttrium nitride: structural and electronic properties

Using first principles total energy calculations within the full-potential linearized augmented plane wave method, we have studied the structural and electronic properties of yttrium nitride (YN) in the three phases, namely wurtzite, caesium chloride and rocksalt structures. The calculations are performed at zero and under hydrostatic pressure. In agreement with previous findings, it is found that the favored phase for YN is the rocksalt-like structure. We predict that at zero pressure YN in the rocksalt structure is a semiconductor with an indirect bandgap of 0.8 eV. A phase transition from a rocksalt to a caesium chloride structure is found to occur at ∼134 GPa. Besides, a transition from an indirect (Γ−X) bandgap semiconductor to a direct (X−X) one is predicted at pressure of ∼84 GPa. For the electron effective mass of rocksalt YN, these are the first results, to our knowledge. The information derived from the present study may be useful for the use of YN as an active layer in electronic devices such as diodes and transistors.     

Ab initio study of structural,electronic and dynamical properties of MgAuSn

The structural and electronic properties of MgAuSn in the cubic AlLiSi structure have been studied, using density functional theory within the local density approximation. The calculated lattice constant for MgAuSn is found to be in good agreement with its experimental value. Our calculated electronic structure is also compared in detail with a recent tight-binding. A linear-response approach to density-functional theory is used to calculate the phonon spectrum and density of states for MgAuSn.     

非晶碳结构建模和电子结构的第一性原理研究

基于密度泛函理论，采用了一种更为精确的交换相关泛函OLYP(OPTX+LYP)，对密度范围从2.0到3.2 g/cm³的非晶碳进行结构建模. 模拟得到的5个碳网络结构无论从径向分布函数还是sp³含量都与实验符合得很好. 对非晶碳电子结构的研究表明费米能级附近的电子态密度主要是sp²碳原子的贡献. 随着密度的增加，sp³碳原子增加，费米能级附近的态密度越来越小. 小环结构增加了费米能级附近的电子态密度，缺陷态在费米能级形     

非晶碳结构建模和电子结构的第一性原理研究

基于密度泛函理论,采用了一种更为精确的交换相关泛函OLYP(OPTX+LYP),对密度范围从2.0到3.2 g/cm³的非晶碳进行结构建模. 模拟得到的5个碳网络结构无论从径向分布函数还是sp³含量都与实验符合得很好. 对非晶碳电子结构的研究表明费米能级附近的电子态密度主要是sp²碳原子的贡献. 随着密度的增加,sp³碳原子增加,费米能级附近的态密度越来越小. 小环结构增加了费米能级附近的电子态密度,缺陷态在费米能级形 关键词： 非晶碳 密度泛函理论 电子结构     

Li_2NH晶体结构建模和电子结构的第一性原理研究

基于密度泛函理论的赝势-平面波方法,采用三种模型分别研究Li,N原子对N—H键方位的影响以及N—H键间的相互影响,得到了Li2NH的晶体结构和H原子的占位位置.计算结果表明:Li2NH晶体为层状结构,空间群为P42,晶胞中4个N—H键分为两层,层内N—H键为反平行排列,层间N—H键为垂直排列.态密度和电子局域函数(ELF)分析表明,N—H键呈明显的共价键特性,Li和N—H键呈明显的离子键相互作用.可逆储氢反应Li2NH+H2/LiNH2+LiH在温度0K时的反应焓为69.6kJ/molH2,与实验结果66kJ/molH2符合得较好.     

Ab initio calculations of structural,elastic and electronic properties of Li2O2

The equilibrium structure of the compound Li₂O₂ has been obtained via minimization of the total energy within the local density approximation (LDA) based on density functional theory (DFT). The calculated lattice parameters are in excellent agreement with available values. The elastic properties of Li₂O₂ are investigated. Values of all independent elastic constants (C ₁₁, C ₁₂, C ₁₃, C ₃₃ and C ₄₄) as well as bulk moduli in the a and c directions are predicted. Our calculated elastic properties suggest that the compound Li₂O₂ is slightly anisotropic but can be considered as an isotropic material compared with other highly anisotropic materials. The electronic properties, including the density of states and energy band structure, are obtained and we find that this compound behaves as a semiconductor.     

Ab initio investigation of the structural,elastic and electronic properties of the anti-perovskite TlNCa3

We have investigated the structural, elastic and electronic properties of the anti-perovskite TlNCa₃ using ab initio calculations within the generalized gradient approximation and the local density approximation for the exchange–correlation potential. The lattice constant, bulk modulus, elastic constants and their pressure dependence, energy band structures, density of states and charge density distribution are calculated and analyzed in comparison with the available experimental and theoretical data. The bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, Lamé’s coefficients, average sound velocity and Debye temperature are numerically estimated for ideal polycrystalline TlNCa₃ aggregates in the framework of the Voigt–Reuss–Hill approximation. This is the first theoretical prediction of the elastic constants and their related properties for TlNCa₃ that requires experimental confirmation.     

Ab initio study of the structural properties of magnesium

Structural properties of magnesium in the hcp structure are calculated using the ab initio pseudopotential method within the local-density-functional formalism. The calculated lattice constants, cohesive energy, bulk modulus, and Poisson's ratio are in good agreement with experimental values. A comparison with the results of beryllium is also discussed.     

Ab initio study of structural,electronic,thermo-elastic and optical properties of Pt_3Zr intermetallic compound

Structural,elastic,electronic and optical properties of the Pt3Zr intermetallic compound are investigated using first principles calculations based on the density functional theory(DFT)within the generalized gradient approximation(GGA)and the local density approximation(LDA).The Pt3Zr compound is predicted to be of cubic L12 and hexagonal D024 structures.The calculated equilibrium ground-state properties(lattice parameters a and c,bulk modulus B and its pressure derivative B',formation enthalpy ΔH)of the Pt3Zr compound,for both cubic and hexagonal phases,show good agreement with the experimental results and other theoretical data.Elastic constants(C11,C12,C13,C33,C44,and C55)are calculated.The predicted elastic properties such as Young's modulus E and shear modulus GH,Poisson ratioν,anisotropic ratio A,Kleinman parameter ξ,Cauchy pressure(C12-C44),ratios B/C44 and B/G,and Vickers hardness Hv indicate the stiffness,hardness and ductility of the compound.Thermal characteristic parameters such as Debye temperature θD and melting temperature Tm are computed.Electronic properties such as density of states(DOS)and electronic specific heat γ are also reported.The calculated results reveal that the Fermi level is on the psedogap for the D024 structure and on the antibonding side for the L12 structure.The optical property functions(real partε1(ω)and imaginary part ε2(ω)of dielectric function),optical conductivity σ(ω),refraction index n(ω),reflectivity R(ω),absorption α(ω)and extinction coefficients k(ω)and loss function L(ω)are also investigated for the first time for Pt3Zr in a large gamme of energy from 0 to 70 eV.     

Ab initio study of structural,electronic and optical properties of ternary CdOl-xSex alloys using special quasi-random structures

The structural, electronic, and optical properties of binary CdO, CdSe, and their ternary CdOi xSex alloys （0 〈 x _〈 1） in the rock salt and zinc blend phases have been studied by the special quasi-random structure （SQS） method. All the calculations are performed using full-potential linearized augmented plane wave plus local orbital＇s （FP-LAPW＋Io） method within the framework of density function theory （DFT）. We use Wu-Cohen （WC） generalized gradient approximation （GGA） to calculate structural parameters, whereas both Wu-Cohen and Engel-Vosko （EV） GGA have been applied to calculate electronic structure of the materials. Our predicted results of lattice constant and bulk modulus show only a slight deviation from Vegard＇s law for the whole concentrations. The obtained band structure indicates that for the rock-salt phase, the ternary alloys present semi-metallic behavior, while for the zinc blend phase, semiconductor behavior with direct bandgap is observed with decreasing order of x except for CdSe. Finally, by incorporating the basic optical properties, we discuss the dielectric function, refractive index, optical reflectivity, the absorption coefficient, and optical conductivity in terms of incident photon energy up to 14 eV. The calculated results of both binaries are in agreement with existing experimental and theoretical values.     

Ab initio studies of the electronic properties of the silica surface

Self-consistent unrestricted Hartree-Fock calculations for the surface of crystalline silica (α-quartz) have been performed. We report here the one-electron energy levels and wavefunctions for several models of surface geometry, and a comparison is then made with experimentally determined densities of states.     

Ab initio calculations of electronic and optical properties in O-doped LiF crystal

We present a first-principle study of electronic and optical properties in pure LiF and O-doped LiF crystals. The pure LiF crystal exhibits a wide band gap while the O-doped LiF crystal shows the less band gap due to the contribution of O 2p. Some optical constants, such as dielectric functions, reflectivity and the refractive index, have been performed. The calculated reflectivity and refractive index from the pure LiF crystal agree with the experimental and recently calculated results in the low-energy range. Meanwhile, the optical properties have also been predicted from the O-doped LiF crystal. The absorption band in 200 nm has been observed, which is relatively close to the experimental result.      

Ab initio studies of the structural,elastic, electronic and thermal properties of NiTi2 intermetallic

First principles calculations were performed in the framework of the density functional theory (DFT) using the Full Potential–Linear Augment Plane Wave method (FP–LAPW) within the generalized gradient approximation (GGA) to predict the structural, electronic, elastic and thermal properties of NiTi₂ intermetallic compound. By using the Wien2k all-electron code, calculations of the ground state and electronic properties such as lattice constants, bulk modulus, presure derivative of bulk modulus, total energies and density of states were also included. The elastic constants and mechanical properties such as Poisson’s ratio, Young’s modulus and shear modulus are estimated from the calculated elastic constants of the single crystal. Through the quasi-harmonic Debye model, the preasure and temperature dependences of the linear expansion coefficient, bulk modulus and heat capacity have been investigated. Finally, the Debye temperature has been estimated from the average sound velocity according to the predicted polycrystal bulk properties and from the single crystal elastic constants.     

Ab initio study of the structural,electronic, elastic and thermal conductivity properties of SrClF with pressure effects

Abstract

SrClF is an important optical crystal and can be used as pressure gauge in diamond anvil cell at high pressure. In this work, we performed a systematic study on the structural, electronic and elastic properties of SrClF under pressure, as well as its thermal conductivity, by first-principles calculation. Different exchange-correlation functionals were tested and PBESOL was finally chosen to study these properties of SrClF. Studies reveal that SrClF has a bulk modulus of about 56.2 GPa (by fitting equation of states) or 54.3 GPa (derived from elastic constants), which agree well with the experimental result. SrClF is mechanically and dynamically stable up to 50 GPa. Its elastic constants increase with the applied pressure, but its mechanical anisotropy deteriorates as the pressure increases. Investigation of its electronic properties reveals that SrClF is a direct band-gap insulator with a gap value of 5.73 eV at 0 GPa, which decreases with the increasing pressure and the reason is found by analysing the partial density of states. Based on the calculated phonon dispersion curves, thermal conductivity of SrClF is predicated. At ambient conditions, the predicted thermal conductivity is about 3.74 Wm^?1 K^?1, while that obtained using the simplified Slack model give a slightly larger value of 4.62 Wm^?1 K^?1.     

Ab initio calculations of phonon transport in ZnO and ZnS

We present an approach to calculate ballistic phonon transport that combines the atomistic Green’s function (AGF) method with ab initio results. For the interatomic potential we use the harmonic approach. The equilibrium positions of the atoms and the interatomic force constants (ifcs) are calculated using the ABINIT program package [X. Gonze et al., Z. Kristallogr. 220, 558 (2005)], which is based on density functional theory. Therefore, the presented approach is parameter free. From the Green’s function of the system we determine the density of states as well as the transmission function. The thermal conductance is obtained within the linear response regime. We apply this approach to bulk ZnO and bulk ZnS. Transmission functions for different transport directions for each material are presented. A comparison of the transmission function shows, that a ZnO/ZnS interface could be a promising phonon blocker. Adding such interfaces in ZnO or ZnS based thermoelectric devices could therefore increase the figure of merit.