Structural, elastic and electronic properties of intermetallics in the Pt–Sn system: A density functional investigation

The structural, elastic and electronic properties of intermetallics in the Pt–Sn binary system are investigated using first-principles calculations based on density functional theory (DFT). The polycrystalline elastic properties are deduced from the calculated single-crystal elastic constants. The elastic anisotropy of these intermetallics is analyzed based on the directional dependence of the Young’s modulus and its origin explained based on the electronic nature of the crystals. All the Pt–Sn intermetallics investigated are found to be mechanically stable, ductile and metallic, and some of them show high elastic anisotropy.     

Calculation of bulk modulus of titanium alloys by first principles electronic structure theory

A theoretical study of the electronic structure and binding energy of some hypothetical Ti-X alloys was carried out using a first-principles discrete variational cluster method. The formation energy of an alloying atom in solution of titanium was estimated based on such calculations, and the case of multi-constituent practical Ti alloys was considered in the dilute limit by a linear superimposition approach. The influences of alloying additions on the bulk modulus of the alloys were evaluated from the variation of the formation energy. The calculated moduli of the Ti alloys were found to vary linearly with the experimental values. This indicates that the present approach is appropriate for the simulation of modulus of titanium alloys. This revised version was published online in July 2006 with corrections to the Cover Date.     

Mechanical properties of U-0.95 mass fraction of Ti alloy quenching and aging treatment:a first principles study

First principles plane wave pseudopotential method was executed to calculate the mechanical properties with respect to the uranium-0.95 mass fraction of titanium(U-0.95 mass fraction of Ti) alloy for quenching and aging,including the elastic modulus,the value of shear modulus to bulk modulus(G/B) and the ideal tensile strength.The further research has also been done about the crack mechanism through Griffith rupture energy.These results show that the elastic moduli are 195.1 GPa for quenching orthorhombic a phase and 201.8 GPa for aging formed Guinier-Preston(G.P) zones,while G/B values are0.67 and 0.56,respectively.With the phase change of uranium-titanium(U-Ti) alloy via the quenching treatment,the ideal tensile strength is diverse and distinct with different crystal orientations of the anisotropic α phase.Comparison of quenching and short time aging treatment,both of the strength and toughness trend to improve slightly.Further analysis about electronic density of states(DOS) in the electronic scale indicates that the strength increases continuously while toughness decreases with the aging proceeding.The equilibrium structure appears in overaging process,as a result of decomposition of metastable quenching a phase.Thereby the strength and toughness trend to decrease slightly.Finally,the ideal fracture energies of G.P zones and overaging structure are obtained within the framework of Griffith fracture theory,which are 4.67 J/m2 and 3.83 J/m2,respectively.These results theoretically demonstrate strengthening effect of quenching and aging heat treatment on U-Ti alloy.     

The electronic, elastic and structural properties of Pd-Zr intermetallic

A first-principles plane-wave pseudopotential method based on the density functional theory was used to investigate the energetic, electronic structures and elastic properties of intermetallic compounds of Pd-Zr system. The Enthalpies of formation, the cohesive energies and elastic constants of these compounds were estimated from the electronic structure calculations and their structural stability was also analyzed. The results show that the PdZr₂ compound is stable, relative to other compounds, and as the concentration of Pd increases, the enthalpy of formation gradually increased except Pd₄Zr_3.The calculated elastic constants are then used to estimate mechanical properties of Pd-Zr intermetallics compounds. The brittle/ductile behavior is assessed by analyzing the phenomenological formula G/B of shear modulus (G) over bulk modulus (B). The new knowledge from this study could be used for future development of Pd-Zr system.     

Zn_(1-x)O_(1-y)Mn_xN_y磁半导体的稳定性,电子结构和半金属性质的第一性原理研究(英文)

利用全势缀加平面波方法计算了Mn和N共掺杂的P型ZnO的8种不同的位置构型.基于总能最低原理我们发现在没有空穴载流子的情况下,第四种构型(N个Mn在同一个层内最近邻位置)是最为稳定的。计算表明,费米面在价带的顶端附近。这个共掺杂体系表现出来的是半金属特性,磁性的起源可以利用BMP理论做出解释。     

Vacancy effects on structural and electronic properties of 4d transition-metal carbides

We present a study of the effect of the vacancies on the structural and electronic properties in substoichiometric NbC_x and MoC_x in the NaCl type structure using ab-initio full-potential linear augmented plane wave method (FP-LAPW). A model structure of 8 and 16 atom supercell with ordered vacancies within the carbon sublattices is used. We find that the lattice parameters of the studied stoichiometries in both MoC_x and NbC_x are smaller than that of ideal stoichiometric MoC and NbC. Our results are found to be in good agreement with experiment and other theoretical ab-initio calculations.     

First principles investigation on the structural, mechanical and electronic properties of OsC2

The structural, mechanical and electronic properties of OsC₂ were investigated by use of the density functional theory. Seven structures were considered, i.e., orthorhombic Cmca (No. 12, OsSi₂), Pmmn (No. 59, OsB₂) and Pnnm (No. 58, OsN₂); tetragonal P4₂/mnm (No. 136, OsO₂) and I4/mmm (No. 139, CaC₂); cubic Fm–3m (No. 225, CaF₂) and Pa-3 (No. 205, PtN₂). The results indicate that Cmca in OsSi₂ type structure is energetically the most stable phase among the considered structures. It is also stable mechanically. OsC₂ in Pmmn phase has the largest bulk modulus 319 GPa and shear modulus 194 GPa. The elastic anisotropy is discussed.     

稀土La掺杂Mg2Si的几何结构、弹性性能和电子结构的第一性原理研究EI北大核心CSCD

采用密度泛函理论(DFT)的第一性原理平面波赝势方法对稀土元素镧(La)掺杂Mg2Si的几何结构、弹性性能和电子结构进行计算与分析。首先,结合形成焓、Born力学稳定性以及差分电荷密度的结果可知,掺杂稀土元素La之后,形成的Mg8Si4La和Mg8Si3La均不能稳定存在,La掺杂的Mg2Si优先占据体系Mg原子的位置;其次,晶体的体模量(B),剪切模量(G),杨氏模量(E),泊松比(ν),以及各向异性系数(A)的计算结果表明本征Mg2Si为脆性相,而Mg7Si4La为韧性相,掺杂La可以提高Mg2Si的延展性;最后,态密度、Mulliken布居数和电荷差分密度的计算结果表明掺杂稀土镧后费米面向高能级区域偏离,进入导带,提高了Mg2Si的导电性。     

First principles investigation of electronic, optical and transport properties of α- and β-phase of arsenic telluride

Crystalline arsenic telluride exists in two stable phases. The monoclinic α-phase transforms to rhombohedral β-phase under high pressure. The electronic, optical and transport properties of the two phases has been investigated using full potential linear augmented plane wave (LAPW) + local orbitals (lo) scheme, in the framework of DFT with generalized gradient approximation (GGA). We present the energy bands, density of states and optical properties like the complex dielectric functions and absorption coefficients. From the dynamic dielectric constant, the structural anisotropy for the monoclinic α-phase is clearly observed, whereas the longitudinal and transverse components are almost identical for the β-phase. The optical absorption profiles clearly indicate that β-phase has possibility of greater multiple direct and indirect interband transitions in the infrared and visible regions compared to the α-phase. The rhomohedral phase which has the Bi₂Te₃ type structure has the possibility of thermoelectric properties, therefore transport properties like electrical and thermal conductivities, Seebeck and Hall coefficients etc. are also calculated. Good agreements are found with the available experimental results.     

Structural, electronic and elastic properties of V5Si3 phases from first-principles calculations

The phase stability, electronic, elastic and thermodynamic properties of V₅Si₃ has been investigated by using first-principles calculations based on the density functional theory (DFT). In the present calculations, three phases of V₅Si₃ (Cr₅B₃-prototype, W₅Si₃-prototype and Mn₅Si₃-prototype) have been taken into account to check the phase stability. The calculated formation enthalpies indicate that the W₅Si₃-prototype is the stable phase which is in agreement with experiments, whereas the Cr₅B₃-prototype is a potential metastable phase. The elastic constants, bulk modulus, shear modulus and Young’s modulus are calculated in the present work, and are very close to that of the Nb₅Si₃. The density of states and bonding charge density of V₅Si₃ within the W₅Si₃-phase are obtained indicating a strong covalent character of the bonds. Finally, using the Debye-model, the Debye temperature, heat capacity, and thermal expansion have also been calculated and are in good agreement with experimental results.     

First principles study on the elastic properties and electronic structures of (Fe, Cr)3C

First principles calculations are performed to investigate the elastic properties and electronic structures of Cr doped Fe₃C carbides, the obtained results are compared with Cr₃C and Fe₃C. The calculated bulk modulus of Fe₁₁CrC₄ and Fe₁₀Cr₂C₄ is 260GPa and 270GPa, respectively, larger than Fe₃C. So the hardness of Fe₃C phase can be enhanced by doped with an appropriate amount of Cr, however, the calculated formation enthalpy and defect formation enthalpy of Fe₁₁CrC₄ and Fe₁₀Cr₂C₄ are positive. On the other hand, the electronic calculations reveal that the ground states of Fe₁₁CrC₄ and Fe₁₀Cr₂C₄ are ferromagnetic. The evaluated local magnetic moments of Fe at 4c sites are larger than that of 8d sites, which is analogous to Fe₃C. Milliken population results indicate that the stabilities of Fe₁₁CrC₄ and Fe₁₀Cr₂C₄ are reduced mainly due to the strong repulsive bonds among metal atoms.     

First principles prediction of structural stability,elastic, lattice dynamical and thermal properties of osmium carbides

Abstract

First principles calculations are performed to investigate the structural stability, elastic, lattice dynamical and thermal properties of osmium carbides with various crystal structures. Our calculation indicates that the I₄Te type structure is energetically the most favourable for Os₄C. Based on stress–strain relationships, elastic constants are obtained, and the relevant mechanical properties are also discussed. The phonon dispersion relation and the dynamical stability are also predicted. We have found that the predicted structures are mechanically stable as well as dynamically stable except for cubic-Os₄C. Through the quasi-harmonic Debye model, the temperature and pressure effects on the bulk modulus, thermal expansion coefficient, heat capacity, Grüneisen parameter and Debye temperature are presented.     

Structural,elastic, electronic and optical properties of new layered semiconductor BaGa2P2

We report the results of a detailed first-principles based density functional theory study of the structural, elastic, electronic and optical properties of a recently synthesized layered semiconductor BaGa₂P₂. The optimized structural parameters are in excellent agreement with the experimental structural findings, which validates the used theoretical method. The single crystal and polycrystalline elastic constants are numerically estimated using the strain–stress method and Voigt–Reuss–Hill approximations. Predicted values of the elastic constants suggest that the considered material is mechanically stable, brittle and very soft material. The three-dimensional surface and its planar projections of Young’s modulus are visualized to illustrate the elastic anisotropy. It is found that Young’s modulus of BaGa₂P₂ show strong dependence on the crystallographic directions. Band structure calculation reveals that BaGa₂P₂ is a direct energy band gap semiconductor. The effective masses of electrons and holes at the minimum of the conduction band and maximum of the valence band are numerically estimated. The density of state, charge density distribution and charge transfers are calculated and analyzed to determine the chemical bonding nature. Dielectric function, refractive index, extinction coefficient, absorption coefficient, reflectivity and electron-loss energy function spectra are computed for a wide photon energy range up to 20 eV. Calculated optical spectra exhibit a noticeable anisotropy.     

First principles study on the structural properties and electronic structure of X2B (X = Cr, Mn, Fe, Co, Ni, Mo and W) compounds

First principles calculations are performed to study the stability, electronic and structural properties of X₂B (X = Cr, Mn, Fe, Co, Ni, Mo and W). The calculated cohesive energy and formation enthalpy of these compounds both have negative values, which indicate that they are thermodynamically stable structures. The ground states of Cr₂B and Mn₂B are anti-ferromagnetic; Fe₂B and Co₂B are ferromagnetic; Ni₂B, Mo₂B and W₂B are paramagnetic. The calculated local magnetic of Fe₂B is 1.962μ_B/Fe, and for Co₂B is 1.182μ_B/Co. They are comparable to the values of Fe₃B (1.97μ_B/Fe) and Co₃B (1.18μ_B/Co), but smaller than pure Fe and Co. The observed magnetic behaviors of X₂B compounds can be explained by Stoner’s model. Two main peaks are observed in the calculated PDOS (partial density of states) of these compounds (P1 and P2). P1 is caused by strong covalent X–B bonds and P2 is attributed to metallic X–X bonds.     

Concerning the brittleness of iridium: An elastic and electronic view

The brittleness of Ir has become a challenging and puzzling problem for decades and its fundamental mechanisms are controversial with each other in the literature. The present first principles calculation aims to get a deep understanding of the brittleness of Ir from an elastic and electronic view. It is found that Ir has normal pressure-dependent mechanical behavior, while the temperature-dependent behavior of Ir is unusual and contrary to that of other FCC metals, and that pressure decreases the brittleness of Ir, whereas temperature increases its brittleness, suggesting that the machining of Ir products should be performed at low temperature and high pressure. Moreover, electronic structure and crystal field theory reveal that Ir has the mainly octahedral bonding, which would be transformed to the mainly cubic bonding under high pressure, while become more octahedral and directional at high temperature. In addition, the implication and importance of the similarities between Ir and semiconductors are also discussed.     

Optical and transport properties and electronic structure of nickel doped arsenic chalcogenides

The electronic structure of monoclinic As₂X₃ (X = S, Se) is investigated using full potential linearized augmented plane wave method in the framework of density functional theory. From energy bands and the density of states it is seen that the lone pair p-states of sulfur/selenium contribute closest to Fermi energy level. Introduction of transition metal impurities such as Nickel modifies the semiconducting gap in As₂S₃ and As₂Se₃. The crystal field splits the Ni 3d bands with t_2g electrons in the valence band and e_g electrons in conduction band. We compute optical properties like the complex dielectric functions, refractive indices, absorption coefficient, reflectance, etc. The low symmetry chalcogenides exhibit optical dichroism. On doping, As₂S₃ and As₂Se₃ show additional losses in the IR regions, which indicates allowed interband transitions due to available 3d-states in the valence and conduction bands. A narrow transparent window in the visible region is available in Ni_0.5As_1.5Se₃ crystals. Important transport properties such as Seebeck and Hall coefficients, and carrier concentration are computed. It is seen that these high resistivity chalcogenides are n-type semiconductors.     

First-principles studies of the electronic and elastic properties of metal nitrides XN (X = Sc, Ti, V, Cr, Zr, Nb)

Electronic and elastic properties of a series of the transition metal ion mononitrides (ScN, TiN, VN, CrN, ZrN, NbN) have been modeled in the framework of ab initio plane wave spin-polarized calculations using the generalized gradient and local density approximations. The calculated band structures are typical for metallic compounds, except for ScN, whose band structure is that one of the gapless semiconductor. Strongly delocalized d states of transition metal ions are spread over a wide region of about 10-12 eV and are strongly hybridized with the nitrogen 2p states. Among the considered nitrides, only CrN exhibits a clear difference between the spin-up and spin-down states, which would manifest itself in magnetic properties. The overall appearance of the calculated cross-sections of the electron density difference changes drastically when going from Sc to Nb in the considered series of compounds. For the first time the calculated tensors of the elastic constants and elastic compliance constants were used for the analysis and visualization of the directional dependence of the Young’s moduli. It was shown that ScN and VN can be characterized as more or less elastically isotropic materials, whereas in TiN, CrN, ZrN, and NbN the Young’s moduli vary significantly in different directions. The maximal values of the Young’s moduli are along the crystallographic axes, the minimal values are along the bisector direction in the coordinate planes; the difference between them in the case of CrN exceeds one order of magnitude. In addition, pressure dependence of the “metal - nitrogen” distance was modeled.     

First principles study on the properties of p-type conducting In:SnO2

Detailed theoretical investigations on the structural, electronic and optical properties of p-type conducting In:SnO₂ have been conducted by first principle calculations. Analysis on the thermal stability via standard enthalpy of formation calculations shows that In:SnO₂ remains stable at very high In concentration, although the lattice constant expands in a distorted rutile structure with the increase of indium content. This can be attributed to the larger ionic radii and the one less 5p electron of In³⁺. Due to the differences in thermal stabilities of the structures with the same indium concentration, the preferred In³⁺ distribution is to occupy the Sn sites in different (110) slabs, followed by occupying the location in the same (110) slab with a maximized distance between indium ions. Indium element in SnO₂ introduces a band in the low energy region originated from the In 4d orbitals and an acceptor energy level slightly above the Fermi energy. While the large effective mass of the electron holes in the valence band results in the small p-type conductivity of In:SnO₂. The tiny changes in the conduction band and band gap lead to the invariability of the optical spectra in the ultraviolet-visible region. On the contrary, the dramatic enhancement of dielectric function, reflectivity and absorption in infrared region can be interpreted by the transition from the occupied states to the empty bands near E_f as well as the exciton effect. These features make In:SnO₂ a good candidate for applications such as transparent conducting materials, infrared reflecting materials and gas sensors.     

Further study on structural and electronic properties of silicon phosphide compounds with 3:4 stoichiometry

Si₃N₄ has been extensively studied, due to its potential applications in electronic devices. Substitution of N by P results in Si₃P₄ which is a relatively unknown material. In this study, we carried out further investigation on the structural and electronic properties of Si₃P₄ using first principles total energy method based on the density functional theory and the generalized gradient approximation. Our calculations show that pseudocubic-Si₃P₄ is energetically favored. However, the present study based on the generalized gradient approximation predicts that the phase is more stable than the γ phase which was predicted to be more energetically stable in an earlier study based on the local density approximation. Other properties such as bulk modulus, band structure, of Si₃P₄ calculated using GGA are consistent with the LDA results.     

First-principles calculations of structural, electronic and elastic properties of Ca2MgSi2O7

We studied the structural, electronic and elastic properties of åkermanite, Ca₂MgSi₂O₇, by using the first-principles method. The structure of åkermanite is constructed by interleaved tetrahedral and Ca cation layers, and this characteristic is perfectly presented by three-dimensional (3D) crystal lattice, as well as two-dimensional (2D) contour plots of total electron densities in this paper. The chemical bonding and interaction are investigated by analyzing the bond population and density of states (DOS) of the crystal. Theoretical elastic constants of åkermanite are consistent with the experimental values. Moreover, significant anisotropy for Young’s modulus can be observed.