Electronic structure and magnetic properties of Laves phase LuFe2

The electronic structure and magnetic properties of the Laves phase of LuFe₂ with C14, C15, and C36 structures has been investigated using the full-potential linearized augmented plane wave method. In order to study the stability of magnetic phases, nonmagnetic and spin-polarized calculations for ferromagnetic ordering were performed. It is found that the ferromagnetic hexagonal C14 phase is the ground-state structure and the C15 phase is an intermediate state between the C14 and C36 structures. There is an increase in the average magnetic moment on the Fe sites in the order of C15 →C14 →C36 structures, whereas the Lu-moment is not significantly different. We also find that there exist both localized and itinerant d electrons, resulting in antiferromagnetic ordering in the three structures. Their density-of-states, equilibrium volumes, and elastic properties are discussed, which is important for the understanding of the physical properties of LuFe₂ and may inspire future experimental research.     

Mg-Al-Ca合金中三元Laves相的稳定性和电子结构

采用基于密度泛函理论的第一性原理方法,应用VASP (Vienna Ab-initio Simulation Package) 计算软件,研究了Mg-Al-Ca合金中三元Laves相,即Ca(Mg1-x,Alx)2和Al2(Ca1-x,Mgx) (x=0, 0.25, 0.50, 0.75, 1)在不同形态结构(C14, C15和C36)下的相稳定性及电子结构。计算所得的晶格常数和实验值吻合很好,形成能和相关能的计算用来研究三元Laves相的合金化能力和稳定性,结果表明：C14-Ca(Mg0.25,Al0.75)2具有很好的合金化能力,而C15- CaAl2具有很好的结构稳定性。态密度和电荷密度的计算用来研究Mg-Al-Ca合金中三元Laves相稳定性的内在微观机制。     

Magnetic properties, Mössbauer effect and first principle calculations study of laves phase HfFe2

The magnetic properties and hyperfine interaction parameters for Laves phase HfFe₂ with C14 type structure are studied using SQUID magnetometer and M?ssbauer measurement. The saturation magnetization, remanent magnetization, coercive field, magnetic moment per unit formula and the hyperfine magnetic field at Fe site are reported. In addition, a detailed theoretical study of the electronic structure and hyperfine magnetic fields of the two possible HfFe₂ structures, C15 and C14, is presented. Using the full-potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN 97 package, the equilibrium volume, bulk moduli, magnetic moments and hyperfine magnetic fields for the two structures are calculated. The obtained results are compared with the measured data.     

Mg–Al-Ca合金中二元Laves相的电子结构和力学性能

采用基于密度泛函理论的第一性原理方法,研究了Mg-Al-Ca合金中AB2型二元Laves相（如CaMg2, CaAl2 和MgAl2 ）在不同形态结构（C14, C15和C36）下的晶体结构、电子结构和力学性能。计算所得的晶格常数与实验数据吻合很好。形成能与相关能的计算表明C15- CaAl2 Laves相具有很好的合金化能力和结构稳定性。态密度和电荷密度的计算从微观上进一步分析了C15-CaAl2具有很好的合金化能力和稳定性的根源。弹性常数的计算和力学性能的分析表明C14-MgAl2具有很好的延展性,C15-MgAl2具有很好的塑性。     

Mg–Al-Ca合金中二元Laves相的电子结构和力学性能

采用基于密度泛函理论的第一性原理方法,研究了Mg-Al-Ca合金中AB2型二元Laves相（如CaMg2, CaAl2 和MgAl2 ）在不同形态结构（C14, C15和C36）下的晶体结构、电子结构和力学性能。计算所得的晶格常数与实验数据吻合很好。形成能与相关能的计算表明C15- CaAl2 Laves相具有很好的合金化能力和结构稳定性。态密度和电荷密度的计算从微观上进一步分析了C15-CaAl2具有很好的合金化能力和稳定性的根源。弹性常数的计算和力学性能的分析表明C14-MgAl2具有很好的延展性,C15-MgAl2具有很好的塑性。     

Electronic structural, elastic properties and thermodynamics of Mg17Al12, Mg2Si and Al2Y phases from first-principles calculations

Electronic structures, elastic properties and thermal stabilities of Mg₁₇Al₁₂, Mg₂Si and Al₂Y have been determined from first-principle calculations. The calculated heats of formation and cohesive energies show that Al₂Y has the strongest alloying ability and structural stability. The brittle behavior and structural stability mechanism is also explained through the electronic structures of these intermetallic compounds. The elastic constants are calculated, the bulk moduli, shear moduli, Young's moduli and Poisson ratio value are derived, the brittleness and plasticity of these phases are discussed. Gibbs free energy, Debye temperature and heat capacity are calculated and discussed.     

Phase stability,anisotropic elastic properties and electronic structures of C15-type Laves phases ZrM2 (M = Cr,Mo and W) from first-principles calculations

Abstract

Systematic investigations of phase stability and mechanical properties of C15-type ZrM₂ (M = Cr, Mo and W) Laves phases were performed using first-principles calculations. The formation enthalpies of ZrM₂ are in good agreement with the theoretical and experimental values. The elastic properties, including elastic constants and moduli, Poisson’s ratio and B/G, were discussed. The elastic anisotropy was also investigated via the anisotropy indexes (A^U, A^Z, A_shear and A_comp), the anisotropy of shear modulus and the 3D construction of bulk and Young’s moduli. The elastic anisotropy of ZrM₂ is in order of ZrCr₂ < ZrMo₂ < ZrW₂. The variations in the shear modulus and hardness show similar trends with increasing values from ZrCr₂ to ZrW₂. The electronic structures for these C15-type Laves phases were analysed to obtain deeper understanding of chemical bonds and phase stabilities. Finally, the sound velocities and Debye temperatures were also investigated.     

Ab initio study of I<Subscript>2</Subscript> and T<Subscript>2</Subscript> stacking faults in C14 Laves phase MgZn<Subscript>2</Subscript>

Based on the synchroshear mechanism, the formation of intrinsic stacking fault I ₂ and twin-like stacking fault T ₂ in C14 Laves phases has been modeled in detail and the generalised stacking fault energy curve of I ₂ and T ₂ for C14 Laves phase MgZn₂ has been calculated from first-principles. The results demonstrate that the unstable stacking fault energy of I ₂ by synchroshear is still very large, and the stable stacking fault energy of I ₂ is higher in comparison with pure Mg implying that the formation of I ₂ stacking fault in MgZn₂ is difficult. Starting with the I ₂ configuration, the T ₂ stacking fault can be formed by an additional synchroshear. The unstable and stable stacking fault energies of T ₂ are only slightly larger than those of I ₂, implying that the formation of T ₂ may be essentially similar to that of I ₂. From the obtained generalised stacking fault energy, the relevant deformation mechanism of MgZn₂ is also discussed. Finally, the electronic structure during synchroshear process is further studied.     

Electronic structures of Laves phase compounds

The electronic structures of Laves phase compounds AB₂ (A = Y, Zr, Nb; B = Cr, Mn, Co, Ni) were investigated systematically by the LMTO method within the framework of the LSD approximation. Comparing the calculated total energy for hypothetical para-, ferro-, and antiferromagnetic state with the C14 and the C15 structure, the theoretical predictions are in good agreement with the experiments concerning the stability of the crystal structure and the magnetism. The magnetic stability critically depends upon the change of the lattice spacing, which leads to the colorful behaviors of the magnetism in Fe Laves phase compounds and the anomalous volume change in the antiferromagnetic state of YMn₂.     

Theoretical predictions of the structural and thermodynamic properties of MgZn2 Laves phase under high pressure

The structural and thermodynamic properties of MgZn₂ Laves phase under hydrostatic pressure have been investigated by using a first-principles method based on the density functional theory within the generalized gradient approximation. The calculated equilibrium structural parameters are consistent with the previous experimental and theoretical data. Especially, we study the pressure dependence of the elastic constants, polycrystalline elastic moduli, Poisson’s ratio, elastic anisotropy, and theoretical Vickers hardness of MgZn₂. It is found that the pressure plays a significant role in the elastic properties of MgZn₂ due to the variations of inter-atomic distance. In addition, the density of states and Mulliken analysis are performed to reveal the bonding characteristics of MgZn₂. It is observed that the total density of states exhibits a certain offset with the increase of external pressure. Finally, the dependences of thermodynamic properties on pressure and temperature of MgZn₂ Laves phase have been also successfully predicted and analyzed within the quasi-harmonic Debye model for the first time.     

First-principles studies of the structural and electronic properties of the C14 Laves phase XCr2 (X = Ti,Zr, Nb,Hf and Ta)

The optimized structures, electronic properties and bonding characteristics of the hexagonal C14 Laves phase XCr₂ (X?=?Ti, Zr, Nb, Hf and Ta) have been investigated using first-principles calculations. Our results reveal that the equilibrium formation enthalpies are not depends entirely on the atomic numbers. The total and the partial density of states and valence charge densities of Laves phases are also calculated and applied to reveal the nature of the bonding character in consideration of the different atomic numbers.     

Dynamical stability and phase transition of ZrC under pressure

A comprehensive first principles study of structural, elastic, electronic, and phonon properties of zirconium carbide (ZrC) is reported within the density functional theory scheme. The aim is to primarily focus on the vibrational properties of this transition metal carbide to understand the mechanism of phase transition. The ground state properties such as lattice constant, elastic constants, bulk modulus, shear modulus, electronic band structure, and phonon dispersion curves (PDC) of ZrC in rock-salt (RS) and high-pressure CsCl structures are determined. The pressure-dependent PDCs are also reported in NaCl phase. The phonon modes become softer and finally attain imaginary frequency with the increase of pressure. The lattice degree of freedom is used to explain the phase transition. Static calculations predict the RS to CsCl phase transition to occur at 308?GPa at 0?K. Dynamical calculations lower this pressure by about 40?GPa. The phonon density of states, electron–phonon interaction coefficient, and Eliashberg's function are also presented. The calculated electron–phonon coupling constant λ and superconducting transition temperature agree reasonably well with the available experimental data.     

The stabilities, electronic structures and elastic properties of Rbben As systems

The structural, electronic and elastic properties of Rb-As systems (RbAs in NaP, LiAs and AuCu structures, RbAs₂ in the MgCu₂ structure, Rb₃As in Na₃As, Cu₃P and Li₃Bi structures, and Rb₅As₄ in the A₅B₄ structure) are investigated with the generalized gradient approximation in the frame of density functional theory. The lattice parameters, cohesive energies, formation energies, bulk moduli and the first derivatives of the bulk moduli (to fit Murnaghan's equation of state) of the considered structures are calculated and reasonable agreement is obtained. In addition, the phase transition pressures are also predicted. The electronic band structures, the partial densities of states corresponding to the band structures and the charge density distributions are presented and analysed. The second-order elastic constants based on the stress-strain method and other related quantities such as Young's modulus, the shear modulus, Poisson's ratio, sound velocities, the Debye temperature and shear anisotropy factors are also estimated.     

The stabilities, electronic structures and elastic properties of Rb-As systems

The structural, electronic and elastic properties of Rb–As systems (RbAs in NaP, LiAs and AuCu structures, RbAs2 in the MgCu2 structure, Rb3 As in Na3As, Cu3 P and Li3Bi structures, and Rb5 As4 in the A5B4 structure) are investigated with the generalized gradient approximation in the frame of density functional theory. The lattice parameters, cohesive energies, formation energies, bulk moduli and the first derivatives of the bulk moduli (to fit Murnaghan’s equation of state) of the considered structures are calculated and reasonable agreement is obtained. In addition, the phase transition pressures are also predicted. The electronic band structures, the partial densities of states corresponding to the band structures and the charge density distributions are presented and analysed. The second-order elastic constants based on the stress-strain method and other related quantities such as Young’s modulus, the shear modulus, Poisson’s ratio, sound velocities, the Debye temperature and shear anisotropy factors are also estimated.     

Theoretical calculations of structural,electronic, and elastic properties of CdSe_(1-x)Te_x:A first principles study

The plane wave pseudo-potential method was used to investigate the structural, electronic, and elastic properties of Cd Se_(1-x)Te_x in the zinc blende phase. It is observed that the electronic properties are improved considerably by using LDA + U as compared to the LDA approach. The calculated lattice constants and bulk moduli are also comparable to the experimental results. The cohesive energies for pure Cd Se and Cd Te binary and their mixed alloys are calculated. The second-order elastic constants are also calculated by the Lagrangian theory of elasticity. The elastic properties show that the studied material has a ductile nature.     

Electronic and mechanical properties,phase stability,and formation energies of point defects of niobium boronitride Nb<Subscript>2</Subscript>BN

The electronic structure, Fermi surface, Sommerfeld and Pauli paramagnetic susceptibility coefficients, cohesive energies, phase and point defect formation energies, elastic constants, bulk, shear, and Young moduli, Poisson ratios, and Vickers microhardness of niobium boronitride Nb₂BN are determined by the ab initio FLAPW-GGA full-potential method. The obtained values are discussed in comparison with similar data for Mo₂BC and other related binary carbides, nitrides, and borides of transition metals, and with available experimental data.     

Electronic structures and elastic properties of monolayer and bilayer transition metal dichalcogenides MX_2(M= Mo,W;X= O,S,Se,Te):A comparative first-principles study

First-principle calculations with different exchange-correlation functionals, including LDA, PBE, and vd W-DF functional in the form of opt B88-vd W, have been performed to investigate the electronic and elastic properties of twodimensional transition metal dichalcogenides(TMDCs) with the formula of MX2(M = Mo, W; X = O, S, Se, Te) in both monolayer and bilayer structures. The calculated band structures show a direct band gap for monolayer TMDCs at the K point except for MoO2 and WO2. When the monolayers are stacked into a bilayer, the reduced indirect band gaps are found except for bilayer WTe2, in which the direct gap is still present at the K point. The calculated in-plane Young moduli are comparable to that of graphene, which promises possible application of TMDCs in future flexible and stretchable electronic devices. We also evaluated the performance of different functionals including LDA, PBE, and opt B88-vd W in describing elastic moduli of TMDCs and found that LDA seems to be the most qualified method. Moreover, our calculations suggest that the Young moduli for bilayers are insensitive to stacking orders and the mechanical coupling between monolayers seems to be negligible.     

Layer-stacking irregularities in C36-type Nb–Cr and Ti–Cr Laves phases and their relation with polytypic phase transformations

Specific layer-stacking irregularities have been identified in C36 (4H) Nb–Cr and Ti–Cr Laves phases on the basis of X-ray diffraction line-profile analysis and high-resolution transmission electron microscopy. Domain boundaries and transformation errors within domains could be distinguished. The layer-stacking irregularities in both C36-NbCr₂ and C36-TiCr₂ can be associated with a preceding C14 (2H) → C36 (4H) phase transformation carried out by glide of mobile synchro-Shockley partial dislocation dipoles in an ordered fashion. The stacking irregularities observed can be interpreted as deviations from such perfect “ordered glide”. The interpretation is supported by the observation that, in the case of C36-NbCo₂, where no preceding C14 → C36 transformation occurs, different layer-stacking irregularities are observed.     

Elastic properties, thermal expansion coefficients and electronic structures of Ti0.75X0.25C carbides

Elastic properties, thermal expansion coefficients and electronic structures of Ti_0.75X_0.25C carbides (X=W, Mo, Ta, Nb, V, Hf, Zr, Cr and Al) were systematically investigated using ab initio density functional theory (DFT) calculations. The calculated elastic moduli, electronic structures and thermal expansion coefficients α(T) of pure TiC are in good agreement with experimental data and other DFT calculations. Based on a phenomenological formula, the trends of elastic properties and ductile/brittle behavior of Ti_0.75X_0.25C were analyzed. It was found that alloying elements W, Mo, Ta, Nb, V and Hf can increase elastic moduli, while Zr, Cr and Al reduce moduli. The nearly free electron model and Debye approximation were applied in the evaluation of α(T). The anharmonic effect was taken into account by including volume-dependent elastic moduli and Debye temperature. Results show that alloying additions of 3d V, 4d Zr and Mo slightly reduce α(T), while 3d Cr increases α(T), Al, 4d Nb, 5d Hf and W almost keep α(T) unchanged in Ti_0.75X_0.25C at high temperatures. The electronic structures of Ti_0.75X_0.25C were calculated and analyzed, and the electronic density of states was used to interpret variations of elastic properties and ductile/brittle behavior induced by alloying additions.