Phase stability of magnesium-rare earth binary systems from first-principles calculations

The comprehensive investigation to cohesive properties of 344 intermetallic compounds in 17 Mg-RE (RE = Sc, Y, and Lanthanide elements) binary system has been carried out systematically in the framework of density-functional theory (DFT) type first-principles with the generalized gradient approximation (GGA). The calculated properties at present work; including total energy, enthalpy of formation, equilibrium volume, bulk modulus, and electronic structure, was consistent with the experimental data. It was convinced that both D0₃-Mg₃RE and B2-MgRE were stable compounds in Mg-RE systems except Mg₃Eu, Mg₃Yb and Mg₃Lu in Mg₃RE series and MgYb for B2-MgRE branch extracting from calculated results.     

Enthalpies of formation of magnesium compounds from first-principles calculations

An energetics database of binary magnesium compounds has been developed from first-principles calculations. The systems investigated include Mg–X (X = As, Ba, Ca, Cd, Cu, Dy, Ga, Ge, La, Lu, Ni, Pb, Sb, Si, Sn and Y). The calculated lattice parameters and enthalpies of formation of binary compounds in these systems are compared with both experimental data and thermodynamic databases.     

Effect of alloying elements on the elastic properties of Mg from first-principles calculations

The influence of different alloying elements on the lattice parameters and elastic properties of Mg solid solution has been studied using first-principles calculations within the generalized gradient approximation. The solute atoms employed herein are Al, Ba, Ca, Cu, Ge, K, Li, Ni, Pb, Si, Y and Zn. A supercell consisting of 35 atoms of Mg and one solute atom is used in the current calculations. A good agreement between calculated and available experimental data is obtained. Lattice parameters of Mg–X alloys are found to be dependent on the atomic radii of the solute atoms. A correlation between the bulk modulus of Mg–X alloys and the nearest-neighbor distance between Mg and X is shown. Addition of solute atoms belonging to the s-block and p-block of the periodic table results in a lower bulk moduli than d-block elements. A strong dependence of the elastic modulus of Mg–X alloys on the elastic properties of the solute atoms is also observed. Using the bulk modulus/shear modulus ratio (B/G), the change in the ductility of Mg due to the addition of the solute atom is briefly described. Linear regression coefficients for the elastic constants of each of the alloys are obtained as a tool for predicting the trend in the elastic properties of Mg as a function of concentration of the solute atoms.     

Half-metallic ferromagnetism in the half-Heusler compounds GeKCa and SnKCa from first-principles calculations

The structural, electronic and magnetic properties of the compounds GeKCa and SnKCa with half-Heusler structure excluding transition metals are investigated by using the first-principles pseudopotential plane wave method based on density functional theory. From the calculated total energies of three possible atomic arrangements we obtain the most stable structure for GeKCa and SnKCa where Ge (Sn), K and Ca occupy the (0, 0, 0), (1/4, 1/4, 1/4) and (3/4, 3/4, 3/4) positions, respectively. It is shown that both GeKCa and SnKCa with the most stable atomic arrangement exhibit half-metallicity with large half-metallic gaps (0.28 and 0.27 eV, respectively) and with an integer magnetic moment of 1.00 μ_B per formula unit. The magnetic moment mainly originates from Ge (Sn) p electrons, and the ferromagnetic state is more favourable in energy than the antiferromagnetic state for both compounds. We also find that the half-metallicity can be maintained up to the lattice contraction of 10% and 13% for GeKCa and SnKCa, respectively.     

Automating first-principles phase diagram calculations

Devising a computational tool that assesses the thermodynamic stability of materials is among the most important steps required to build a “virtual laboratory,” where materials could be designed from first principles without relying on experimental input. Although the formalism that allows the calculation of solid-state phase diagrams from first principles is well established, its practical implementation remains a tedious process. The development of a fully automated algorithm to perform such calculations serves two purposes. First, it will make this powerful tool available to a large number of researchers. Second, it frees the calculation process from arbitrary parameters, guaranteeing that the results obtained are truly derived from the underlying first-principles calculations. The proposed algorithm formalizes the most difficult step of phase diagram calculations, namely the determination of the “cluster expanison,” which is a compact representation of the configurational dependence of the alloy’s energy. This is traditionally achieved by a fit of the unknown interaction parameters of the cluster expansion to a set of structural energies calculated from first principles. We present a formal statistical basis for the selection of both the interaction parameters to include in the cluster expansion and the structures to use to determine them. The proposed method relies on the concepts of cross-validation and variance minimization. An application to the calculation of the phase diagram of the Si-Ge, CaO-MgO, Ti-Al, and Cu-Au systems is presented.     

Automating first-principles phase diagram calculations

Devising a computational tool that assesses the thermodynamic stability of materials is among the most important steps required to build a “virtual laboratory,” where materials could be designed from first principles without relying on experimental input. Although the formalism that allows the calculation of solid-state phase diagrams from first principles is well established, its practical implementation remains a tedious process. The development of a fully automated algorithm to perform such calculations serves two purposes. First, it will make this powerful tool available to a large number of researchers. Second, it frees the calculation process from arbitrary parameters, guaranteeing that the results obtained are truly derived from the underlying first-principles calculations. The proposed algorithm formalizes the most difficult step of phase diagram calculations, namely the determination of the “cluster expanison,” which is a compact representation of the configurational dependence of the alloy’s energy. This is traditionally achieved by a fit of the unknown interaction parameters of the cluster expansion to a set of structural energies calculated from first principles. We present a formal statistical basis for the selection of both the interaction parameters to include in the cluster expansion and the structures to use to determine them. The proposed method relies on the concepts of cross-validation and variance minimization. An application to the calculation of the phase diagram of the Si-Ge, CaO-MgO, Ti-Al, and Cu-Au systems is presented.     

Electronic structure and optical properties of plutonium dioxide from first-principles calculations

The electronic structure and optical properties of plutonium dioxide were calculated using the generalized gradient approximation with a Hubbard parameter U(GGA+U) for considering the strong coulomb correlation between localized Pu 5f electrons based on the first-principles density functional theory. The calculated results show that PuO_2 is a semiconductor material with the band gap of 1.8 eV, which is in good agreement with the corresponding experimental data. Furthermore, the dielectric function, reflectivity, refractive index, and extinction coefficient were calculated and analyzed using the Kramers–Kronig relationship for PuO_2. The calculated results were compared with the experimental data from the published literature.     

Point defect structures of YAl2 and ZrCo2 Laves phase compounds by first-principles calculations

In Laves phase alloys with prominent size mismatch between constituent atoms and/or large negative enthalpy of formation, the existence of vacancies as the dominant point defect type is often suggested. However, there are not enough experimental data to prove or disprove these arguments. Employing first-principles calculations, we study the point defect structures of YAl₂ and ZrCo₂ C15 Laves phases, as both compounds exhibit large size mismatch between constituent atoms, and large negative enthalpy of formation. We find that one must go beyond the simple geometrical or enthalpy arguments in determining the point defect structures of these alloys. In both compounds, the point defect structure is found to be dominated by the anti-site defects on the larger atom-rich side of the stoichiometry.     

Crystal structure of Mg3Pd from first-principles calculations

Crystal structure of Mg3Pd alloy was studied by first-principles calculations based on the density functional theory. The total energy, formation heat and cohesive energy of the two types of Mg3Pd were calculated to assess the stability and the preferentiality. The results show that Mg3Pd alloy with Cu3P structure is more stable than Na3As structure, and Mg3Pd alloy is preferential to Cu3P structure. The obtained densities of states and charge density distribution for the two types of crystal structure were analyzed and discussed in combination with experimental findings for further discussion of the Mg3Pd structure.     

Study on the binary intermetallic compounds in the Mg–Ce system

Electron-probe microanalysis (EPMA) and X-ray diffraction (XRD) studies conducted on Mg–38 wt%Ce and Mg–66 wt%Ce alloys demonstrated the existence of two distinct intermetallic phases, Mg_3.6Ce, Mg₃Ce, with the compositions Mg_3.6–3.7Ce, Mg_3.0–3.2Ce, respectively. XRD indicates that Mg_3.6Ce is likely a defect-vacancy structure of Mg₃Ce. The μ-Mg₃Ce phase, with the Mg_3.0–3.3Ce composition and a possible orthorhombic structure, has also been discovered, which is considered a metastable high-temperature form of the Mg₃Ce phase. Based on these results a version of the phase diagram is suggested for the Mg–Ce system in the composition range of 38–70 wt%Ce which correlates well with the solidification microstructures and phases of the two alloys.     

Composition-dependent elastic properties and electronic structures of off-stoichiometric TiNi from first-principles calculations

The composition-dependent elastic properties and electronic structure of off-stoichiometric TiNi with a B2 structure are investigated by using the first-principles exact muffin-tin orbitals method in combination with coherent potential approximation and first-principles plane-wave pseudopotential method (for computing bonding charge densities). The Zener anisotropy, c₄₄/c′, increases with increasing Ni contents, but is quite small, indicating a strong correlation between the softening of c₄₄ and c′ with decreasing temperature during martensitic transformation (MT). For the Ni-rich TiNi, c₄₄ increases with increasing Ni content whereas c′ decreases. On the Ti-rich side, both c₄₄ and c′ are insensitive to the composition. It was observed that larger c₄₄ corresponds to higher MT temperature, and the composition dependence of elastic modulus is discussed on the basis of the bonding charge densities and electronic density of states. We propose that the strong composition dependence of the elastic modulus of the Ni-rich TiNi can be attributed to the Coulomb static electronic repulsion between the antisite Ni atoms and their surroundings. The insensitivity of the elastic modulus of the Ti-rich TiNi to the composition is due to the absence of such repulsion between the Ti antisites and their nearest neighbors.     

Diffusion mechanisms of vacancy and doped Si in Al3Ti from first-principles calculations

First-principles calculations were employed to study the migration of vacancy in clean and Si-doped Al₃Ti. The effect of Si doping on the formation of vacancy and the diffusion of doped-Si atom in the Al₃Ti were also investigated. It is found that, under Al-rich condition, the formation energies of Al vacancies in Al₃Ti with Si doping are decreased compared with those in clean Al₃Ti. The preferred migration paths of vacancies are not changed when Si occupies Al site, but the migration energy barriers for the majority of paths are decreased after Si doping. The doped-Si atom on Al site prefers to diffuse via the nearest Al vacancy.     

Temperature-dependent mechanical properties of alpha-/beta-Nb5Si3 phases from first-principles calculations

The temperature-dependent structural properties and anisotropic thermal expansion coefficients of α-/β-Nb₅Si₃ phases have been determined by minimizing the non-equilibrium Gibbs free energy as functions of crystallographic deformations. The results indicate that the crystal anisotropy of α-Nb₅Si₃ phase is much more temperature dependence than that of β-Nb₅Si₃ phase. The total/partial density of states of α-/β-Nb₅Si₃ phases are discussed in detail to analyze their electronic hybridizations. It is demonstrated that the bonding of the two phases is mainly contributed from the hybridization between Nb-4d and Si-3p electronic states. The temperature-dependent mechanical properties of α-/β-Nb₅Si₃ phases are further investigated via the quasi-harmonic approximation method in coupling with continuum elasticity theory. The calculated single-crystalline and polycrystalline elasticity shows that both phases are mechanically stable and exhibit the intrinsic brittleness. The results also suggest that α-Nb₅Si₃ phase possesses a superior ability of compression resistance but an inferior ability of high-temperature resistance of mechanical properties than those of β-Nb₅Si₃ phase. The bonding features of α-/β-Nb₅Si₃ phases are discussed by means of charge density difference analysis in order to explain the difference of the temperature-dependent mechanical properties between the two phases.     

Thermodynamic properties of Al,Ni, NiAl,and Ni3Al from first-principles calculations

The thermodynamic properties of Al, Ni, NiAl, and Ni₃Al were studied using the first-principles approach. The 0-K total energies are calculated using the ab initio plane wave pseudopotential method within the generalized gradient approximation. The contribution to the free energy from the lattice vibration was calculated using the phonon densities of states derived by means of the ab initio linear-response theory. The thermal electronic contribution to the free energy was obtained from the one-dimensional numerical integration over the electronic density of states. With the deduced Helmholtz free-energy, the thermal expansion and enthalpy as a function of temperature were calculated and compared with the experimental data. Our calculations show that the enthalpies of formation are slightly temperature dependent with a slope of −1.6 J/mol/K for NiAl and −1.2 J/mol/K for Ni₃Al. For Ni, the inclusion of thermal electronic excitation results in a 10% increase in thermal expansion and 15% increase in enthalpy at 1600 K.     

Dynamical charge tensors and infrared spectra of the crystalline α-quaterthiophene polymorph phases from first-principles calculations

Infrared intramolecular vibrations and lattice modes for both the crystalline α-quaterthiophene (4T) polymorph phases are investigated by using the modern theory of polarization combined with density functional theory. For the first time, Born effective charge tensors and far- and mid-infrared responses for both the polymorph phases of 4T have been calculated. We have also proposed an assignment of the main spectral infrared signatures of the 4T polymorph phases. These assignments could be of particular interest in the far-infrared domain for the understanding of the electronic transport of these materials.     

Structural,half-metallic and elastic properties of the half-Heusler compounds NiMnM (M = Sb,As and Si) and IrMnAs from first-principles calculations

The structural, half-metallic and elastic properties of the half-Heusler compounds NiMnM (M = Sb, As and Si) and IrMnAs were investigated using first-principles calculations within the generalized gradient approximation (GGA) based on density function theory (DFT). The most stable lattice configurations about site occupancy are (Ni)_4a(Mn)_4c(Sb)_4d, (Ni)_4a(Mn)_4c(As)_4d, (Ni)_4a(Mn)_4c(Si)_4d and (Ir)_4a(Mn)_4c(As)_4d, respectively, and the exchange of elements in Wyckoff position 4c and 4d results in an identical (symmetry-related) phase. The half-Heusler compounds show half-metallic ferromagnetism with a half-metallic gap of 0.168 eV, 0.298 eV, 0.302 eV and 0.109 eV, respectively, and the total magnetic moments (M_tot) are 4.00 μ_B, 4.00 μ_B, 3.00 μ_B and 3.00 μ_B per formula unit, respectively, which agree well with the Slater–Pauling rule based on the relationship of valence electrons. The compound (Ir)_4a(Mn)_4c(As)_4d with half-metallic ferromagnetic character was reported for the first time. The individual elastic constants, shear modulus, Young's moduli, ratio B/G and Poisson's ratio were also calculated. The compounds are ductile based on the ratio B/G. The Debye temperatures derived from the average sound velocity (ν_m) are 327 K, 332 K, 434 K and 255 K, respectively. The predicted Debye temperature for NiMnSb agrees well with the available experimental value, and the Debye temperatures for the rest three compounds were reported for the first time.     

No miscibility gap in Pt-Rh binary alloys: A first-principles study

Recently, diffraction experiments and bond-order calculations gave given cause for distrust in the widely accepted phase diagram of binary Pt-Rh, suggesting it forms a solid solution below 1100 K. This is in contrast to the phase diagram published by Raub in 1959. In order to clarify this situation, we use a first-principles approach to conduct an exhaustive ground-state search and set up a cluster-expansion Hamiltonian. This allows for grand-canonical Monte-Carlo simulations in order to investigate the phase behavior. We show that the miscibility gap does not in fact exist. We also present simulated diffraction patterns extracted from the cluster-expansion Hamiltonian, which are in good agreement with experimental data.     

Phase stability and elastic moduli of Cr2Nb by first-principles calculations

The phase stability and elastic moduli of Cr₂Nb are investigated by first-principles calculations. Heats of formation are calculated and compared for the three Laves phases (C15, C14, and C36). It is found that the C15 phase is the ground-state structure with the lowest energy and the C36 phase is an intermediate state between C15 and C14. These three phases, however, are very close in energy, indicating low stacking fault energies in this system. For the ground-state C15 phase, we calculate three elastic constants from which the shear and Young’s moduli are obtained. It is found that these calculated moduli are smaller than the experimental values obtained from polycrystals.