Calculated Equilibrium Volume Research Articles

Structural properties, electronic band structure, magnetic and mechanical characteristic of XFeGe (X = Co, Cr, Ni) half Heusler compounds: Insights from DFT calculation

The structural, electronic, magnetic and thermodynamic properties of XFeGe (X = Co, Cr, Ni) half Heusler compounds are studied using first-principles calculation within density functional theory in five different atomic configurations. For each configuration, their optimized lattice constants are determined. It showed that the configuration of type 3 is energetically the most stable. From the spin-polarized band structure and density of states, it is found that CoFeGe alloy exhibits a half-metallic character with integer magnetic moments of 3.00μBper formula unit at their equilibrium volume, whereas NiFeGe and CrFeGe exhibit a metallic character. These results indicate that the magnetic moment mainly originates from the Fe atoms. The calculated equilibrium volume for the three compounds CoFeGe, CrFeGe, and NiFeGe at various temperatures is predicted. The heat capacity at constant volumeCVhas been calculated as a function of temperature and pressure. Additionally, the calculated elastic constants for the three compounds confirm the mechanical stability criteria for cubic systems.

Structural and Thermodynamic Properties of the High-Entropy Alloy AlCoCrFeNi Based on First-Principles Calculations

During the past two decades, the high-entropy alloy AlCoCrFeNi has attracted much attention due to its outstanding thermal and mechanical properties under ambient conditions. However, the exploration on the thermodynamic properties of this alloy under high temperatures and high pressures is relatively insufficient. Combining structural modeling with the similar atomic environment (SAE) method and first-principles simulations with the modified mean-field potential (MMFP) approach, we studied the lattice and magnetic structure as well as the thermodynamic properties of the body-centered-cubic AlCoCrFeNi, through supercell simulations. AlCoCrFeNi was found to display a strong local lattice distortion compared with typical 3d high-entropy alloys; the ferromagnetic structure stable at 0 K was predicted to transform to the paramagnetic structure at the Curie temperature TC = 279.75 K, in good agreement with previous calculations; the calculated equilibrium volumes, bulk modulus, and shock Hugoniot all agree well with available experimental data and other theoretical values. These results demonstrate the validity and reliability of our methods used to study the dynamic properties of AlCoCrFeNi, providing a promising scheme for accessing the dynamic properties of sophisticated high-entropy alloys.

Electronic Structure of Intermetallic Antiferromagnet GdNiGe

The electronic structure of the GdNiGe ternary intermetallic compound was investigated in this work. We carried out the spin-polarized DFT+U calculations of its band structure within generalized gradient approximation accounting for strong electronic correlations in the 4f-shell of gadolinium ions. The antiferromagnetic ordering was reproduced in the calculations, in agreement with experimental data. The calculated equilibrium volume is within 2% accuracy to the experimental crystal structure data, which demonstrates the reliability of the method chosen. The 4f-shell of Gd was demonstrated to substantially contribute to the spectral and magnetic properties of the GdNiGe compounds, whereas other ions were found nonmagnetic, in agreement with experimental data.

Prediction on elastic properties of Nb-doped Ni systems

ABSTRACTOn the basis of first-principles simulation, the structure, formation enthalpy and mechanical properties (elastic constant, bulk and shear modulus and hardness) of five Nb-doped Ni systems are systematically studied. The calculated equilibrium volume increases with the Nb concentration increasing. The computational elastic constants and formation enthalpy indicate that all Nb-doped Ni systems are mechanically and thermodynamically stable in our research. The hardness of these systems was predicted after the bulk modulus and shear modulus had been accurately calculated. The results show that the hardness increases with the Nb concentration increasing when the Nb concentration was below 4.9%, beyond which the hardness will decrease; this is within the scope of our study.

Ground-state properties of rare-earth metals: an evaluation of density-functional theory

The rare-earth metals have important technological applications due to their magnetic properties, but are scarce and expensive. Development of high-performance magnetic materials with less rare-earth content is desired, but theoretical modeling is hampered by complexities of the rare earths electronic structure. The existence of correlated (atomic-like) 4f electrons in the vicinity of the valence band makes any first-principles theory challenging. Here, we apply and evaluate the efficacy of density-functional theory for the series of lanthanides (rare earths), investigating the influence of the electron exchange and correlation functional, spin-orbit interaction, and orbital polarization. As a reference, the results are compared with those of the so-called ‘standard model’ of the lanthanides in which electrons are constrained to occupy 4f core states with no hybridization with the valence electrons. Some comparisons are also made with models designed for strong electron correlations. Our results suggest that spin–orbit coupling and orbital polarization are important, particularly for the magnitude of the magnetic moments, and that calculated equilibrium volumes, bulk moduli, and magnetic moments show correct trends overall. However, the precision of the calculated properties is not at the level of that found for simpler metals in the Periodic Table of Elements, and the electronic structures do not accurately reproduce x-ray photoemission spectra.

Concentration-dependent crystal structure, elastic constants and electronic structure of Zr x Ti1−x alloys under high pressure

The physical properties of Zr x Ti1−x (x = 0.0, 0.33, 0.5, 0.67, 0.75 and 1.00) alloys were simulated by virtual crystal approximation (VCA) methods which is generally used for disordered solid solutions modeling. The elastic constant, electronic structure and thermal Equation of state (EOS) of disordered Zr x Ti1−x alloys under pressure are investigated by plane-wave pseudo-potential method. Our simulations reveal increasement of variations of the calculated equilibrium volumes and decreasement of Bulk modulus as a function of the alloy compositions. Lattice parameters a and c of alloys with different Zr concentrations decrease linearly with pressure increasing, but the c/avalues are increasing as pressure increases, indicating no phase transitions under pressure from 0 GPa to 100 GPa. The elastic constants and the Bulk modulus to the Shear modulus ratios (B/G) indicate good ductility of Zr, Zr0.33Ti0.67, Zr0.5Ti0.5, Zr0.75Ti0.25 and Ti, but the Zr0.67Ti0.33 alloy is brittle under 0 K and 0 GPa. The metallic behavior of these alloys was also proved by analyzing partial and total DOS.

First-principles study of the structural properties and magnetism of R2CoIn8 (R=Y, Pr, Nd, and Dy) intermetallic compounds

The electronic structure of R2CoIn8 (R=Y, Pr, Nd, and Dy) intermetallic compounds is calculated from first principles based on the density functional theory (DFT). The Kohn–Sham single-particle equations of the DFT are solved using two independent computational methods, namely APW+lo and FPLO. First the structural properties of Y2CoIn8 are studied. Good agreement of calculated equilibrium volume and c/a ratio with the experimental data is found. Also we minimize the forces at equilibrium volume and calculate the symmetry-free structural parameters of the space group P4/mmm for Y2CoIn8. In Y2CoIn8, the Co 3d states are almost fully occupied and situated below the Fermi level. We applied the fixed-spin-moment method and a stable paramagnetic ground state for Y2CoIn8 was found. Finally, the crystal field (CF) parameters were calculated for R=Pr, Nd and Dy from first principles. The microscopic tetragonal CF Hamiltonian was diagonalized and the obtained eigenvalues and eigenfunctions were used to predict the anisotropy of the magnetic properties of R2CoIn8 single crystals.

Why UFe xAl 12− x phase does not crystallize with ThMn 12-structure type, when x = 2?

The crystallographic structure of recently reported orthorhombic UFe 2Al 10 phase, which belongs to the YbFe 2Al 10-structure type is different from the well known tetragonal UFe x Al 12− x (3 < x < 8) having ThMn 12-structure type. Comparative Density Functional Theory (DFT) study of the relative stability of the orthorhombic UFe 2Al 10 phase with respect to different models of tetragonal ThMn 12-type structures with the same composition is carried out to explain this unusual phenomenon. It is shown that the fine interplay between the overlapping of 3d-states of Fe and 5f-states of U in the region −2.5 ÷ −0.3 eV below the Fermi energy dictates the lower total energy of orthorhombic UFe 2Al 10 phase in comparison with tetragonal UFe 2Al 10 phase crystallized in ThMn 12 structure. On the example of UFe 2Al 10 it is also directly demonstrated that DFT calculations in the framework of the Full Potential + Linear Augmented Plane Wave method within the local density approximation may be used for refining the structure of complicate compounds. The calculated equilibrium volume, lattice parameters and atomic Wyckoff positions of UFe 2Al 10 intermetallic compound are in very good agreement with experimental data.

Theoretical investigations of the physical properties of tetragonal CaSiO3 perovskite

Abstract The first principles study of the physical properties of tetragonal CaSiO 3 perovskite is reported using the pseudopotential plane wave method within the local density approximation. The calculated equilibrium volume is in good agreement with the available experimental data. A complete elastic tensor of the tetragonal CaSiO 3 perovskite is determined in the wide pressure range. The geologically important quantities: sound velocity, Young’s modulus, Poisson’s ratio, and crystal anisotropy, are derived from the calculated data. A systematic decrease of crystal’s anisotropy with pressure is noticed except symmetry plane (001). Finally, by using a quasiharmonic Debye model, the Debye temperature, the heat capacity, the coefficient of thermal expansion, and the Gruneisen parameter are also obtained in the present work.

Effect of external pressure on the Fe magnetic moment in undoped LaFeAsO from density functional theory: Proximity to a magnetic instability

We investigate the effect of external pressure on the Fe magnetic moment in undoped LaFeAsO within the framework of density-functional theory and show that this system is close to a magnetic instability. The Fe moment is found to drop by nearly a factor of 3 within a pressure range of $\ifmmode\pm\else\textpm\fi{}5\text{ }\text{GPa}$ around the calculated equilibrium volume. While the Fe moments show an unusually strong sensitivity to the spin arrangement (type of antiferromagnetic structure), the low-temperature structural distortion is found to have only a minor influence on them. Analysis of the Fermi-surface topology and nesting features shows that these properties change very little up to pressures of at least 10 GPa. We discuss the magnetic instability in terms of the itinerancy of this system.

Phase transition, and elastic and thermodynamic properties of InN derived from first‐principles and the quasi‐harmonic Debye model

AbstractThe transition phase of InN from wurtzite (WZ) structure to rocksalt (RS) structure is investigated via the minimization of the total energy within Local Density Approximation (LDA) based on Density Functional Theory (DFT), the calculated equilibrium volume are in agreement with available experimental value and other calculated data. In the meantime, the thermodynamic properties of different phase under high temperature and high pressure are investigated applying nonempirical Debye model combining with the first principle theory in the quasi‐harmonic approximation. The transition phase from WZ structure to RS structure occurs at the pressure of 10.1 GPa, which agrees well with experimental value. The evaluated equilibrium volume using this model agrees with the value obtained from ab intio and from experiment. The results demonstrate that this method can provide reliable predictions for the temperature and pressure dependence of these quantities such as the equation of state (EOS), the isothermal bulk modulus, the heat capacity, and the thermal expansion in detail. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

DFT studies on the thermodynamic properties of Na2Ti3O7 under high temperature and high pressure

AbstractThe equilibrium structure of the compound Na2Ti3O7 has been obtained via the minimization of the total energy within Local Density Approximation (LDA) based on Density Functional Theory (DFT), the calculated equilibrium volume are in agreement with available experimental value. In the meantime, the thermodynamic properties under high temperature and high pressure are investigated applying nonempirical Debye model combining with the first principle theory in the quasi‐harmonic approximation. The evaluated equilibrium volume using this model agrees with the value obtained from ab intio and from experiment. The results demonstrate that this method can provide reliable predictions for the temperature and pressure dependence of these quantities such as the equation of state (EOS), the entropy, the isothermal bulk modulus, the heat capacity, and the thermal expansion in detail. In addition, the calculated thermodynamic properties are all in agreement with available experimental data. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Self-interaction-corrected local spin density theory of5felectron localization in actinides

The electronic structures of the actinide elements U, Np, Pu, Am, Cm, and Bk are investigated within the self-interaction-corrected local spin density approximation. This method allows us to describe a dual character of the $5f$ electrons, some of which occupy localized and corelike states, while the remaining $5f$ electrons hybridize and form bands. Based on energetics, the calculations predict delocalization and/or paramagnetism in the early actinides and localization and/or antiferromagnetism in the later actinides. The corresponding calculated equilibrium volumes are in agreement with the experimental values. For Pu and Am, the method wrongly predicts magnetic ordering, but we find that the paramagnetic state gives a better description of cohesive properties. Under compression, in the later actinides, a localization-delocalization transition happens gradually as more and more $f$ electrons become bandlike with decreasing volume. Pu is already at this transition point at ambient conditions. Delocalization sets in for Am and Bk at a compression of $V\ensuremath{\sim}0.75{V}_{0}$, for Cm at $V\ensuremath{\sim}0.60{V}_{0}$, where ${V}_{0}$ is the equilibrium volume, and the transition is complete for $V\ensuremath{\sim}(0.4--0.5){V}_{0}$ in these three elements.

On mechanical stability of molybdenum

Investigations of the strength properties of materials under different loading conditions are of practical importance in many engineering applications. The knowledge of elastic moduli as a function of strain is required for determination of strength properties. In the present work, we have determined the elastic moduli of molybdenum through first principles study of the energy changes under three different loading conditions namely ‘uni-axial tensile deformation’ along [0 0 1] direction, ‘uni-axial tensile loading’ along [0 0 1] direction and ‘hydrostatic tensile loading’. The stability conditions for the system are expressed in terms of the elastic moduli and analyzed along the deformation paths corresponding to these three loading modes. The theoretical spall strength (σS), tensile strength (σT) along [0 0 1] direction and hydrostatic tensile strength (σH), are evaluated as a stress at the first onset of the instability for three loading conditions, respectively. The calculated equilibrium volume and elastic moduli are compared with that reported from experimental and other theoretical works.

First principle study on phase stability and electronic structure of YCu

The phase stability and electronic structure of YCu were studied by self-consistent full-potential linearized augmented plane wave method (FP_LAPW) on the basis of the density functional theory (DFT). The calculated equilibrium volumes are 41.963 and 173.21Å3 for B2 and B27 structures respectively, which are in good agreement with the experimental values. The total energy of the B27 phase is about 0.03 eV lower than that of the B2 phase. The formation energies are −1.173 and −1.204eV for B2 and B27 structures respectively. The density of state at the Fermi energy, N(EF), is 1.08states/eV for B2 phase and 0.92states/eV for B27 phase, respectively. These results indicate that the B27 phase is the thermodynamic ground state equilibrium phase of YCu at low temperatures, as observed experimentally. However, our calculations also predict that a pressure-induced B27 to B2 phase transition exists in YCu.

First‐principles study of density, viscosity, and diffusion coefficients of liquid MgSiO3 at conditions of the Earth's deep mantle

Constant‐pressure constant‐temperature ab initio molecular dynamics simulations at high temperatures have been used to study MgSiO3, the major constituent of the Earth's lower mantle to conditions of the Earth's core‐mantle boundary. The calculated equilibrium volumes and densities are compared with simulations using an orthorhombic perovskite configuration under the same conditions. For molten MgSiO3, we have determined the diffusion coefficients and shear viscosities at different thermodynamic conditions. Our results provide new constraints on the properties of molten MgSiO3 at conditions near the core‐mantle boundary. The volume of the liquid is greater than that of the solid throughout the pressure‐temperature conditions examined, and the volume change on fusion ranges from 5% at 88 GPa and 3500 K to 2.9% at 120 GPa and 5000 K. Existing experimental constraints on solid‐liquid partition coefficients for Fe suggest that Fe is preferentially partitioned into the liquid. Such enrichment of Fe increases the density of the liquid, thus allowing the possibility of negatively buoyant melts from (Mg,Fe)SiO3 perovskite compositions at deep lower mantle conditions for plausible values of solid‐liquid partition coefficients for Fe. At 120 GPa and 4500–5000 K, the diffusion coefficient of liquid MgSiO3 is 2–3 × 10−5 cm 2/s and the diffusion rates of the different chemical species are similar. The shear viscosity is estimated using Zwanzig's formula to be 19–31 cP under these conditions. On the basis of our calculated diffusivities, MgSiO3 is above the glass transition temperature at 120 GPa and 4500 K.

Ab initio study of Nb 2SC and Nb 2S 2C: Differences in coupling between the S and Nb–C layers

Using ab initio calculations, we have studied the structurally related compounds Nb 2SC and Nb 2S 2C. In Nb 2S 2C (space group R 3 ¯ m , prototype Bi 2Te 3), S atoms are nearest neighbours, while in Nb 2SC (space group P6 3/ mmc, prototype Cr 2AlC) this is not the case. The calculated equilibrium volume for these two phases deviates by 1.6–3.7% to previously-published experimental data and the bulk modulus-to- c 44 ratios obtained are 1.5 and 5.9, respectively. These results indicate a resemblance of Nb 2S 2C to hexagonal BN and graphite. Furthermore, we have demonstrated that the uniform compression method is adequate for estimating the elastic properties of Nb 2SC, a so-called MAX phase. It is our ambition that these calculations will stimulate further experimental research on these compounds.

Ab initiocalculation of the elastic properties ofAl1−xLix(x⩽0.20)random alloys

Ab initio total energy calculations, based on the exact muffin-tin orbital (EMTO) theory, are used to determine the elastic properties of ${\mathrm{Al}}_{1\ensuremath{-}x}{\mathrm{Li}}_{x}$ random alloys $(x\ensuremath{\leqslant}0.20)$ in the face-centered-cubic crystallographic phase. The compositional disorder is treated within the framework of the single-site coherent potential approximation (CPA). The effect of the local lattice relaxation on the elastic constants is estimated using a supercell technique. We study the effect of the single-site approximation by comparing the theoretical ground-state properties calculated using different corrections to the Madelung energy. We find that the calculated equilibrium volumes and alloy formation energies strongly depend on the approximations employed in the Poisson equation, in accordance with former observations. At the same time, the experimental trends of the elastic moduli of disordered $\mathrm{Al}\penalty1000-\hskip0pt\mathrm{Li}$ alloys are well reproduced by the EMTO-CPA method. Using these theoretical results we show that the nonlinear effect of Li addition on the elastic constants originates from the detailed band structure of Al near the Fermi level.

LSDA+U Calculations of UIrAl and UPtAl

Ternary uranium compounds UPtAl and UIrAl both crystallize in the layered ZrNiAl structure. Despite of the same structure and the fact that Pt and Ir are neighbors in periodic table, these compounds display rather different magnetic properties. By electronic structure calculations in the framework of density-functional theory (DFT) with LSDA+U exchange-correlation functional, we reproduced uranium magnetic moments and improved the agreement between calculated equilibrium volume and experiment for both compounds in comparison to existing calculations based on relativistic LSDA with or without orbital-polarization corrections.

The Electronic Structure of MgY&lt;SUB&gt;2&lt;/SUB&gt;H&lt;SUB&gt;6+&amp;delta;&lt;/SUB&gt; High-Pressure Hydride

The electronic structure of the MgY2H6+δ high-pressure hydride has been investigated by means of the full potential linearized augmented plane wave method, and the stability of the octahedral and tetrahedral sites for H atoms has been discussed. The calculation models used were MgY3H8–12, corresponding to H/M = 2–3. From the calculation of the density of states for MgY3H11, it was found that the states originating from H-s and Y-d mainly formed valence and conduction bands, respectively. In the valence band, bonding states originating from H-s of tetrahedral sites, Y-d, Y-p, and Mg-p can be observed in a wide energy range, especially between −3∼−5 eV below the Fermi level. In addition, the contour plots of the valence electron charge density revealed that the bonding between Y and H atoms on the tetrahedral sites was predominantly covalent, while the H atom on the octahedral site showed a weak interaction with Y and H atoms on the tetrahedral sites. The cell volume optimization indicated that the calculated equilibrium volume linearly increased with increasing the number of vacancies on the octahedral sites, while it decreased in the case of introducing the vacancy on the tetrahedral site. From these observations, it can be concluded that the H atoms on the tetrahedral sites seem to be responsible for holding its crystal structure, while ones on the octahedral sites had a certain level of contribution to enhancing the bond strength. These observations support the experimental results that the crystal structure of MgY2H6+δ is stable even after the partial desorption of hydrogen, which presumably occupy the octahedral sites, at around 600 K.