Comparison of the electronic states of alloys from the coherent potential approximation and an order-N calculation

For historical reasons, the coherent potential approximation was originally proposed for isomorphous models of alloys in which all the atoms of a given species are assumed to have the same charge distribution. Order-N methods for density-functional theory local-density approximation calculations of the electronic structure for clusters of hundreds or thousands of atoms demonstrate that a polymorphous model, in which each atom is different, is more realistic. The predictive ability of isomorphous coherent potential approximations is studied by comparison with order-N calculations.     

Magnetic and transport properties in the Heusler series Ni2−xMn1+xSn affected by chemical disorder

Crystal structure, magnetic and transport characteristics of Ni_2−x Mn_1+x Sn Heusler series have been studied with the emphasis on chemical disorder effects. It is shown that the structure and the disorder character in these series can be predicted by using simple rules. Ni₂ MnSn is a ferromagnetic, congruent melting phase, which crystallizes cubic in the L2₁ structure type. By increasing x, Ni and Mn atoms randomly mix and occupy the heterocubic sites of the regular Heusler structure, and the magnetic structure becomes ferrimagnetic. The total magnetic moment m_sat decreases linearly in the range 0.2 ≤ x ≤ 1, while the Curie temperature T_C increases. At low Mn content (x < 0.2), the unit cell volume shows anomalous behavior, characterized by constant m_sat and T_C. Electrical resistivity, Seebeck coefficient, and thermal conductivity strongly depend on the amount of disorder, which increases with the Mn content. Results of first-principle calculations based on the coherent potential approximation (CPA) alloy theory for the magnetic and electrical properties are in reasonable agreement with the simple rules and all experimental data.     

The effects of substituting oxygen for terminal nitrogen in aniline oligomers 2: A DFT comparison of diamino,dihydroxyl, and monoamino/monohydroxyl terminated aniline trimers

The variant aniline trimers, N,N′-bis (3′,4′-diaminophenyl)-1,4-quinonediimine; N,N′-bis (3′-hydroxy-,4′-aminophenyl)-1,4-quinonediimine; and N,N′-bis (3′,4′-dihydroxyphenyl)-1,4-quinonediimine were synthesized, and the properties of the compounds compared to the results of density functional theory (DFT) calculations using the B3LYP functional. The calculations are complicated by the large number (five) of isomeric forms of each compound. Each compound energetically preferred the syn-, syn- (outer) isomer. The computed vertical ionization energies for the same isomer of each compound are 75.0, 76.10, and 83.1 kcal mol⁻¹, respectively. The trimer with two oxygen atoms on each outer ring thus holds electrons more tightly than the trimers with either one or zero oxygen atoms on the outer rings. The electronic spectra computed by the ΔSCF method are in excellent agreement with experimental values, though inclusion of solvent effects does not improve the predictions. A red-shift upon change to more polar solvents confirms that the principal absorption band for each trimer is π → π*, in agreement with electronic structure calculations.     

Mechanical alloying of Ni3Al with molybdenum and arrangement of Mo atoms in sublattices of the intermetallic

Mechanochemical synthesis (MS) of Ni₇₀Al₂₅Mo₅ (composition 1) and Ni₇₅Al₂₀Mo₅ (composition 2) mixtures, in which 5 at % Mo substitutes for the equal amount of Ni or Al, leads to the formation of Ni-based nanocrystalline (coherent domains are ～7–12 nm in size) solid solutions; in this case, some amount of molybdenum remains free. A comparison of the lattice parameters of solid solutions, which were determined experimentally, with the magnitudes determined theoretically using Vegard law and Bozollo-Ferrante simulation, which takes into account volume modules of elasticity of elements, showed an increase in interactions between atoms composed the solid solution and the formation of regions characterized by short-range order. The heating of mechanically synthesized three-component Ni(Al, Mo) solid solutions to 720°C in a calorimeter chamber forms the ordered γ′ phase (L1₂) at T ～ 450°C. An analysis of the ratio of relative intensities of superlattice and fundamental reflections showed that, whatever the composition of initial mixture, Mo atoms always occupy positions in the Al sublattice. This arrangement of Mo atoms was confirmed by calculations of coefficients of concentrational variations of the lattice parameters. When molybdenum is added to Ni₃ Al, Mo atoms, rather than Ni atoms, complete the Al sublattice. In this case, vacancies compensate for the lack of atoms in the Ni sublattice.     

Electronic structure of antiferromagnetic PbCrO3 (0 0 1) surfaces

Surfaces of cubic perovksite PbCrO₃ in (0 0 1) plane are investigated through density functional theory. The plane wave pseudopotential method is applied with generalized gradient approximation scheme. Hubbard U correction (GGA + U) is included in all calculations in order to simulate on-site Coulomb interactions between Cr-d states. Two types of terminations, namely, PbO- and CrO₂-terminations are considered in construction of the surfaces. Surfaces of both terminations show convergence at 9-layer slab geometry. The density of states calculations on the converged slab geometry yield a metallic behavior for both PbO- and CrO₂-terminations. Both metal atoms, Pb and Cr, in the uppermost layer of the respective terminations, have inward atomic relaxations much larger in magnitude than the oxygen atoms of the respective layer. However, Cr atoms which are labeled as up and down according to their spin orientation show different relaxations. The interlayer distance between the uppermost layer and the first one next to it decreases in both PbO- and CrO₂-terminated surface geometries. The calculations of the relative movement of the oxygen atom with respect to the Pb or Cr atom in each terminations give a positive rumpling in the uppermost layer.     

Electronic structure of UO<Subscript>2.12</Subscript> calculated in the coherent potential approximation taking into account strong electron correlations and spin-orbit coupling

Based on the coherent potential approximation, the method of calculating the electronic structure of nonstoichiometric and hyperstoichiometric compounds with strong electron correlations and spin-orbit coupling has been developed. This method can be used to study both substitutional and interstitial impurities, which is demonstrated based on the example of the hyperstoichiometric UO_2.12 compound. The influence of the coherent potential on the electronic structure of compounds has been shown for the nonstoichiometric UO_1.87 containing vacancies in the oxygen sublattice as substitutional impurities, for stoichiometric UO₂ containing vacancies in the oxygen sublattice and oxygen as an interstitial impurity, and for hyperstoichiometric UO_2.12 with excess oxygen also as interstitial impurity. In the model of the uniform distribution of impurities, which forms the basis of the coherent potential approximation, the energy spectrum of UO_2.12 has a metal-like character.     

First principles study of structural, electronic and magnetic properties of Mg1−xMnxTe alloys

Density functional FP-LAPW + lo calculations have been performed to study the structural, electronic and magnetic properties of Mg_1−xMn_xTe for compositional parameter x = 0.25, 0.50, 0.75 and 1. Our calculations reveal the occurrence of ferromagnetism in these compounds in which the transition-metal atom is ordered in a periodical way thereby interacting directly with the host atoms. Results extracted from electronic band structure and density of states (DOS) of these alloys show the existence of direct energy band gap for both majority- and minority-spin cases, while the total energy calculations confirm the stability of ferromagnetic state as compared to anti-ferromagnetic state. The total magnetic moment for Mg_1−xMn_xTe for each composition is found to be approximately 5 μ_B, which indicates that the addition of Mn content does not affect the hole carrier concentration of the perfect MgTe compound. Moreover, the s-d exchange constant (N₀α) and p-d exchange constant (N₀β) are also calculated which are in accordance with a typical magneto-optical experiment. The estimated spin-exchange splitting energies originated by Mn 3d states energies, i.e. Δ_X(s-d) and Δ_X(p-d), show that the effective potential for minority-spin is more attractive than that of the majority-spin. Also, the p-d hybridization is found to cause the reduction of local magnetic moment of Mn and produce small local magnetic moments on the nonmagnetic Mg and Te sites.     

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.     

Role of vibrational optical phonons in the heat capacity of K3C60

The reported heat capacity C(T) data of alkali doped fulleride K₃C₆₀ is theoretically investigated in the temperature domain 5 ≤ T ≤ 25 K. Calculations of C(T) have been made within the two component scheme: one is the phonon (optic and acoustic) and the other is electronic contribution. We begin with the intercage interactions between the adjacent C₆₀ cages and expansion of lattice due to the intercalation of alkali atoms based on the spring model to estimate vibrational optic and acoustic phonon frequencies from the dynamical matrix for the intermolecular alkali-C₆₀ phonon mode. Lattice specific heat is well estimated from the Debye and Einstein approximation. Fermionic component as the electronic specific heat coefficient is deduced using the band structure calculations for metallic phase. Comparison of the coefficient of the normal state electron contribution to C with band structure calculations gives an estimate of the electron–phonon coupling strength. It is notice that electron correlations are essential to enhanced density of state over simple Fermi liquid approximation in the metallic phase. The present numerical analysis of specific heat shows dominating role of vibrational optical phonons.     

Changes in the potential energy upon the formation of vacancies in the volume and in the cores of crystallite-conjugation regions in cubic polycrystalline metals

Changes in the potential energy of atoms that constitute the nearest neighborhood of vacancies formed in the bulk of d transition and precious cubic metals have been determined. These changes agree with the available first-principles calculations of changes in the potential energy of atoms of the nearest neighborhood of vacancies. In the cores of crystallite-conjugation regions (CCRs) of bcc polycrystalline d transition metals, the formation of vacancies is accompanied by positive changes in the potential energy of atoms of their nearest neighborhood. The absolute magnitudes of these changes are several times less than the changes in the potential energy of atoms of the nearest neighborhood of vacancies in the bulk of these metals, in accordance with the relationship between the enthalpies of formation of vacancies in these regions of polycrystals. The changes in the potential energy of atoms of the nearest neighborhood of vacancies formed in the cores of CCRs of fcc polycrystalline metals are negative because of the split structure of vacancies in the CCR cores of such metals.     

Effect of chemical interaction on the stability of metal clusters in FCC metals

Structural stability of small Ni, Al, and Au metal clusters with a number of atoms close to N = 55 and 147 has been studied by the method of molecular dynamics with the use of realistic potentials of interatomic interaction. It has been shown that in Ni, in which the icosahedral configuration is most stable, the mass spectrum predominantly contains peaks, which correspond to N = 55 and 147. At the same time, for Au and Al clusters, the consequence of magic numbers differs from that specified by the close packing of atoms, and its realization depends on experimental conditions. The results obtained allow concluding that the position of peaks in the mass spectrometric experiments with small clusters is determined by morphological features of the structural state, which depend on the character of interatomic interaction.     

Investigation on poly(vinylene sulphide): Ab initio energy band structure,use of effective core potentials,lattice sum truncation

The energy band structures of two structural isomers of poly(vinylene sulphide) are calculated using the ab initio Hartree-Fock crystal orbital method. The basis set dependence and the convergence of quantities like the total energy per elementary cell and the atomic electron population with the number of interactions with neighbours are investigated. The band structure calculations are repeated employing effective core potentials first for sulphur atoms only and then for all non-hydrogen atoms. The results of the valence-electron only and the mixed computations agree very well with the all-electron calculations. A lattice sum truncation scheme, based on symmetric and electrostatically-balanced cut-off radii, is investigated. It turns out that the convergence of the total energy per elementary cell is very slow using these cut-off schemes, whereas the electronic and energetic properties rapidly converge in the strict interaction with neighbours approximation. Part of the explanation is found in the incorrect treatment of the nuclear-nuclear repulsion terms, and the inconsistent handling of the electron-nuclear repulsion integrals.     

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.     

First-principles study of electronic structure and magnetic properties of orthorhombic CrGa2Sb2 and MnGa2Sb2

By the first-principles calculations, we present the results of electronic structure and magnetic properties on bulk CrGa₂Sb₂ and MnGa₂Sb₂ in an orthorhombic structure with the linear chains of transition-metal Cr and Mn atoms, using four different exchange correlation potentials: the local density approximation (LDA), the generalized gradient approximation (GGA), GGA + U, and the Tran-Blaha modified Becke-Johnson functional (mBJ). The electronic structure calculations from four exchange correlation potentials show that CrGa₂Sb₂ is a pseudogap (negative gap) material with very small density of states (DOS) at the Fermi level, while MnGa₂Sb₂ has notably higher DOS at the Fermi level compared to CrGa₂Sb₂, exhibiting stronger metallic conductivity, although the mBJ potential obtains lower DOS at the Fermi level than LDA and GGA for both CrGa₂Sb₂ and MnGa₂Sb₂. The GGA + U method with a small value (1 eV) of the on-site Coulomb interaction parameter U obtains lower DOS at the Fermi level compared to the large value of U. In agreement with the measurement data, the total energy calculations reveal that both CrGa₂Sb₂ and MnGa₂Sb₂ have a stable ferromagnetic ground state with lower energies relative to antiferromagnetic state. Based on the Heisenberg model, the magnetic exchange constants between the nearest-neighbor Cr–Cr and Mn–Mn along transition-metal linear chains are calculated to be 48.6 meV and 27.5 meV for CrGa₂Sb₂ and MnGa₂Sb₂, respectively. By the mean-field approximation method, we calculated the Curie temperature of two compounds to be above room-temperature.     

Calculation of the electronic structure of the vanadium dioxide VO<Subscript>2</Subscript> in the monoclinic low-temperature phase <Emphasis Type="Italic">M</Emphasis><Subscript>1</Subscript> using the generalized transition state method

It is known that the ground state of the vanadium dioxide in the low-temperature monoclinic phase M ₁ is a nonmagnetic insulator. The calculations in the local-density approximation (LDA) predict the metallic nonmagnetic state, whereas the calculations in terms of the LDA + U approach (local-density approximation with explicit allowance for on-site Coulomb correlations U) predict the insulating antiferromagnetic state. In terms of the method of generalized transition state, the nonmagnetic insulating state of VO₂ in the M ₁ phase with a band gap of 0.3 eV has been reproduced for the first time.     

First-principles investigation of dual substitutional impurity-induced electronic structural modulation of PbTe on cationic and anionic sites

In view of recent experimental results, enhanced thermoelectric performance can be achieved in PbTe via dual doping at both cationic and anionic sites. However, little information is available for a deep understanding of how the various dual dopants tune the electronic structures of PbTe. In this work, the lattice geometry and band structures of (M, N) (M = {K, Ag, Ge, Sn, Sb, Bi}, N = {S, Se, I}) dually doped pairs are systematically clarified using first-principles calculations. The results indicate that the dual-dopant pairs tend to cluster in PbTe and impose a remarkable effect on the band structures near the band gap by the splitting of bands. The alkali K re-enlarges the nearly closed band gap in S- or Se-doped PbTe, while Ag introduces additional bands near the Fermi level in both solely and dually doped configurations. Unexpectedly, (Ag, S), (Ge, Se), (Sn, S) and (Sn, Se) co-doping produce camel’s-back-like structures with multiple extrema due to abnormal bending of bands near the band gap, which are anticipated to enhance the power factor of PbTe. Similarly, the Sb- and Bi-induced bands are also bent on the bottom of the conduction band. The band gaps in (Sb, S), (Sb, Se) and (Bi, N) are closed to drive the system towards metallicity. However, a re-opened band gap is obtained in the SbI-3 doping configuration. Supplementary calculations on larger supercells for the Ag–S doping configuration indicate that the single (Ag, S) dual-dopant pair behaves in a similar way to that in the small supercells with lower local atomic relaxation, weak band splitting and reduced band gap. The formation of dual-dopant nanoclusters can widen the band gap due to the introduction of large local strain in the vicinity of clusters. The impurity cluster size exerts a prominent effect on the band-edge states of PbTe. Our calculations reveal that it is possible to intentionally modulate the band structures of PbTe by appropriate collocation of dual-doping atoms for improved thermoelectric properties.     

Symmetry analysis of the magnetic structure of Mn3Sb

To estimate the reliability of the previously published results concerning the magnetic structure of Mn₃Sb, a symmetry analysis of possible magnetic structures characterized by a wave vector k ₁₂ = 0 has been performed. In this work, we quite briefly describe and apply the previously developed technique for the calculations of the basis functions of the irreducible representations of the space group O _h ¹ (Pm $\bar 3$ m) entering into the composition of the magnetic representation with the above wave vector. Results of the calculations of these basis functions for the Mn atoms located in 3c positions are given and discussed in detail. A conclusion is made that the magnetic moments of Mn atoms in this intermetallic compound can be oriented along any crystallographic direction with the formation of collinear or canted magnetic structures. The total magnetic moments of separate Mn atoms in the unit cell of the crystal structure can coincide or differ in magnitude.     

On the effect of hydrogen on plastic instabilities in metals

Experimental observations and theoretical calculations have demonstrated that hydrogen solute atoms increase the dislocation mobility in metals and alloys, thus promoting highly localized plastic processes which eventually lead to localized ductile rupture. While the underlying mechanism for hydrogen-enhanced dislocation mobility is well understood, little is known on how this mechanism acting at the microscale can lead to macroscopic plastic instability. In this paper, a theoretical investigation is carried out in a specimen under plane-strain tension in an effort to understand how hydrogen-induced softening and lattice dilatation at the microscale can lead to macroscopic i) shear localization (shear banding bifurcation) or ii) necking bifurcation.     

The effect of variable chain length on the electronic properties of electroluminescent polymers

We have used self-consistent molecular dynamics calculations with semi-empirical quantum chemistry at the complete neglect of differential overlap (CNDO) level to discuss some of the issues relating to the electronic processes involved in electroluminescence of a particular conjugated polymer: poly(p-phenylene-vinylene) (PPV). Specifically addressed are the effects of chain length and molecular charge on the electronic properties of individual PPV strands, such as chemical potential, charge induced defects and charge rearrangement among the polymer atoms. The effect of high electric fields on charge-induced defects already formed is also discussed.