Atomistic simulations of [001] symmetric tilt grain boundaries in Ni, Al, and Ni3Al are presented. The atomistic structures of the simulated grain boundaries have been analyzed in terms of the structural unit model, which is found to be of limited utility for intermetallics. Simulation results show that boron segregates more strongly to grain boundaries than to free surfaces, and strengthens the grain boundary. Good cohesive properties of the grain boundaries occur when both boron and some segregated Ni are present. The Ni and B are found to co-segregate to the Ni3AI boundary with an energy advantage of ∼ 0.5eV.

Intermetallic compounds have been extensively studied because of their superior strength, low creep rate, and high melting point [1,2]. However, room temperature ductility for the L12 and B2 phases are a continuing problem. Both L12 Ni3Al [3,4] and B2 NiAl [5,6] exhibit an intergranular fracture mode. Understanding grain boundaries in these materials is of particular importance since intergranular fracture limits the applicability of these otherwise promising materials. In an effort to understand the fracture mechanism, we have used embedded atom potentials [7] to study the properties of Ni 3Al [8,9,10] and NiAl [11]. We also consider the effect of boron, sulfur, and nickel segregation on the strength of grain boundaries in Ni3Al and NiAl. Many of the results presented here appear in literature elsewhere [8,9,10,11].

The augmented-plane-wave (APW) method was used with a spin-dependent potential to calculate the one-electron energies in solid sodium. The charge density from which this potential was obtained was derived from spin-polarized Hartree-Fock-Slater atomic wave functions with fractional occupation of the valence spin orbitals. The atoms were placed on a cesium chloride Bravais lattice with alternating spin density. This “antiferromagnetic” arrangement was maintained with a decreasing moment as the calculation was carried to self-consistency. The possibility of this state having a lower total energy than the “paramagnetic” state is discussed. A “spin-density-wave” potential was introduced into the Hamiltonian and a perturbation calculation was performed. The unperturbed functions used in this calculation were self-consistent solutions of the “paramagnetic” state. By fitting the bands so obtained to the above “antiferromagnetic” bands the amplitude of the “spin-density-wave” in that configuration was determined. The possibility of obtaining a state with a lower total energy by using a “spin-density-wave” with this amplitude and varying wavelength is discussed.

THE crystal structures of the light actinides have intrigued physicists and chemists for several decades(1). Simple metals and transition metals have close-packed, high-symmetry structures, such as body-centred cubic, face-centred cubic and hexagonal close packing. In contrast, the structures of the light actinides are very loosely packed and of low symmetry-tetragonal, orthorhombic and monoclinic. To understand these differences, we have performed total-energy calculations, as a function of volume, for both high- and low-symmetry structures of a simple metal (aluminium), a non-magnetic transition metal (niobium), a ferromagnetic transition metal (iron) and a light actinide (uranium). We find that the crystal structure of all of these metals is determined by the balance between electrostatic (Madelung) interactions, which favour high symmetry, and a Peierls distortion of the crystal lattice, which favours low symmetry. We show that simple metals and transition metals can adopt low-symmetry structures on expansion of the lattice; and rye predict that, conversely the light actinides will undergo transitions to structures of higher symmetry on compression.

We present first-principles calculations of the equilibrium volumes and crystal structures of the light actinides (ThPu). The calculated equilibrium volumes for f.c.c. Th, b.c.t. Pu, α-U, and β-Np are found to agree reasonably well with the experimental data, and when comparing the total energies of the b.c.c., f.c.c., b.c.t., α-U, and β-Np structures we obtain the correct crystal structures for all studied systems. Also, the calculated equilibrium volumes for ThPu, using a hypothetical f.c.c. structure, have been calculated and it is demonstrated that although spin—orbit coupling is included in these calculations the calculated equilibrium volume of Pu is smaller than for Np, in disagreement with experiment. Moreover, the calculated tetragonal elastic constant, C′, is shown to be negative for b.c.c. U, b.c.c. Np, b.c.c. Pu and f.c.c. Pu. Thus, our zero temperature calculations suggest that the b.c.c. structure is unstable for these elements and that f.c.c. Pu is also unstable. This is in conflict with experiment and we are led to the conclusion that temperature effects must be of crucial importance for stabilizing cubic structures in U, Np, and Pu. Further, as a function of decreasing volume we predict a crystal structure sequence f.c.c. → b.c.t. → f.c.c. in Th, a sequence α-U → b.c.t. → b.c.c. in U, and a sequence β-Np → b.c.t. → b.c.c. in Np. Also, a sequence of transitions in Sc as a function of decreasing volume have been calculated, namely h.c.p. → f.c.c. → ω → β-Np → b.c.c.

The total energy has been calculated as a function of volume for various crystal structures in neptunium metal. Around the equilibrium volume the experimentally observed low-temperature structure, alpha -Np, is found to have the lowest energy. As a function of compression we predict Np to undergo a crystallographic phase transition from alpha -Np to sec. The zero-temperature calculations also show that at volumes close to the alpha -Np to BCC transition the BCT structure is very close in energy, and high-pressure experiments performed at elevated temperatures on Np may allow observation of this structure as well.

Based on all-electron self-consistent fully relativistic Korringa-Kohn-Rostoker coherent potential approximation results, concentration- and lattice-parameter-dependent effective pair interactions up to the fifth nearest neighbors are calculated in terms of the fully relativistic embedded cluster method for the Au-Pd system. The obtained effective pair interactions are then used to evaluate a simplified (V 1 , V 2 ) phase diagram, ordering energies, and related quantities

The crystal structures of some selected f band metals (Ce, Th, Pa, U and Am) have been calculated from first principles. It is demonstrated that the onset of low symmetry structures correlates with the increasing f occupation of the valence states. It is also shown that pressure is an important parameter for these types of studies, and that the occupation of the various orbitals of the valence band depends on it. One can thus modify (increase) the f occupation of a certain system, simply by applying a pressure. Since the crystal structure depends on the f occupation, for these systems, one can drive different crystal structures as function of pressure. We present one such example, the FCC→BCT phase transition at ∼ Mbar for Th.

Accurate full-potential, relativistic, total energy electronic structure calculations have been carried out over a range of volumes for selected rare-earth and actinide elements in crystal structures experimentally observed in these elements. Correct zero temperature crystal structures are obtained, and calculated equilibrium properties are in reasonable agreement with experiment. Results of these calculations suggest that the unusual equilibrium crystal structures occurring in the light actinides are characteristic of narrow band metals in general and that the light actinides will exhibit structural phase transitions to high symmetry structures under compression. A balance between one-electron bandwidths and band fillings and the electrostatic energy of the crystal lattice produces a tendency toward high-symmetry structures at broad bandwidths and low-symmetry structures at narrow bandwidths. Allowing for expansion as well as contraction, simple metals, transition metals and actinides can be ``stabilized`` in low- and high-symmetry crystal structures.

Using the all-electron fully relativistic Karringa-Kohn-Rostoker coherent-potential-approximation method self-consistent total-energy calculations were performed for the system Au/Pd. The calculated equilibrium lattice constants are in fairly good agreement with the experimental data. In particular, microscopic reasons for the breaks in the variation of the lattice constants with concentration can be derived from the results. Since the calculations were carried out self-consistently for a maximum angular momentum quantum number of 2 as well as 3, an interesting quantitative comparison for the alloy total energies, spectral densities, and properties related to the Fermi energy or Fermi surface can be presented.

The electronic structure of the semiconducting YBiPt intermetallic compound is compared with the isostructural, but metallic, YbBiPt compound. The difference in the transport properties is found to be governed by the volume, since the smaller volume in YbBiPt results in broader Yb d and Pt d bands and a closing of the semiconducting gap. The closing of the gap in YbBiPt is partly a relativistic effect, since scalar relativistic calculations do not reproduce the metallic ground state, whereas fully relativistic (Dirac) calculations do. In the semiconducting YBiPt intermetallic compound the valence band is found to be dominated by the Pt d states and the conduction band by the Y d states.

In this paper, the authors present theoretical evidence for ferromagnetism in Ru and Rh overlayers on Ag(001). These predictions are based on ab-initio, spin-polarized, electronic-structure calculations within the framework of the local spin-density approximation. For Tc, Ru, Rh, and Pd overlayers chemisorbed on Ag(001), only Ru and Rh exhibited ferromagnetism. Several metamagnetic spin states were found for the Ru overlayers.

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We present theoretical evidence for ferromagnetism in Ru and Rh overlayers on Ag(001). These predictions are based on ab-initio, spin-polarized, electronic-structure calculations within the framework of the local spin-density approximation. For Tc, Ru, Rh and Pd overlayers chemisorbed on Ag(001), only Ru and Rh exhibited ferromagnetism. Several metamagnetic spin states were found for the Ru overlayers.

The total energies of the observed crystal structures of Ce [face-centered cubic (fcc), orthorhombic, and body-centered tetragonal (bct)] under pressure have been calculated, using the local-density approximation. The linear-muffin-tin-orbital calculations were full potential, all electron, and fully relativistic. The experimental data for the different crystallographic transitions are well reproduced by the calculations and we have extracted two terms that are mainly responsible for the alpha --> alpha' transition: a one-electron term and a Madelung term. The alpha --> alpha' transition is driven by the increasing importance of the 4f contribution with decreasing volume. This finding is also supported by a calculation without the 4f contribution to the cohesion which yields the alpha' phase unstable. The alpha' --> bct transition is found to be somewhat more complex in nature since it is quite heavily influenced also by the 5d electrons. The calculated ground state is (correctly) found to be fcc and the equilibrium volume as well as the bulk modulus are in good agreement with experiment. The present ab initio calculation of a crystallographic phase diagram of an f electron system suggests delocalized 4f electrons exist in the high-pressure phases, including the alpha phase, of Ce.

We have, for the first time, theoretically determined the self-consistent orbital contribution to the magnetic moment on the (001) surfaces of Fe, Co and Ni. We used a surface geometry that corresponds to the bulk crystal structure with no relaxation of the surface. In addition to enhanced spin moments at the surface, we find that the orbital moment for surface states is greatly enhanced (sometimes by over 100%).

Results of a fully relativistic study of angle-resolved photoemission (ARPES) for the (100) surfaces of Ag and Au are presented. The calculations are based on FLAPW thin film potentials and a single-step model of photoemission. The results for Ag are compared with experimental data and a previous non-relativistic calculation. In the case of Au-DELTA-6 and DELTA-7 initial state symmetries are investigated using linearly and circularly polarized light.

Results of a fully relativistic study of angle-resolved photoemission (ARPES) for the (1 0 0) surfaces of Ag and Au are presented. The calculations are based on FLAPW thin film potentials and a single-step model of photoemission. The results for Ag are compared with experimental data and a previous non-relativistic calculation. In the case of Au Δ6 and Δ7 initial state symmetries are investigated using linearly and circularly polarized light.

The importance of including all of Hund’s rules in self-consistent electronic-structure calculations in solids is pointed out. We present a scheme that accounts for the interactions that lead to these rules, and we give examples of when these interactions become important. Calculated ground state properties (chemical bonding, magnetic moment, magnetic form factor) of actinide and rare earth systems are found to agree better with experiment when all Hund’s rules are accounted for. On the other hand, but also in agreement with experiment, we find that Hund’s second and third rule contributions for Fe, Co, and Ni are small.

The fully relativistic spin-polarized embedded-cluster method is discussed for magnetic impurities in nonmagnetic host metals. We apply the method to the calculation of the density of states and the magnetic energy of the cases of a single Fe impurity in Au, and of a pair of Fe atoms located within a first-neighbor shell cluster in Au. In addition to allowing for different distances between the two Fe impurities, we also allow for different spin orientations, with respect to the lattice symmetry of the host. We have also analyzed the spin polarization induced on a central Au site by the Fe atom(s). Thus, we have shown that the method is appropriate to obtain effective spin Hamiltonians for local moment systems, which contain local anisotropy terms and anisotropic exchange interactions.