Fcc Transition Metals Research Articles

High-spin and low-spin states in Invar and related alloys.

Total-energy band calculations that show the coexistence of a high-spin and low-spin state in fcc transition metals and alloys are presented. The energy difference between the two states is shown to be a function of the electron concentration and to vanish at 8.6. At larger electron concentrations the low-temperature state is the high-spin state, and the thermal expansion is shown to pause at a system-dependent characteristic temperature. At lower electron concentrations the low-temperature state is the low-spin state, and enhanced thermal expansion is expected. An analysis that leads to a qualitative understanding of the thermal properties of Invar and that implies a connection with martensitic transformations and spin glasses in related alloys is presented. For Invar a magnetic collapse from the high-spin to the low-spin state at a pressure of 55 kbar is predicted.

Multilayer relaxation and reconstruction in bcc and fcc transition and noble metals

Using a tight-binding scheme and a ‘quenched-molecular dynamics’ method, we calculate the multilayer relaxation of bcc transition metals for the (110), (100) and (111) faces (which present only perpendicular relaxations) and the (211), (310) and (210) faces (for which both parallel and perpendicular components are present). Moreover, we account for the multilayer reconstruction of W(100) and its non-occurrence for Ta(100). Then performing similar calculations in the case of all fcc transition and noble metals, we obtain the multilayer relaxation of the low index surfaces: (111), (100) and (110). Finally, we explain the material dependence of the (1 × 2) reconstruction of the (110) face, which is only observed for the 5d metals (Ir, Pt, Au). The model allows us to predict order-disorder surface phase transition for the (110) face of all these fcc metals.

Thermodynamic properties of fcc transition metals as calculated with the embedded-atom method.

Solid and liquid Gibbs free energies of Cu, Ag, Au, Ni, Pd, and Pt have been calculated through the use of the embedded-atom method, and the results are in good agreement with experiment. The melting points of the materials are calculated from the intersection of the solid and liquid free-energy curves. The values of the melting point are in excellent agreement for the noble metals and in reasonable agreement for Ni, Pd, and Pt. The thermal expansions of the solids are computed and the results are in excellent agreement with experiment over the entire temperature range.

Many-body surface strain and surface reconstructions in fcc transition metals.

Bonding in the near-surface region is strongly influenced by the truncation of the lattice at the surface. This many-body effect is examined quantitatively by use of the embedded-atom method on low-index transition-metal surfaces. An unreconstructed metal surface is found to have a tensile strain of several percent. This many-body surface strain is an important factor in the energetics of surface structure, serving as a parameter combining surface stress and nonlinear elastic effects. In particular, it is demonstrated that the surface strain drives surface reconstruction, and also produces asymmetry in the stability properties of thin mismatched epitaxial overlayers.

Many-body forces in the lattice dynamics of some FCC transition metals

In the present analysis, the interaction system of an fcc d-band metal is considered to be composed of two-body and many-body parts. We use a short-range three-body potential to deduce the contribution of many-body forces to the dynamical matrix of fcc structure. The inclusion of multi-particle forces in the transition metal model potential scheme of Animalu improves, in general, the transverse branches of phonon dispersion curves of Cu, Ag, Au and Pd and gives good agreement with the neutron data for all the metals.

Surface segregation in dilute binary fcc transition-metal alloys.

We present a tight-binding study of the surface segregation energy of transition-metal impurities in fcc transition metals. Various approaches to the self-consistent determination of the impurity potentials are discussed and their results are compared. When size effects can be neglected, the trends followed by the segregation energy are qualitatively given by the difference of surface energies between the solute and the solvent species if electronic correlation effects are not too large. These size and electronic correlation effects are finally discussed.

Theoretical study of the Debye-Waller factor in surface extended x-ray-absorption fine structure: Influence of an adsorbed monolayer.

We calculate mean-square relative displacements which are required for the determination of the Debye-Waller factor in surface extended x-ray-absorption fine-structure spectroscopy. The lattice dynamical model involves central and angular forces treated within the harmonic approximation. The projected density of modes contributing to relative vibrational motion is obtained from a continued-fraction method. Systematic results for mean correlated displacements between first neighbors are presented for the three low-index faces of fcc transition and noble metals: We predict a strong anisotropy between pairs belonging to the surface and pairs with just one surface atom. Finally, in order to get experimental checks of our results, we also study the case of a monolayer of cobalt on the simple faces of copper. The agreement with the existing experimental results for Cu(111) is very good.

Variation at the surface of mean correlated displacements in fcc transition and noble metals studied by a continued fraction technique

We calculate mean square and mean correlated displacements which are required for the determination of the Debye-Waller factors in LEED and SEXAFS experiments. The crystal model involves central and angular forces treated within the harmonic approximation. The phonon densities of states are obtained from a continued fraction method. Results concerning the mean square displacements are given for the three low index faces of fcc metals: some general trends, in agreement with experiments, are put forward. Finally, preliminary results on the mean correlated displacements between first neighbours are presented in the particular case of Cu(111).

Variation at the surface of mean correlated displacements in fcc transition and noble metals studied by a continued fraction technique

Variation at the surface of mean correlated displacements in fcc transition and noble metals studied by a continued fraction technique

Triple-Qand Single-QStates in Antiferromagnetic fcc Transition Metals with the First-Kind Ordering

By using a real space expansion approach to calculate the Green function of d -electrons, the four-spin interaction is derived and the relative stability among various multiple spin density wave states is discussed for antiferromagnetic fcc transition metals with the first-kind ordering. The transition from a triple- Q state to a single- Q state is shown to occur with increasing or decreasing the number of d -electrons from the nearly half-filled band region.

Empty electronic states at the (100), (110), and (111) surfaces of nickel, copper, and silver.

We have performed a systematic investigation of empty electronic surface states on the (100), (110), and (111) faces of nickel, copper, and silver using momentum-resolved inverse photoemission spectroscopy at \ensuremath{\Elzxh}\ensuremath{\omega}=9.7 eV. We present the measured two-dimensional energy-band dispersions E(${k}_{?}$) and discuss common trends. All the observed surface bands can be classified as being either crystal induced or image-potential induced. The former class of surface states is well known from ordinary photoemission and occurs in bulk band gaps as a consequence of the termination of the three-dimensional crystal periodicity. Image-potential states arise from the long-range forces which describe the screening of an electron approaching a metal surface by the conduction electrons and have no filled counterpart. The relationship between the two kinds of states can be qualitatively understood in terms of a one-dimensional model proposed by Echenique and Pendry. Numerical calculations are necessary for quantitative understanding. Comparison of the experimental data to such calculations is made where available. We conclude that the systematics of unoccupied sp-like surface states on fcc transition metals are now well established.

Lattice vibrations at (111) and (001) surfaces of fcc transition metals by using the surface green function matching (SFGM) method

Lattice vibrations at (111) and (001) surfaces of fcc transition metals by using the surface green function matching (SFGM) method

Lattice vibrations At (111) and (001) surfaces of fcc transition metals by using the surface green function matching (SFGM) method

The dispersion relations of surface phonons at (111) and (001) surfaces of fcc metals (Ni. Ag) have been obtained by applying the extension of the SGFM method to discrete systems, with nearest neighbours central interactions. The dispersion relations thus obtained agree quite well with experimental data and with those obtained by other numerical methods. The discrete SGFM method performs quite well in practice, both to obtain the secular determinant (in two alternative ways) and to obtain the surface projected mode density.

Bulk and surface vibrational and thermodynamical properties of fcc transition and noble metals : a systematic study by the continued fraction technique

We present a systematic study of vibrational and thermodynamical properties of fcc transition and noble metals, using a model including central and angle-bending forces. The phonon frequency spectra are computed by means of the continued fraction technique; their knowledge enables us to calculate the contribution of phonons to various thermodynamical quantities such as the specific heat at constant volume, the entropy and the mean square displacements. For the bulk, our calculations reproduce all the experimental trends both qualitatively and quantitatively. Then we give results for the three low index faces of these metals. The surface spectra exhibit drastic deformations due to the existence of surface phonons. The variation of thermodynamical quantities due to the surface reveals some general trends. For example, the variation of specefic heat is maximum at a temperature which is a constant fraction (∼0.175) of the bulk Debey temperature. On the other hand, the lateral mean square displacement is much more dependent on the surface density of atoms than the perpendicular one Etude systematique des proprietes de vibration et thermodynamiques dans le cadre d'un modele comportant des forces centrales et des forces angulaires. Calcul des spectres de frequence de phonons, determination de la contribution des phonons a diverses grandeurs thermodynamiques: chaleur specifique a volume constant, entropie, deplacement quadratique moyen. Etude des trois faces de bas indice de ces metaux ou l'on constate l'influence de phonons de surface

Evidence for fcc-like short range order in Fe-Ni-B metallic glasses

Soft X-ray appearance potential spectra have been measured for iron and nickel in (Fe 0.5Ni 0.5) 100− x B x metallic glasses with x = 16, 18, 20, 22, 24, 26. For x = 16 both iron and nickel spectra exhibit structural features characteristic for the electronic structure of fcc transition metals. It is concluded that in the amorphous state Ni and Fe are arranged in fcc-like geometry beyond nearest neighbours.

A calculation of the surface energies of fcc transition metals

The surface energies of six transition metals — nickel, gold, silver, platinum, palladium and copper — have been calculated using the Baskes and Melius interatomic potential. The results obtained fall within 10% of the experimental values, thus indicating that the Baskes and Melius potential could provide a valuable tool for the computer modelling of surface phenomena.

A calculation of the surface energies of fcc transition metals

The surface energies of six transition metals — nickel, gold, silver, platinum, palladium and copper — have. been calculated using the Baskes and Melius interatomic potential. The results obtained fall within 10% of the experimental values, thus indicating that the Baskes and Melius potential could provide a valuable tool for the computer modelling of surface phenomena.

Electronic energy bands and optical properties of Ir and Pt

Electronic energy bands of the FCC transition metals Ir and Pt have been calculated by the KKR method for a large number of k points inside the Brillouin zone in the energy range 0.-35 eV. The method of Segall and Yang (1980) was used for efficient calculation of the structure constants. Gaspar's (1954) universal potential and l-dependent muffin-tin constants have been used to obtain the correct positioning of the d bands. From the band structure so obtained, the density of states and epsilon 2 spectra were calculated for both metals. The rich variety of structures found in the spectra was compared with the latest available experimental results. The agreement was found to be much better than other previous theoretical calculations. The structures have been successfully interpreted in terms of various band-to-band transitions inside the Brillouin zone.

Charge densities in FCC transition metals

A simple and accurate means of reproducing the band structure of an FCC transition metal is by means of a model Hamiltonian which depends on a set of parameters determined from known energy eigenvalues at symmetry points in the Brillouin zone. It is shown that the model Hamiltonian can also reproduce the metal charge density with good accuracy if care is taken with the choice of atomic orbitals to be used in the calculations. Calculations are performed for Ni, Cu, Rh, Pd and Ag and the spherically averaged charge densities obtained from the model Hamiltonian are compared with those of Moruzzi et al. (1978). Agreement to better than 10% is obtained in all cases out to the muffin-tin radius.

Magnetic Phase Diagrams at Zero Temperature of bcc and fcc Transition Metals

The method for calculating the electronic structure of transition metals which was developed before is applied to ferromagnetic, antiferromagnetic and helical spin density wave states. By use of the method, the magnitude of local moments, energy, etc. are calculated within the Hartree-Fock approximation. The relative stability of those states is investigated for a given valence, and phase diagrams of bcc and fcc transition metals are constructed. It is found that the most stable state changes continuously from an antiferromagnetic one to a ferromagnetic one via a helical spin density wave one when the valence changes from five to ten. The correspondence between the obtained phase diagrams and experimentally observed ones for 3 d metals is discussed.