Binary Substitutional Alloys Research Articles

Ordered Structures of Interstitial Solutes in BCC Lattice

By using the method developed by Kaburagi and Kanamori (Progr. theor. Phys. 54 (1975) 30) the ground state ordered structures and the conditions for their existence for the case of BCT substitutional binary alloys were obtained, which can be applied to the case of BCC based interstitial binary alloys. The method of graphical solution of the problem were devised. The six kinds of pairwise interactions, J 1 , …, J 6 were assumed. For the case of J 4 = J 5 = J 6 =0, the problem could be solved completely for the whole range of solute concentration. The other two important cases of J 5 = J 6 =0 and J 1 , J 2 ≫| J i ( i ≠1,2) were also solved completely but for the range of low concentration. Almost all the structures experimentally observed could be obtained and the conditions for their existence were determined.

Self-consistent continued-fraction theory of the conductivity of disordered systems: linear (TTF-TCNQ salts, polymers, proteins)

A new theory of conductivity in disordered materials is presented. The theory is based on the self-consistent continued-fraction method (SCCF) for the density of states calculation and a Kubo-type formula. It is formulated entirely in r space. The general theory is applied to linear binary substitutional alloys and the results demonstrate that the theory provides an alternative explanation, from first principles, of metal-insulator transitions, such as the Peierls-Frohlich transition in TTF-TCNQ salts and related quasilinear materials.

Quantitative prediction of surface segregation

An assessment has been made of the 32 available equilibrium measurements for surface segregation in substitutional pure binary alloy systems. These measurements have been analyzed in relation to the various terms proposed for the free energy of segregation. Three terms have been selected, the surface energies of the constituents, their enthalpy of mixing and a term related to the release of strain energy on segregation. A least squares fitting of the measurements to the three terms shows that some of the current theories are valid but that the importance of each term may not have been correctly emphasized. The assessment leads to a simple predictive relationship, valid for both dilute and concentrated alloy systems, which correlates with the experimental results over the range of free energies of segregation from −80 to 20 kJ/mol, with a standard deviation of less than 9 kJ/mol.

L-dependent pseudopotential in the theory of band structure of disordered solids-an application to binary substitutional alloys MgLi, MgCd and MgIn

In this work the authors extend the KKRZ method of band structure calculation for crystalline solids to the disordered systems of substitutional alloys and amorphous solids. In the case of substitutional alloys the average T-matrix approximation was employed whereby the l-dependent pseudopotential became complex, making the secular determinant complex. For amorphous solids, a positional averaging was taken following Kramer (1970), whereby a complex function phi nalpha (k+Kn,E) of a disorder parameter alpha appeared in the diagonal term of the secular determinant, again making the secular determinant complex. In both cases the energy eigenvalues obtained are complex in nature, the imaginary part being interpreted by a lifetime broadening of the energy state. Finally, the authors have applied this method to three binary substitutional alloys MgLi, MgCd and MgIn to obtain the complex energy band of these alloys. From these band structure data, density of states and optical properties have also been calculated and compared with the pure case.

Theoretical investigation of the electron energy spectrum in binary substitutional alloys with an arbitrary long-range order II. Crystal potential

The Green function method is used to obtain the crystal potential for substitutional alloys with an arbitrary long-range order. The particular case of a binary alloy ordered in a lattice of the CsCl type is considered.

Coherent potential approximation for the electrical resistivity of partially ordered binary magnetic alloys

AbstractThe dc electrical resistivity is calculated for a b.c.c. based random binary substitutional magnetic alloy with partial long‐range order for a single‐band tight‐binding model with diagonal disorder, two kinds of localized spins, arbitrary concentrations and arbitrary order parameter. The spin‐disorder contribution to the resistivity is treated by means of the sd exchange model. Within the framework of the CPA up to second order in the scattering potential a general closed resistivity formula is given in terms of alloy parameters and spin parameters. The resitivity is discussed for various special cases in dependence on the long‐range order parameter.

Kinetics of Short Range Ordering and Behavior of Vacancies in Binary Substitutional Alloys

Kinetics of Short Range Ordering and Behavior of Vacancies in Binary Substitutional Alloys

Electrical conductivity of two‐band models for disordered binary substitutional alloys theory

AbstractThe static electrical conductivity for a two band S–D matrix generalization of the Shiba model is determined for a disordered binary substitutional alloy within the single‐site approximation. Unlikely the previous works, the conductivity is expressed in an analytic form allowing for an interpretation of the Boltzmann equation limit in terms of the scatterings in complex, hybridized bands.

Electrical conductivity of two‐band metals for disordered binary substitutional alloys: Numerical investigation

Electrical conductivity of two‐band metals for disordered binary substitutional alloys: Numerical investigation

Ferromagnetic alloys: A Cluster-Bethe-lattice approach

We apply the Cluster-Bethe-lattice method to calculate the local density of one-magnon states for a binary substitutional ferromagnetic alloy. Short range order effects for the alloy are included. Simple clusters of one atom and one atom plus its nearest neighbors are studied. We show that, for the latter type of cluster, there are localized excitations the energies of which fall outside the continuum of excitations.

Recent Progress in the Experimental Determination of Magnetization Density Distributions

Some recent experimental determinations of the magnetization density distributions in crystalline solids are reviewed. There have been a number of investigations of the magnetic form factors of 3d and 4d paramagnetic metals and the measurements on the ferromagnetic 3d elements have been extended to binary substitutional alloys between two 3d elements. The review also includes sections devoted to first group transition metal intermetallic compounds, covalency effects and magnetization densities of rare earth and actinide materials. In conclusion, some new contributions to the experimental technique itself are discussed.

Electrical conductivity of electrons in a model binary disordered alloy with long range order

The CPA d. c. electrical conductivityσ for a bcc based random binary substitutional alloy with long range ordering is determined for a single band model with diagonal disorder and arbitrary alloy parameters. The CPA vertex corrections toσ vanish, off-diagonal elements of the Green function are most important. The conductivity obeys general symmetries with respect to alloy parameters and can be brought to closed form resembling the standard expressionσ = ne2τ/m. To interpret various terms inσ correctly, limits with available Boltzmann equation solutions are analyzed in detail.

General relationship between concentration derivatives and correlation functions for alloys

A general relationship is derived between the concentration derivative of any ensemble-averaged quantity and the correlation function of that quantity with fluctuations in the concentration. This relationship is then used to obtain general formulas for the concentration derivatives of the magnetization M (kappa-arrow-right), and of the Warren short-range order parameters, ..cap alpha.. (R), for the case of binary substitutional alloys. The use of these formulas in the analysis of diffuse elastic neutron-scattering data and the analysis of the effects of short-range order is discussed briefly. (AIP)

Densities of states of disordered Cu-Ni alloys

A method for calculating the density of states of substitutional binary alloys is presented. It essentially consists of successive applications of Beeby'st-matrix formulation of the KKR theory to the single-impurity problem. This method is applied to calculate thed-densities of states of Cu-Ni alloys of various concentration. The results are in reasonable agreement with both experimental photoemission and soft-X-ray emission data and with the theoretical calculations of Stockset al. and Bansilet al. via different approaches.

Vacancies in binary alloys

General expressions for the concentrations of divacancies and single vacancy-atom complexes in substitutional binary alloys is derived, using a model taking into account the interaction of neighbouring atoms as well as the interaction between vacancies and neighbouring atoms. The results of this paper are compared with those obtained with help of former theories, based on more restricted conditions.

Vacancies in binary alloys: III. The energy of vacancy formation

A general expression for the energy of formation of a single vacancyEF in substitutional binary alloys having two types of lattice sites is derived, which takes into account the interaction of neighbouring atoms. It is shown, that near the Curie temperature Θ the formation energy exhibits an anomaly.

The Two Band Models in the Theory of Disordered Substitutional Alloys

AbstractFor description of the electronic structure of a binary substitutional alloy, a two band model with hybridized S and D bands is introduced, which uses the concept of multiplicative off‐diagonal randomness thus representing a matrix generalization of the single band model of Shiba. All matrix elements may be random permitting to interpolate between the band structure of both pure component crystals with different band locations and/or widths, and with different hybridization strength and k‐dependence as well. All previously described two‐band alloy models are special cases of the present scheme. Despite the high flexibility of the model, its numerical implementation is easy, because of the special Hamiltonian structure.

Electronic structure of alloy surfaces: Coherent-potential approximation

The electronic structure of alloy surfaces is investigated within the framework of the coherent-potential approximation, a scheme originally derived for infinite alloy systems and here generalized to the case of the semi-infinite cleaved alloy crystal. The theory is applied to substitutional binary alloys, described by a single-band Wannier model one-electron Hamiltonian. Numerical results for the configurationally averaged surface density of states are discussed for a variety of system parameters. Among the cases considered are alloys of uniform composition and alloys in which the surface composition differs from that of the interior.

Electron correlation in the narrow-band model for disordered ferromagnetic alloys

The ferromagnetism of a random disordered substitutional binary alloy described by the narrow-energy-band model of Hubbard is discussed within the framework of the coherent-potential approximation with strong correlation effects included. The treatment of Coulomb correlations used is an improved version of the first Hubbard approximation which seems to adequately describe those correlation effects that have a major influence on the stability of the ferromagnetic phase for large intra-atomic Coulomb repulsion. Disorder is considered in the site-diagonal terms of the Hubbard model. Numerical illustrations are presented which describe the ferromagnetic solutions in terms of the component magnetizations and component densities of states as functions of electron concentration in the band for several cases of disorder and alloy compositions. In these examples the intra-atomic Coulomb repulsion is assumed to be infinite. Comparisons are made with results obtained using a Coulomb repulsion of the order of the bandwidth treated by the Hartree-Fock approximation. One finds that results are qualitatively similar for small electron concentrations and the examples detail the differences at larger concentrations. Finally a brief discussion with respect to experiment is made.

Generalized Flinn operators with applications to multi-component alloys

The relationship between the occupation operator, spin operator and Flinn operator formalisms for a substitutional binary alloy is presented. These three methods are generalized to multicomponent alloys with irreducible static n-body potentials. In this manner the concept of generalized Flinn operators is introduced. The configurational Hamiltonians and generalized Flinn operators are then explicitly presented for ternary and quaternary alloys with static two-body potentials. The merits of the generalized Flinn operator approach are discussed.