F Brouers

Off-diagonal disorder and energy of formation in transitional alloys

The authors show that in most cases the energy of formation of disordered transitional alloys is mainly determined by the deformation of the electronic bands upon alloying and that the difference in bandwidth between the metals must be taken into account together with the difference between their energy levels.

The theory of metal-non-metal transitions in gold alkali liquid alloys

By incorporating calculated liquid pair correlation functions into a tight binding Cayley tree model for the electronic structure of binary alloys, the authors have developed a self-consistent theory of the electronic and atomic structure of liquid alloys. Model calculations for the Au alkali systems show the existence of a gap in the density of states if the ratio of the electronegativity difference to the mean overlap integral exceeds a certain value. For alloys exhibiting a gap, a metal-non-metal transition occurs for concentrations in the neighbourhood of stoichiometry. This metal-non-metal transition occurs for values of the atomic parameters lying between those of Au-K and Au-Rb, in agreement with experimental findings. The transition is shown to be very sharp due to electron-electron interaction effects.

Energy of formation, band structure and local environment effects in transitional binary alloys

The electronic contribution to the energy of formation of transitional alloys AcB1-c is determined from their band structure. Preliminary numerical results are presented for nickel and iron paramagnetic alloys and it is shown that the variation of the energy of formation with concentration is not very sensitive to the detailed local environment, but is mainly determined by the average surrounding of A and B atoms.

Local environment and magnetic properties in transitional binary alloys. I. (theory)

A theory is presented for the study of the local environment effects on the magnetic properties in transitional binary alloys; the local environment effects are taken into account for the calculation of the one electron properties by an extension of the cpa and the electron-electron interactions are taken into account in the random phase approximation.

Charge transfer and ordering energy in a model binary alloy

Intra- and inter-atomic Coulomb interactions have been accounted for in the band theory of the order-disorder transition in binary alloys. The variation of the energy levels of the two kinds of atoms on the two sublattices is determined self-consistently using an extension of the coherent potential approximation (CPA). As an application, the authors investigate the role of the charge transfer in the order-disorder transition of CsCl type structure transitional alloys such as VMn, TiFe and ScCo.

Local environment effects on the electronic structure of disordered alloys

An approximate selfconsistent scheme generalizing the single site CPA is derived in order to account for the effect of local environment on electronic and magnetic properties of substitutional alloys.

Local environment and magnetic properties in transitional binary alloys. II. (numerical results)

Numerical results are presented from the study of local environment effects on the magnetic properties of atoms in simple models corresponding to the three typical alloys rh-ni, rh-pd and cu-ni. It is shown that (i) the dependence of the susceptibilities upon concentration and environment, (ii) the importance of the selfconsistent calculation of the charge transfers and the energy levels, (iii) the connection between the relative position of the energy levels and the relative magnitude of the susceptibilities, and (iv) that the susceptibility of the central atom is mainly determined by the electron-electron interactions on the central atom for rh-ni and rh-pd but that in cu-ni atoms further than the first neighbours need to be considered.

Order-disorder transition in a model binary alloy

The authors discuss the connection between the usual Bragg-Williams theory and some recent extension of the coherent potential approximation derived to deal with the order-disorder transition in alloys.

Ionic-metallic transition in gold-based liquid alkali metal alloys

The authors have calculated the density of states and conductivity of a series of gold-based liquid alkali metal alloys using a tight-binding model that includes short-range order and charge transfer in a self-consistent way. They take account of the variation of the alkali band widths, electronegativities and Coulomb interactions. The model can account for the diversity of conductivities observed at the equiatomic concentrations which range from metallic values for LiAu to ionic values for CsAu and RbAu. The calculations show that the difference of electronegativity between the alloy components is not the essential factor giving rise to the ionic to metallic transition in this series of liquid alloys, but rather it is the variations in the band widths and the interatomic Coulomb interactions that are the dominant factors.

Theory of metal-non-metal transitions in liquid-metal alloys

A technique for calculating the physical properties of liquid-alloy systems that undergo metal-insulator transitions as their concentrations are varied is presented. A simple tight-binding model is used, which includes short-range order and charge transfer self-consistently. The technique is used to investigate the liquid Cs-Au alloy system in detail. The variation of chemical order with concentration and temperature is calculated by minimising the free energy. This allows the calculation of the variation of conductivity with concentration and temperature, and the variation of magnetic susceptibility and Knight shift with concentration. The results are compared to existing experimental data and the agreement is found to be good.