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The authors investigate the common qualitative idea that a solid is particularly stable if the electron states are filled up to some energy band gap. Such ideas have previously been advanced to explain the occurrence of a certain family of M5X4 compounds (M=transition metal, X=chalcogen) only for particular ranges of valence electron concentration (VEC). In these materials octahedral metal clusters condense to form infinite chains. The authors study the stability of such chains against break-up on two models, (i) a regular octahedral chain and (ii) the chain of the compound Nb 5Te4 and discuss the question of why only the compounds with 2.4<or=VEC<or=3.6 exist.

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Journal of Physics F: Metal Physics

The fusion of metal clusters in M5X4 compounds

Inorganic Chemistry

Relation between the Electronic Structure and the Condensation of Metal Clusters in M5X4 Compounds

Using the cluster-variation method in the tetrahedron approximation, we study order-disorder transitions at the (100) surface of Cu-Au alloys. Depending on surface segregation, the order-disorder transition may be first or second order. Disordering of the surface may occur at temperatures above or below the bulk transition temperature. The recent low-energy ion-scattering data on Cu3Au is interpreted as evidence for stronger long-range order at the surface than in the bulk. {\textcopyright} 1984 The American Physical Society.

Physical Review Letters

Order-disorder transitions and segregation at the (100) surface of Cu-Au alloys

With the use of a self-consistent tight-binding scheme, local electronic densities of states (LDOS) are calculated for commensurate (C) Pd* (110) and incommensurate (IC) Pd(111) layers on Nb(110). At low coverages (<1) Pd induces a Pd-Nb resonance near the bottom of the Nb d band. In agreement with the experimental results this resonance shifts to lower binding energy as the coverage is increased. Comparing detailed features of the LDOS with the photoemission data we predict a C-IC structural transformation for approximately one monolayer of Pd. This we also conclude from a simple interface energy calculation. The d-electron charge transfer across the interface and the LDOS of Pd at the Fermi energy (EF) are small. The latter, however, increases with the Pd coverage. The LDOS of the top Pd layer is nearly the same for Pd(111) on Pd* (110)-Nb(110) or Pd(111)-Nb(110) interfaces and in all cases there is a significant decrease of Nb d states near the EF. This shows that the LDOS is mainly perturbed at the interface. Implications of these results for hydrogen uptake and work-function changes are discussed. {\textcopyright} 1983 The American Physical Society.

Physical Review B

Electronic structure of transition-metal-transition-metal interfaces: Pd on Nb(110)

AIChE Journal

Simple method for solution of a class of reaction‐diffusion problems

The present paper analyzes the consequences of coupling two independent CSTRs operating under different stability conditions. A variety of behavioural patterns of the coupled system have been shown to exist for different types of reactor combinations, viz. stable-stable, oscillatory-stable and oscillatory-oscillatory. The reactor output is in general sensitive to the exchange coefficient, a proper choice of which can be of use in practice. The results obtained also provide an explanation for the occurrence of aperiodic behaviour. More importantly, the present simple model, viz. the linear coupling of the lumped parameter systems, offers guidelines for the analysis of more complex distributed parameter systems such as catalyst pellet or the fixed bed. © 1983.

Chemical Engineering Science

Dynamic behaviour of coupled CSTRs operating under different conditions

The authors have developed a self-consistent coherent potential approximation generalised to take the effect of clusters into account. Off-diagonal disorder and short-range order are taken into account. A graphical method married to the recursion technique enables them to work on realistic three-dimensional lattices. Calculations are shown for a binary alloy on a diamond lattice.

Journal of Physics C: Solid State Physics

Electronic structure of disordered alloys I. Self-consistent cluster CPA incorporating off-diagonal disorder and short-range order

Numerical results are given for the surface core-level shifts at clean single crystal surfaces, stepped and reconstructed surfaces and surfaces with chemisorbed atoms. These results demonstrate that surface core-level shifts can be used as a diagnostic tool to study various surface effects. {\textcopyright} 1982.

Solid State Communications

Semiempirical theory for surface core-level shifts

The interactions of ad-atoms on a surface may be modeled with the Ising Hamiltonian conveniently extended to accomodate couplings beyond the simplest nearest neighbor. We use real space renormalization group methods to study the phase diagram for such a model for hydrogen on Pd(100). Confirming the previous Monte Carlo results we find qualitative agreement with experiment at high temperatures but with a too narrow ordered region. We report also the finding of a second order transition to a (2×2) superlattice structure at lower temperatures. {\textcopyright} 1982.

Surface Science

Phase diagrams for a square lattice with two- and three-body interactions

Surface segregation is studied for CuxNi1-x alloys using the tight-binding Hartree-Hamiltonian and a mixed Bethe-lattice model to determine the electronic energy. Results for the surface segregation at (100) and (111) surfaces of the alloy are presented and compared with existing experimental results. Also, results are given for the temperature dependence of the segregation. Furthermore, the effect of the surface segregation on the local electronic density of states and on the surface core-level shifts is calculated on the basis of our electronic theory. {\textcopyright} 1982 The American Physical Society.