Spontaneous L1(2) order at Ni(90)Al(10)(110) surfaces: an x-ray and first-principles-calculation study

Influence of defect distribution on the thermoelectric properties of FeNbSb based materials

Cooperative effects of a solid solution and phase separation could strongly scatter phonons and improve the performance of thermoelectric materials.

Nanoalloy electrocatalysis: simulating cyclic voltammetry from configurational thermodynamics with adsorbates

Simulated 2-dimensional cyclic voltammetry for nanoalloys with a hybrid ensemble scheme in Monte Carlo simulation based on the cluster expansion method.

Phase relationships and structures of inorganic crystals by a combination of the cluster expansion method and first principles calculations

Properties of crystalline solutions are generally dependent not only on their chemical composition but also on the configurations of solute atoms and/or point defects. Quantitative knowledge of the configuration-dependent properties is therefore essential for materials design. The cluster expansion (CE) method has been widely used to describe the configurational properties. Increases in computational power and advances in numerical techniques enable us to perform a large set of systematic first principles calculations based on density functional theory (DFT) to be combined with CE calculations. In this paper, our procedure of CE with optimal selections of clusters and DFT structures is described. Two examples of such calculations are then shown. One is the cation arrangement in a series of spinel oxides. The other is arrangement of the oxygen vacancy in a series of tin sub-dioxides.

Predicting the segregation profile of the Pt25Rh75(100) surface from first-principles

The segregation profile of the Pt(25)Rh(75)(100) surface is studied by the combination of density functional theory calculations with the cluster-expansion method and Monte Carlo simulations. We construct the stability diagram for the surface layers, which allows the prediction of the most stable atomic configuration for a given average concentration in those layers. On this basis, we apply the cluster-expansion Hamiltonian in grand-canonical Monte Carlo simulations for the prediction of the temperature-dependent concentration profile. The experimentally found enrichment of Pt in the top layer and depletion in the second layer is nicely confirmed by the calculations.

Cluster expansion approach for transmutative lattice systems

We propose a cluster expansion (CE) technique that can express any function of atomic arrangement on any given lattice with the same number of lattice points in a single formalism. In the proposed CE, two types of spin variable, σ and τ, on the base lattice and virtual lattice, respectively, are introduced. The former spin variable specifies the occupation of the constituent elements for each lattice point. The latter specifies the positions of each lattice point. Basis functions constructed from the two types of spin variable satisfy completeness and orthonormality for any atomic arrangement on given lattices. As examples, the proposed CE is applied to one- and three-dimensional lattices in a binary system, which clarifies the concept of base and virtual lattices, how the functions of atomic arrangements are expressed in terms of the two types of spin variable, and the efficiency and convergence of the proposed CE with a finite number of clusters and input structures.

Thermodynamic theory of epitaxial alloys: first-principles mixed-basis cluster expansion of (In, Ga)N alloy film

Epitaxial growth of semiconductor alloys onto a fixed substrate has become the method of choice to make high quality crystals. In the coherent epitaxial growth, the lattice mismatch between the alloy film and the substrate induces a particular form of strain, adding a strain energy term into the free energy of the alloy system. Such epitaxial strain energy can alter the thermodynamics of the alloy, leading to a different phase diagram and different atomic microstructures. In this paper, we present a general-purpose mixed-basis cluster expansion method to describe the thermodynamics of an epitaxial alloy, where the formation energy of a structure is expressed in terms of pair and many-body interactions. With a finite number of first-principles calculation inputs, our method can predict the energies of various atomic structures with an accuracy comparable to that of first-principles calculations themselves. Epitaxial (In, Ga)N zinc-blende alloy grown on GaN(001) substrate is taken as an example to demonstrate the details of the method. Two (210) superlattice structures, (InN)(2)/(GaN)(2) (at x = 0.50) and (InN)(4)/(GaN)(1) (at x = 0.80), are identified as the ground state structures, in contrast to the phase-separation behavior of the bulk alloy.

Reliable first-principles alloy thermodynamics via truncated cluster expansions

In alloys cluster expansions (CE) are increasingly used to combine first-principles electronic-structure calculations and Monte Carlo methods to predict thermodynamic properties. As a basis-set expansion in terms of lattice geometrical clusters and effective cluster interactions, the CE is exact if infinite, but is tractable only if truncated. Yet until now a truncation procedure was not well defined and did not guarantee a reliable truncated CE. We present an optimal truncation procedure for CE basis sets that provides reliable thermodynamics. We then exemplify its importance in Ni3V, where the CE has failed unpredictably, and now show agreement to a range of measured values, predict new low-energy structures, and explain the cause of previous failures.

Absence of 2kF splitting in the diffuse scattering from Cu3Au at the (001) surface

We report a new type of short-range order correlations at the (001) surface of Cu3Au which no longer produces the 2k(F)-splitting characteristic for the bulk short-range order scattering. We present the temperature dependence of this phenomenon and a theoretical interpretation of its origin. We argue that this new surface effect is caused by a drastic change of the strain-induced interactions at the surface.

Antisite-defect-induced surface segregation in ordered NiPt alloy

By means of first principles simulations we demonstrate that tiny deviations from stoichiometry in the bulk composition of the NiPt-L1(0) ordered alloy have a great impact on the atomic configuration of the (111) surface. We predict that at T=600 K the (111) surface of the Ni51Pt49 and Ni50Pt50 alloys corresponds to the (111) truncation of the bulk L1(0) ordered structure. However, the (111) surface of the nickel deficient Ni49Pt51 alloy is strongly enriched by Pt and should exhibit the pattern of the 2x2 structure. Such a drastic change in the segregation behavior is due to the presence of different antisite defects in the Ni- and Pt-rich alloys and is a manifestation of the so-called off-stoichiometric effect.