P. A. Korzhavyi

Theoretical investigation of sulfur solubility in pure copper and dilute copper-based alloys

Dilute Cu-based alloys containing S, P, and Ag impurities and also vacancies are studied theor- etically on the basis of total energy calculations. This is done within the supercell approach by using the locally self-consistent Greens function (LSGF) method. The impurity solution energies, volume misfits, and interaction energies for these defects are calculated and used to study the microscopic mechanism behind the eAect of these impurities on embrittlement of copper at intermediate temperatures. It is shown that the solubility of S in Cu is low due to precipitation of the highly stable Cu2S phase. A large binding energy of a sulfur-vacancy defect pair in the first coordination shell (ˇ0.46 eV) and a sulfur-sulfur defect pair in the second coordination shell (ˇ0.12 eV) seem to favor this precipitation. The eAect of phosphorus and silver impurities on the bulk S solubility has also been studied, and was found to depend on the com- petition of these impurities with sulfur for vacancies, as well as probably for other lattice defects. # 1999

First-principles investigation of thermal point defects in B2 NiAl

The equilibrium concentrations of thermal defects are calculated in the mean-field approximation for the configurational entropy. The stable configurations of point defects in NiAl are discussed.

Ab Initio Study of Vacancies in Metals and Compounds

The results of ab initio calculations of the vacancy formation energies in all the transition and noble metals are presented. We also report on the formation energies of native point defects in the NiAl intermetallic compound. The calculations are performed within the locally self-consistent Green’s function method and include multipole electrostatic corrections to the atomic sphere approximation. The results are in excellent agreement with experiment and existing full-potential calculations. We also perform a qualitative analysis of constitutional and thermal defects in NiAl within the Wagner-Schottky model of a lattice gas of non-interacting defects.

Point Defects in NiAl Alloys Under Pressure

We investigate the effect of elevated pressures on the point defect thermodynamics in NiAl alloys. A particular motivation for this study is due to the expected elimination of structural vacancies on the Al-rich side at high pressure. We employ the density functional theory to compute point defect energies as a function of pressure, which are in turn used as input to the Wagner-Schottky model. We find that at about 200 kbar a change in the constitutional defect from V Ni to Al Ni does take place. The extension of the Wagner-Schottky model by introducing elastic interactions between defects leads to the prediction of a qualitatively new phenomenon in the system, namely the appearance of an isostructural phase transition terminated at a critical point. Similar behaviour is expected in some other ordered off-stoichiometric compounds.

Locally Self-Consistent Green’s Function Method and Its Application in the Theory of Random Alloys

A formulation of the order-N locally self-consistent Green’s function, LSGF, method in conjunction with the linear muffin-tin orbital (LMTO) basis set is discussed. The method is particularly suitable for calculating the electronic structure of systems with an arbitrary distribution of atoms of different kinds on an underlying crystal lattice. We showthat in the framework of the tight-binding representation it can be generalized to systems without ideal three-dimensional symmetry of the underlying lattice, like, for instance, alloys with local lattice relaxations or surface alloys. We also showthat multipole corrections to the atomic sphere approximation can be easily incorporated into the formalism. Thus, the method represents a powerful tool for studing different problems within alloy theory.

Constitutional and Thermal Defects in Nickel Aluminides

The formation energies of intrinsic point defects and the interaction energies of possible defect pairs in NiAl are calculated from first principles within an order-N, locally self-consistent Greens function method in conjunction with the multipole electrostatic corrections to the atomic sphere approximation. The theory correctly reproduces the ground-state properties of the off-stoichiometric NiAl alloys. The constitutional defects (antisite Ni atoms in Ni-rich and Ni vacancies in Al-rich NiAl) are shown to form ordered structures in the ground state, in which the defects of the same kind tend to avoid each other at the shortest separation distance on their sublattice. A mean-field theory is applied to calculate the equilibrium concentrations of thermal defects. The statistics of thermal defects is interpreted in terms of dominant composition-conserving complex defects which are shown to be triple defects in Ni-rich and nearly stoichiometric NiAl. In the Al-rich region a novel thermal excitation dominates where two constitutional Ni vacancies are replaced by one antisite Al atom. The number of vacancies, as well as the total number of point defects decrease with temperature in Al-rich NiAl. The boundary between the two regions is treated analytically. The vacancy concentration exhibits a minimum in its temperature dependence at themorexa0» boundary. Similar analysis is applied to study constitutional and thermal defects in Ni{sub 3}Al as a function of concentration is in excellent agreement with recent experimental data.«xa0less

Distribution of silicon over sublattices in semiconducting A3B5 compounds

The stable site of Si substitutional impurities in GaAs and AlAs at T=0 K is determined on the basis of an analysis of the energy of solution of silicon, and of the energies of formation of intrinsic defects and the reaction energies of their interaction obtained by calculating the total energy of the disordered compounds. These calculations indicate that amphotericity and vacancies have an effect on the distribution of Si. At low Si concentrations, Si in GaAs is located on the sublattice of the group III element, and in AlAs, on the sublattice of the group V element.

Theoretical analysis of impurity distributions in crystalline silicon

Calculations of the total energy of oxygen and carbon impurities in silicon at T=0 K are presented. The equilibrium position of point defects is determined for low (10−3–10−2 at. %) concentrations.