A first-principles study of helium storage in oxides and at oxide--iron interfaces

Density-functional theory calculations based on conventional as well as hybrid exchange-correlation functionals have been carried out to study the properties of helium in various oxides (Al2O3, TiO2, Y2O3, YAP, YAG, YAM, MgO, CaO, BaO, SrO) as well as at oxide-iron interfaces. Helium interstitials in bulk oxides are shown to be energetically more favorable than substitutional helium, yet helium binds to existing vacancies. The solubility of He in oxides is systematically higher than in iron and scales with the free volume at the interstitial site nearly independently of the chemical composition of the oxide. In most oxides He migration is significantly slower and He--He binding is much weaker than in iron. To quantify the solubility of helium at oxide-iron interfaces two prototypical systems are considered (Fe|MgO, Fe|FeO|MgO). In both cases the He solubility is markedly enhanced in the interface compared to either of the bulk phases. The results of the calculations allow to construct a schematic energy landscape for He interstitials in iron. The implications of these results are discussed in the context of helium sequestration in oxide dispersion strengthened steels, including the effects of interfaces and lattice strain.Comment: 13 pages, 10 figures, 4 table

First principles study of intrinsic point defects in hexagonal barium titanate

Density functional theory (DFT) calculations have been used to study the nature of intrinsic defects in the hexagonal polymorph of barium titanate. Defect formation energies are derived for multiple charge states and due consideration is given to finite-size effects (elastic and electrostatic) and the band gap error in defective cells. Correct treatment of the chemical potential of atomic oxygen means that it is possible to circumvent the usual errors associated with the inaccuracy of DFT calculations on the oxygen dimer. Results confirm that both mono- and di-vacancies exist in their nominal charge states over the majority of the band gap. Oxygen vacancies are found to dominate the system in metal-rich conditions with face sharing oxygen vacancies being preferred over corner sharing oxygen vacancies. In oxygen-rich conditions, the dominant vacancy found depends on the Fermi level. Binding energies also show the preference for metal-oxygen di-vacancy formation. Calculated equilibrium concentrations of vacancies in the system are presented for numerous temperatures. Comparisons are drawn with the cubic polymorph as well as with previous potential-based simulations and experimental results

Contributions of point defects, chemical disorder, and thermal vibrations to electronic properties of Cd1-xZnxTe alloys

We present a first-principles study based on density functional theory of thermodynamic and electronic properties of the most important intrinsic defects in the semiconductor alloy Cd1-xZnxTe with x < 0.13. The alloy is represented by a set of supercells with disorder on the Cd/Zn sublattice. Defect formation energies as well as electronic and optical transition levels are analyzed as a function of composition. We show that defect formation energies increase with Zn content with the exception of the neutral Te vacancy. This behavior is qualitatively similar to but quantitatively rather different from the effect of volumetric strain on defect properties in pure CdTe. Finally, the relative carrier scattering strengths of point defects, alloy disorder, and phonons are obtained. It is demonstrated that for realistic defect concentrations, carrier mobilities are limited by phonon scattering for temperatures above approximately 150 K

Nucleation of Al3Zr and Al3Sc in aluminum alloys: from kinetic Monte Carlo simulations to classical theory

Zr and Sc precipitate in aluminum alloys to form the compounds Al3Zr and Al3Sc which for low supersaturations of the solid solution have the L12 structure. The aim of the present study is to model at an atomic scale this kinetics of precipitation and to build a mesoscopic model based on classical nucleation theory so as to extend the field of supersaturations and annealing times that can be simulated. We use some ab-initio calculations and experimental data to fit an Ising model describing thermodynamics of the Al-Zr and Al-Sc systems. Kinetic behavior is described by means of an atom-vacancy exchange mechanism. This allows us to simulate with a kinetic Monte Carlo algorithm kinetics of precipitation of Al3Zr and Al3Sc. These kinetics are then used to test the classical nucleation theory. In this purpose, we deduce from our atomic model an isotropic interface free energy which is consistent with the one deduced from experimental kinetics and a nucleation free energy. We test di erent mean-field approximations (Bragg-Williams approximation as well as Cluster Variation Method) for these parameters. The classical nucleation theory is coherent with the kinetic Monte Carlo simulations only when CVM is used: it manages to reproduce the cluster size distribution in the metastable solid solution and its evolution as well as the steady-state nucleation rate. We also find that the capillary approximation used in the classical nucleation theory works surprisingly well when compared to a direct calculation of the free energy of formation for small L12 clusters.Comment: submitted to Physical Review B (2004

Free energy and point defect distribution for heavily doped AgCl by an integral equation method

Au moyen des équations hypernetted chain on a calculé l'énergie libre et les fonctions de distribution radiale dans le cas de AgCl dopé par CdCl2 (1-5 %). On a comparé la fraction d'impuretés qui existe en paires lacune-impureté et l'énergie libre avec la théorie basée sur la loi d'action de masse.The free energy and radial distribution functions for vacancies and impurities were calculated from the hypernetted chain integral equations for AgCl doped with CdCl2 (1-5 %). The fraction of impurities in nearest-neighbour vacancy-impurity pairs (but not in larger clusters) and the free energy were compared with the mass action theory description

CONDUCTIBILITÉ IONIQUETHEORY OF THE EFFECTS OF POINT DEFECT INTERACTIONS ON IONIC CONDUCTIVITY

Le domaine de validité des hypothèses relatives aux interactions des défauts dans la théorie de Lidiard-Teltow de la conductivité ionique est discuté en deux parties et une tentative est effectuée dans chacune pour relier cette théorie à des travaux plus récents dans les domaines des solutions électrolytiques et de l'état solide. Dans la première partie, une nouvelle théorie de l'effet des interactions entre défauts sur les équations conventionnelles de loi d'action de masse pour la formation d'associations de défauts est soulignée. La nature des corrections à l'équation de la loi d'action de masse pour la formation de paires impureté-lacune dans la théorie de Lidiard-Teltow est soulignée et son domaine probable de validité discuté dans le cas de AgCl dopé par CdCl2. Une extension des équations générales de formation de défauts de Schotiky est effectuée mais sans calcul détaillé. Dans la deuxième partie, la théorie de la conductivité de Fuoss-Omsager est présentée sous une forme comparable à la théorie dans l'état solide. La grandeur des corrections de l'effet de relaxation dans les deux théories est comparée en même temps qui est discuté le domaine d'utilité des approximations.The range of validity of the assumptions about defect interactions in the Lidiard-Teltow ionic conductivity theory is discussed in two parts and an attempt made in cach to relate this theory to more recent work in electrolyte solution and solid state fields. In the first part a new theory of the effect of defect interactions on the conventional mass action equations for defect cluster formation is outlined. The nature of the corrections to the mass action equation for impurity-vacancy pair formation in the Lidiard-Teltow theory is outlined and its probable range of validity discussed for AgCl doped with CdCl2. An extension of the general equations to Schottky defect formation is made but without detailed calculations. In the second part the Fuoss-Onsager conductance theory is put into a form comparable to the solid state theory. The magnitude of the relaxation effect corrections in the two theories is compared and the range of usefulness of the available approximations discussed