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Trends in the growth of extended interstitial defects are extracted from extensive tight-binding and ab inito local density approximation simulations. With an increasing number of interstitials, the stable defect shape evolves from compact to chainlike to rodlike. The rodlike 311 defect, formed from (011) interstitial chains, is stabilized as it grows, elongating in the chain direction. Accurate parametrization of the defect-formation energy on the number of interstitials and interstitial chains, together with the anisotropy of the interstitial capture radius, enables macroscopic defect-growth simulations.

We studied the dynamical properties of Au using our previously developed tight-binding method. Phonon-dispersion and density-of-states curves at T=0 K were determined by computing the dynamical-matrix using a supercell approach. In addition, we performed molecular-dynamics simulations at various temperatures to obtain the temperature dependence of the lattice constant and of the atomic mean-square-displacement, as well as the phonon density-of-states and phonon-dispersion curves at finite temperature. We further tested the transferability of the model to different atomic environments by simulating liquid gold. Whenever possible we compared these results to experimental values.

We propose a di-interstitial model for the P6 center commonly observed in ion implanted silicon. The di-interstitial structure and transition paths between different defect orientations can explain the thermally activated transition of the P6 center from low-temperature C 1h to room-temperature D 2d symmetry.The activation energy for the defect reorientation determined by ab initio calculations is 0.5 eV in agreement with the experiment. Our di-interstitial model establishes a link between point defects and extended defects, di-interstitials providing the nuclei for the growth.

Ab initio molecular-dynamics simulations have been used to investigate the structure, dynamics and electronic properties of the liquid alloy Ag1−xSex at 1350 K and at the three compositions x = 0.33, 0.42 and 0.65. To provide a point of reference, calculations are also presented for the equilibrium structure and the electronic structure of the α-Ag2Se crystal. The calculations are based on density-functional theory in the local density approximation and on the pseudopotential plane-wave method. For the solid, we find excellent agreement with experiment for the equilibrium lattice parameters and the atomic coordinates of the 12-atom orthorhombic unit cell, and we present an analysis of the electronic density of states and density distribution. The reliability of the liquid simulations is confirmed by detailed comparisons with very recent neutron diffraction results for the partial structure factors and radial distribution functions (RDF) of the stoichiometric liquid Ag2Se. Comparison with the predictions of an empirical interaction model due to Rino et al. is also given for ℓ-Ag2Se. The ab initio simulations show a dramatic change of the Se-Se RDF with increasing Se content. This change is due to the formation of Se clusters bound by covalent bonds, the Se-Se bond length being almost the same as in pure c-Se and ℓ-Se. The clusters are predominantly chain-like, but for higher x there is a significant fraction of 3-fold coordinated Se atoms. It is shown that the equilibrium fractions of Se present as isolated atoms and in clusters can be understood on a simple charge-balance model based on an ionic interpretation. The Ag diffusion coefficient in the simulated stoichiometric liquid is consistent with experimental values measured in the hightemperature superionic solid. The Ag and Se diffusion coefficients both increase with Se content, in spite of the Se clustering. An analysis of the Se-Se bond dynamics reveals surprisingly short bond lifetimes of less than 1 ps. The electronic density of states (DOS) for ℓ-Ag2Se strongly resembles that of the solid. Some of the changes of DOS with composition arise directly from the formation of Se-Se covalent bonds. Results for the electronic conductivity σ obtained using the Kubo-Greenwood approximation are in adequate agreement with experiment for ℓ-Ag2Se, but for the high Se contents the simulation results for σ are 3-4 times greater than experimental values. Possible reasons for this are discussed.

First-principles molecular-dynamics simulations of liquid selenium at the temperatures 570, 870, and 1370 K are presented. It is shown that calculations based on the local-density approximation are not satisfactory, because they seriously overestimate the equilibrium density of the solid and the liquid, and they give only rough agreement with measured structural data when the liquid is simulated at the experimental density. The inclusion of gradient corrections mitigates these problems, and most of the results presented are based on the generalized gradient approximation. The simulations are used to investigate the three-dimensional structure of the liquid, and results are presented for the bond-angle and dihedral-angle distributions, the concentration and structure of different Se n rings, and the temperature-dependent length of chains. The self-diffusion constants and vibrational spectra are shown to agree satisfactorily with experimental data, and an analysis of the lifetime of covalent bonds gives insight into the diffusion mechanism. The electronic density of states of the liquid displays all the features known for the solid, and this indicates that the electronic structure changes little on melting.

%'e have studied the structural and bonding properties of the equilibrium and high-pressure phases of tellurium by means of Srst-principles total-energy calculations, performed within the localdensity approximation.The calculated characteristics of the various polymorphs under pressure show good general agreement with existing experiments. However, some systematic discrepancies occur between computed and measured structural parameters for the open linear-chain (trigonal) and layer-type (orthorhombic) phases. The interchain and interlayer distances are underestimated within the local-density approach, and the relative stability of the trigonal and of the orthorhombic phase with respect to compact metallic structures is lower in the calculations than indicated by the experimental phase diagram. The complex structural changes of Te under pressure are discussed in terms of the trends under pressure of the di6erent types of bonds involved in the various crystal structures.

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