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The conversion of heat to electricity by thermoelectric devices may play a key role in the future for energy harvesting. In order to meet that purpose, a variety of more efficient thermoelectric materials are needed. Intense research has been conducted over the past decade on type-I thermoelectric clathrates, and further developments may arise with successful introduction of trivalent rare-earth elements into the cages of this class of materials. This paper is dedicated to study the hypothetical charge-balanced compound La 2 Ga 6 Ge 40 , using first-principles calculations, in order to evaluate the direct effects of introducing La 3+ into Ge 46 without occupying all the other cages with other chemical species. Here we present first-principles calculations on the structural, electronic, vibrational and thermoelectric properties of type-I Ge clathrates hosting La 3+ guest ions. Our results indicate that the structures with the lowest formation energies are those in which the La 3+ guests are inside the Ge 46 dodecahedral cages ( 2a Wyckoff positions), with Ga substitution for Zintl charge balance. Furthermore, our calculations show that several features of the system, such as, the equilibrium position of the La 3+ guests inside the cages, the electronic structure near the band edges, the vibrational properties and anharmonic effects are significantly affected by the way the Ga atoms are distributed in the lattice. The influence of these features on the thermoelectric properties of these systems is discussed.

We present a density-function theory (DFT) study of Bjerrum-defect trapping centers involving the molecular vacancy in ice Ih. As a first step, we compute the intrinsic migration barrier to D-defect motion using the nudged elastic band (NEB) method and find them to be of the same order of magnitude as the energy barriers involving intrinsic L-defect motion. This finding suggests that intrinsic mobility factors cannot explain the experimentally observed inactivity of D defects, supporting the idea that D defects are trapped at other lattice-defect sites. Next we study the defect complexes formed by the combination of isolated D and L defects with a molecular vacancy. The corresponding geometries show that the formation of these aggregates significantly reduces elastic distortions that are present in isolated Bjerrum defects. An analysis of the energetics involved in the formation of both defect complexes reveals a significant binding energy, indicating that the molecular vacancy represents a strong trapping center for Bjerrum defects. On the other hand, the fact that there is no difference between the absolute values of the binding energies for both D and L defects suggests that the vacancy affects both species of Bjerrum defects in a similar fashion, possibly ruling out the vacancy trapping centers as an explanation for the experimentally observed inactivity of D defects.

Divacancies are among the most important defects that alter the charge transport properties of single-walled carbon nanotubes (SWNT), and we here study, using ab initio calculations, their properties. Two structures were investigated, one that has two pentagons side by side with an octagon (585) and another composed of three pentagons and three heptagons (555777). We investigate their stability as a function of tube diameter, and calculate their charge transport properties. The 585 defect is less stable in graphene due to two broken bonds in the pentagons. We estimate that the 555777 becomes more stable than the 585 for a diameter of about 40 A (53 A) for an armchair (zigzag) SWNTs, indicating that they will prevail in large diameter multiwalled carbon nanotubes and graphene ribbons.

We studied the effects of arsenic on properties of dislocations in silicon. The theoretical investigation was carried out using ab initio total energy methods, based on the density functional theory. We find that the interaction of an arsenic impurity in the crystal with a dislocation results in a charge exchange, driving the dislocation core to a negative charge state. This interaction is essentially electrostatic and attractive, and leads to arsenic segregation. Although arsenic segregation to the core is energetically favourable, formation of arsenic pairs inside the core is energetically unfavourable. We also investigated the role of vacancies in arsenic diffusion inside the dislocation core.

We investigated through ab initio total energy calculations the interaction of arsenic impurities with the core of a 30° partial dislocation in silicon. It was found that when an arsenic atom sits in a crystalline position near the dislocation core, there is charge transfer from the arsenic towards the dislocation core. As a result, the arsenic becomes positively charged and the core negatively charged. The results indicate that the structural changes around the impurity are very small in both environments, namely, the crystal and the dislocation core. In this scenario, the interaction between arsenic and the core is essentially electrostatic, which eventually leads to arsenic segregation. The segregation energy was found to be as large as 0.5 eV/atom. Additionally, it was found that arsenic pairing inside the core is not energetically favorable.

Agradecimentos: The MD simulations were performed on a Digital Alpha 3000/900 of the CENAPAD-SP Computer Center. The authors gratefully acknowledge the financial support granted by the Brazilian funding agencies CAPES, FAPESP, and CNPq Abstract: We study the application of the adiabatic switching molecular dynamics method to determine bulk and vacancy-formation Gibbs free energies as a function of temperature at zero pressure for copper. The bulk free energy has been determined through isochoric isothermal switching procedures in which a system consisting of 500 copper atoms interacting through a semiempirical tight-binding potential is turned into a system of 500 independent identical three-dimensional harmonic oscillators. The equilibrium volumes of these-simulations were determined from equilibrium isobaric isothermal molecular dynamics simulations. The frequency of the oscillators is chosen to be of the order of a principal phonon frequency of copper in order to achieve competitive convergence. The resulting bulk free energy and entropy are in excellent agreement with experimental values. The vacancy-formation Gibbs free energy has been computed from isobaric isothermal switching procedures in which the interactions of a single copper atom are switched off. Considering the limited accuracy of the interatomic potential and the numerical noise present in the small energy differences measured, the estimated formation enthalpies and entropies agree remarkably well with experimental data. The method has shown to be computationally efficient. Typically, 6 h of CPU time on a Digital Alpha 3000/900 were required per data point for the bulk as well as the vacancy-formation parameters COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPES FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ Fechado

Agradecimentos: We gratefully acknowledge support from the Brazilian agencies CNPq, Fapesp, Capes and the Center for Computational Engineering and Sciences-Fapesp/Cepid #2013/08293-7. Part of the calculations were performed at CCJDR-IFGW-UNICAMP and CENAPAD-SP. We would like to thank the anonymous reviewers for their helpful suggestions Abstract: Using molecular dynamics simulations we analyze the dynamics of two atomic liquids that display a liquid-liquid phase transition (LLPT): Si described by the Stillinger-Weber potential and Ga as modeled by the modified embedded-atom model. In particular, our objective is to investigate the extent to which the presence of a dip in the self-intermediate scattering function is a manifestation of an excess of vibrational states at low frequencies and may be associated with a fragile-to-strong transition (FTST) across the LLPT, as suggested recently. Our results suggest a somewhat different picture. First, in the case of Ga we observe the appearance of an excess of vibrational states at low frequencies, even in the absence of the appearance of a dip in the self-intermediate scattering function across the LLPT. Second, studying the behavior of the shear viscosities traversing the LLPTs we find that both substances are fragile in character above and below their respective LLPT temperatures. Instead of a FTST in an absolute sense these findings are more in line with a view in which the LLPTs are accompanied by a transition from a more fragile to a less fragile liquid. Furthermore, we do not find this transition to correlate with the presence of a dip in the intermediate scattering function CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPES Fechado

Electronic applications require the ability to dope diamond p- and n-type. Boron is well known to dope both natural and synthetic diamond p-type. N-type doping, however, has proven exceedingly difficult. In this work, the suitability of several impurities for n-type doping is investigated theoretically. We also examine the well-known nitrogen deep impurity as well as the effect of simultaneous doping with N and B on the thermodynamic equilibrium between diamond and graphite. The calculations were carried out using local density theory, the pseudopotential formalism, and the Car- Parrinello method. The impurities were embedded in a large supercell and atomic relaxations were computed using ab initio forces. The impurities Li, Na and P are shown to be shallow donors, but they have very low solubilities. This makes their incorporation via in-diffusion difficult and leaves ion implantation and possibly incorporation during growth as the only alternatives. Once incorporated, Li is found to be a fast diffuser whereas Na will be stable up to moderate temperatures. The most suitable shallow donor is Na, which occupies an interstitial site. It is particularly appropriate for ion implantation, since no self-implantation step to create vacant sites is necessary.

The properties of native defects and impurities in diamond and SiC are investigated via large-scale band structure and Car-Parrinello calculations. In diamond, the activation energy for self-diffusion is very high in the intrinsic material (9 eV) but decreases by up to 3 eV in either p- or n-type material. Phosphorus, lithium and sodium are shallow donors, but their solubilities are very low, which makes them unsuitable for incorporation into diamond via in-diffusion. Instead, kinetic trapping during growth or ion implantation must be used. Considering the stability at the dopant site, substitutional phosphorus is expected to diffuse by the vacancy mechanism and to have a high activation energy by analogy to self-diffusion. Both lithium and sodium diffuse through the interstitial channel. Lithium is a relatively fast diffuser while sodium should be stable up to moderately high temperatures. For nitrogen in diamond, the well known (111) distortion is found to be due to the interaction of the fully occupied nitrogen lone pair with the dangling bond of the C(111) atom. The single electron associated with the center resides in an antibonding orbital formed from the dangling hybrid and the nitrogen lone pair. This orbital has most of its amplitude on the carbon atom. In SiC, the lowest energy defect in n-type and intrinsic material is the electrically inactive silicon antisite, while the lowest energy defect in p-type SiC is the doubly positive carbon vacancy. The electrons released by the vacancies compensate acceptor dopants, leading to strong self-compensation effects when doping occurs during crystal growth. In carbon-rich SiC, the dominant defect for all Fermi level positions is the electrically inactive carbon antisite. In boron-doped SiC, BC is preferred for silicon-rich material while, in carbon-rich SiC, BC and BSi have similar formation energies.

Tetrahedral substances, such as silicon, water, germanium, and silica, share various unusual phase behaviors. Among them, the so-called polyamorphism, i.e., the existence of more than one amorphous form, has been intensively investigated in the last three decades. In this work, we study the metastable relations between amorphous states of silicon in a wide range of pressures, using Monte Carlo simulations. Our results indicate that the two amorphous forms of silicon at high pressures, the high density amorphous (HDA) and the very high density amorphous (VHDA), can be decompressed from high pressure (∼20 GPa) down to the tensile regime, where both convert into the same low density amorphous. Such behavior is also observed in ice. While at high pressure (∼20 GPa), HDA is less stable than VHDA, at the pressure of 10 GPa both forms exhibit similar stability. On the other hand, at much lower pressure (∼5 GPa), HDA and VHDA are no longer the most stable forms, and, upon isobaric annealing, an even less dense form of amorphous silicon emerges, the expanded high density amorphous, again in close similarity to what occurs in ice.

We propose a new microscopic model for the $\{001\}$ planar defects in diamond commonly called platelets. This model is based on the formation of a metastable stacking fault, which can occur because of the ability of carbon to stabilize in different bonding configurations. In our model the core of the planar defect is basically a double layer of three-fold coordinated $sp^2$ carbon atoms embedded in the common $sp^3$ diamond structure. The properties of the model were determined using {\it ab initio} total energy calculations. All significant experimental signatures attributed to the platelets, namely, the lattice displacement along the $[001]$ direction, the asymmetry between the $[110]$ and the $[1\bar{1}0]$ directions, the infrared absorption peak $B^\prime$, and broad luminescence lines that indicate the introduction of levels in the band gap, are naturally accounted for in our model. The model is also very appealing from the point of view of kinetics, since naturally occurring shearing processes will lead to the formation of the metastable fault., Comment: 5 pages, 4 figures. Submitted for publication on August 2nd, 2004

The free energy of the concerted exchange mechanism for self-diffusion in silicon is estimated using the thermodynamic integration method and Monte Carlo (MC) simulations with an interatomic potential fitted to reproduce local-density-approximation calculations. Anharmonicity and relaxation are fully taken into account in the calculations, since the phase space is extensively explored by the MC simulations. The results indicate that the concerted exchange mechanism can have a significant contribution to the self-diffusion constant in silicon. \textcopyright{} 1996 The American Physical Society.

Agradecimentos: We gratefully acknowledge support from the Brazilian agencies CNPq, Fapesp, and Capes. Part of the calculations were performed at CCJDR-IFGW-UNICAMP and CENAPAD-SP Abstract: We use molecular simulation to analyze liquid dynamics in the vicinity of the liquid-liquid phase transition (LLPT) recently discovered in the modified embedded-atom model for elemental gallium. For this purpose we analyze the diffusive behavior in terms of the mean-squared displacement and self-intermediate scattering functions for two systems obtained by cooling the stable liquid through the LLPT at different cooling rates. The results show a pronounced heterogeneity of the dynamics upon the onset of the LLPT. Furthermore, it is found that this heterogeneity is closely correlated to the structural properties of the 9-fold coordinated high-density and 8-fold coordinated low-density liquid forms involved in the transition, showing a mixture of domains with very different diffusion time scales. The dynamics of the low-density liquid is found to be much more sluggish than that of the high-density form. Analysis of the energetics suggests that the origin of this difference is rooted in the fact that the cohesion in the former is significantly stronger than that in the latter CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPES Fechado

Electronic applications of diamond require control over native defects as well as the ability to dope it p- and n-type. B is an excellent p-type dopant, but n-type doping has proven very difficult. Diamond films have also been very difficult to anneal, indicating a high activation energy for self-diffusion. We have investigated the properties of native defects and impurities through large-scale band structure and Car-Parrinello calculations. We indeed find that the activation energy for self-diffusion is very high in the intrinsic material, but it decreases by as much as 3 eV in either p- or n-type material. P, Li, and Na are shallow donors, but their solubilities are too low for incorporation into diamond through in-diffusion. It is energetically favorable for B and N to dissolve in diamond, which explains their prevalence in natural diamond. The calculations explain for the first time the reasons for the distortion of atoms around N from the fully tetrahedral site, as well as why N is a deep rather than a shallow donor. We also consider the effects of simultaneous doping with N and B on the thermodynamic equilibrium between diamond and graphite.

Many of the important properties of semiconductors are affected by the presence of native defects. These include diffusion of native atoms and impurities, electronic properties, structural rearrangements during growth or processing, etc. We have used local density theory and non-local pseudopotentials to study trends in the relative abundance of native defects and in self-diffusion (diffusion of native tracer atoms) in diamond, SiC, Si and Ge. Both point-defect-mediated diffusion and direct exchange of lattice atoms were considered. All calculations were carried out using the same methodology and similar convergence criteria.

We have investigated, via first principles total energy calculations, the energetics of elementary native defects in group IV semiconductors. Its implications on the relative abundance of these defects and self-diffusion phenomena are analyzed. The results show that in diamond the self-diffusion is dominated by vacancies, because the interstitial and direct exchange mechanisms have much greater activation energy. In SiC stoichiometry plays an important role. For Si-rich compound, Sic-antisite is the dominant defect in the intrinsic and p-type material, while the carbon vacancy is dominant in the n-type material. For C-rich material, the Csi-antisite is dominant regardless the position of the Fermi level. In Si, it well-known that the vacancy, interstitial and direct exchange mechanisms have very similar activation energies. Our results suggest that self-diffusion experiments carried out at various pressures can determine the relative contribution of each of these mechanisms.

The effects of hydrostatic pressure on the energetics of self-diffusion in silicon are investigated via parameter-free total-energy calculations. The three microscopic mechanisms, vacancy, interstitial, and concerted exchange, which have very similar activation energies in Si, exhibit different pressure dependences. The results suggest that a set of experiments carried out at different pressures can unravel their relative contributions by a comparison to the present results. In addition, it is shown that in contrast to the (111) surface, the nearest neighbors of the Si vacancy relax inwards, rather than outwards.

We have investigated, via parameter-free calculations, the effects of hydrostatic and nonhydrostatic strains on the energetics of defect formation and self-diffusion in silicon. The three microscopic mechanisms, vacancy, interstitial, and concerted exchange, have very similar activation enthalpies at zero pressure but exhibit different behavior with hydrostatic pressure. Our results suggest that experiments performed at different pressures can determine the relative contributions of each of these mechanisms. The calculations also show that the neutral Si vacancy has a negative relaxation volume, with the nearest neighbors of the vacancy relaxing inwards, in contrast to the Si (111) surface. Large nonhydrostatic strains, which are present in e.g. Si/GexSi1-x pseudomorphic heterostractures, substantially reduce the formation energy of the tetrahedral interstitial, but do not affect the formation energy of the vacancy. These findings suggest that, aside from being another useful tool for the investigation of self-diffusion in Si, nonhydrostatic strain may significantly affect annealing and impurity diffusion in strained heterostructures. In particular, the interstitial-assisted impurity diffusion may proceed more rapidly in Si lattice-matched to GexSi1-x, but be slowed down in GexSi1-x lattice-matched to Si. The compressed GexSi1-x layers may thus act as diffusion barriers for impurities diffusing with help of native interstitials, such as B or P.