Effect of pentagons in sp3 systems on electronic, elastic, and vibrational properties: Case of chiral structures

We present first-principles calculations of carbon and silicon chiral framework structures (CFSs). In this system, proposed recently by Pickard and Needs [Phys. Rev. B 81, 014106 (2010)], atoms form only pentagonal cycles. This configuration enables unambiguous analysis of the effects of pentagons on electronic, vibrational, and thermodynamic properties. The local density approximation electronic band gaps in CFSs were found to be equal to or greater than those of clathrates using the same formalism, as confirmed by GW calculations: 1.8 and 5.5 eV for Si and C-CFS, respectively. We show that, as in clathrates, an increasing electronic band gap is correlated with the contraction of the valence bands, resulting from the frustration of the p shells. The electron localized function and Wannier analysis confirm the sp3 nature of the bonds. Finally, we discuss vibrational and related properties. We show that CFSs present singularities, in particular, that the higher frequencies are not located at the Γ point

First-principles calculations of carbon clathrates: comparison to silicon and germanium clathrates

We employ state-of-the-art first-principles calculations based on density-functional theory and density-functional perturbation theory to investigate relevant physical properties and phase diagram of the free guest type-I (X-46) and type-II (X-34) carbon clathrates. Their properties and those of silicon and germanium diamonds, and clathrates have been computed and compared within the same approach. We briefly present and discuss their structural, cohesive, and electronic properties. In particular, we present different results about electronic properties of carbon clathrates. From the symmetry analysis of electronic states around the band gap, we deduce their optical properties, and we forecast the effects of hypothetical-doped elements on their electronic band gap. We then report first-principles calculations of vibrational, thermodynamical, and elastic properties. Whereas vibrational properties of Si and Ge systems can be linked through their atomic weight ratio, we show that the vibrational properties of carbon structures differ strongly. Raman and infrared spectra of all clathrates are also calculated and compared. The effects of pressure and temperature on thermodynamical properties (heat capacity, entropy, thermal expansion, etc.) within static and quasiharmonic approximations are investigated. It is shown that thermodynamical properties of carbon clathrates and diamond present a similar evolution up to high pressures (100 GPa) and over a large range of temperatures ([0, 1500] K). Then we deduce the equilibrium phase diagram (P,T) of C-2/C-34/C-46. We conclude the paper with a presentation of elastic properties computed from acoustic slopes

Structural and electronic properties of p-doped silicon clathrates

We present an ab initio study of the structural and electronic properties of type-I and type-II silicon clathrates doped by elements chosen to be more electronegative than silicon. Depending on the intercalated element, we show that the electronic properties of doped silicon clathrates can exhibit metallic, semiconducting, or insulating behavior. It is found in particular that doping can lead to silicon-based materials with a band gap in the visible range and that, in type-II clathrates, the gap can be direct. However, the analysis of the selection rules show that the optical transitions are forbidden in type-I and type-II clathrates. Concerning the structural properties, the bonding between the dopant atom and silicon can significantly decrease the compressibility of the host network to values equivalent to the one of the much denser diamond phase. The present results are complemented and rationalized by the study of endohedrally doped SinHn n=20,24,28 silicon clusters

First-principles study of nickel-silicides ordered phases

We present a study of nickel-silicides ordered alloys by means of first-principles calculations. Emphasis was put on the phases (low and high temperatures) identified in the binary phase diagram, namely: Ni3Si-β1, -β2, and -β3, Ni31Si12-γ, Ni2Si-δ, -θ, Ni3Si2-ɛ, NiSi-MnP and NiSi2-α. In addition, some common structures are computed for information: L12, D03 and D022. The simulations reproduce with a high accuracy lattice parameters and formation energies of main experimental structures, except for β2 and β3. Our results clarify the crystallographic nature of the γ structure, and the comparison of experimental Raman spectra and vibrational calculations will help experimentalists to identify without ambiguity NiSi3 structures

Electronic and superconducting properties of silicon and carbon clathrates

We review the electronic properties of pure and doped silicon and carbon clathrates. Using accurate quasiparticle calculations within the GW approximation, we show that undoped clathrates are similar to 1.8 eV band gap semiconducting compounds. Further, the effect of doping by elements more electronegative than Si is shown to lead to p-type doped semiconductors with a similar to2.3-2.5 eV band gap in the visible energy range. Similar results are observed under doping of hydrogenated Si(n) (n = 20, 24, 28) clusters and rationalized on the basis of group theory analysis. Finally, the superconducting properties of doped clathrates are discussed. We show that superconductivity is an intrinsic property of the standard silicon sp(3) environment provided that efficient doping can be achieved. (C) 2003 Published by Elsevier B.V

Lattice instabilities in hexagonal NiSi: A NiAs prototype structure

We report a first-principles study of the hexagonal NiSi phase with the B81 strukturbericht designation. This structure, reported by Föll Philos. Mag. A 45, 31 1982, d’Heurle J. Appl. Phys. 55, 4208 1984, and Dai Appl. Phys. Lett. 75, 2214 1999, is actually only observed during annealing of Ni films on 111 silicon crystals. We discuss, in this paper, about its structural, energetic, vibrational, electronic, and elastic properties, computed by means of the density-functional and density-functional perturbative theory within the spinpolarized Perdew-Burke-Ernzerhof functional. Two configurations with this crystallographic structure have been studied, noted h-NiSi and h-SiNi in the following. We show that theoretical and experimental lattice parameters are not compatible for both systems. A large discrepancy 8–10 % is evidenced, much larger than both experimental and simulation accuracies obtained for others Ni-Si systems. Moreover the vibrational spectra of h-NiSi and h-SiNi present both soft modes, indicating that in their ground states these systems are dynamically unstable. Using a band folding approach, we have analyzed modes for h-NiSi on a supercell, permitting us to identify eigenvectors associated to these instabilities. We have then relaxed the cell in accordance to these eigenvectors, and a final structure is thus proposed. To understand the mechanism at the origin of these negative frequencies in h-NiSi, electronic states around the Fermi level have been plotted, and we identify in the Fermi-surface potential nesting vectors, suggesting that an electron-phonon coupling mechanism could be at the origin of the instability. Whereas the ground state of “h-NiSi” seems not to be associated to the B81 system, we show that a stress in the basal plane could induce an increasein the c axis, restoring the agreement with experimental data

First principle energies of binary and ternary phases of the Fe–Nb–Ni–Cr system

We present first principles enthalpies of formation and lattice parameters of iron, nickel, chromium and niobium alloys. Some of these results have been partially used in a recent assessment of the Fe–Ni–Cr–Nb quaternary phase diagram. Emphasis has been put on the fcc (A1) and bcc (A2) unary structures, the X3Y-D022, -L12, -D03, -D0a, X2Y-C14(MgZn2), -C15(MgCu2) and -C36 (MgNi2) Laves and X7Y6-D85 (μ) binary phases, and the X8Y4Z18-D8b (σ) ternary phase. We employed the state of the art to compute their properties by means of the DFT (PBE functional and PAW pseudo potentials). A comparison with experimental and theoretical data is also provided

First-principles study of diffusion and interactions of vacancies and hydrogen in hcp-titanium

We present a study of the stability of n-vacancies (Vn) and hydrogens in the hexagonal close-packed titanium system computed by means of first-principles calculations. In this work, performed by using the generalized gradient approximation of density functional theory, we focused on the formation energies and the processes of migration of these defects. In the first part, the calculated formation energy of the monovacancy presents a disagreement with experimental data, as already mentioned in the literature. The activation energy is underestimated by almost 20%. The stability of compact divacancies was then studied. We show that a divacancy is more stable than a monovacancy if their migration energies are of the same order of magnitude. We also predict that the migration process in the basal plane of the divacancy is controlled by an intermediate state corresponding to a body-centered triangle(BO site). The case of the trivacancies is finally considered from an energetic point of view. In the second part, the insertion of hydrogen and the processes of its migration are discussed. We obtain a satisfactory agreement with experimental measurements. The chemical nature of the interactions between hydrogen and titanium are discussed, and we show that the H-atom presents an anionic behavior in the metal. The trapping energy of hydrogen in a monovacancy as a function of the number of hydrogen atoms is finally presented

Optimisation of the parameters of an extended defect model applied to non-amorphizing implants

In this paper, we present the optimisation of the parameters of a physical model of the kinetics of extended defects and applied the model with the optimised parameters to non-amorphizing implants. The model describes the small clusters, the {113} defects and the dislocation loops. In the first part, we determine the formation energies of the small clusters, the fault energy of the {113} defects, their Burgers vector and the self-diffusivity of silicon using TEM measurements and extractions of the supersaturation from the spreading of boron marker layers in low-dose implanted silicon. The improvements of the simulations are presented for the fitted experiments and for other wafers annealed at intermediate temperatures. In the second part, we increase the dose and energy of the non-amorphizing implant, leading to the transformation of {113} defects into dislocation loops. The predictions obtained with the optimised model are shown to be in agreement with the measurements. (c) 2005 Elsevier B.V. All rights reserved

Atomic-scale study of low-temperature equilibria in iron-rich Al-C-Fe

The capability of the thermodynamic approach based on the independent point defect approximation to describe low-temperature phase equilibria is investigated and applied to the Al-C-Fe system. The method gives a reasonable description of the multicomponent and multisublattice Fe-rich corner and evidences numerous peculiarities concerning the ordered phases as well as the density-functional-theory (DFT) energy models. The study of Fe3Al(-C), revealing strong defect-induced instabilities, rules out the LDA, SLDA and GGA schemes and leaves (spin-polarized) SGGA as the only valid one. C stabilizes L12 Fe3Al with respect to D03, which justifies the fcc-type structure of the kappa Fe3AlC compound. The present work also helps in justifying the experimentally observed depletion of C in the kappa phase. Finally, a correct description of both Fe3C and kappa requires inclusion of interstitial carbon at low temperature, emphasizing the unexpected importance of interstitial defects in ordered phases