Quantum Theory of Advanced Materials

We present a first-principles investigation to study the possible alloy phases of sodium and lithium alanates. Structural and energetics properties of alloy systems Na(1-x)Li(x)AlH(4) and Na(3(1-x))Li(3x)AlH(6) are studied via phase interpolation. Alloy system Na(1-x)Li(x)AlH(4) is found to have a small mixing energy (<5 kj/mol). The equilibrium structure undergoes a transition from a tetragonal structure to a monoclinic structure between x = 0.25 and 0.5. Within each structure the cell volume decreases with increasing x, which can be explained by Li having a smaller ion size than Na. Alloy system Na(3(1-x))Li(3x)AlH(6) is also studied, and one intermediate composition Na(2)LiAlH(6) is found to be stable in agreement with experimental findings. (C) 2009 Elsevier B.V. All rights reserved.

The Li-Mg-N-H system has been identified as a promising hydrogen storage material due to its moderate operation conditions as well as the high capacity and reversibility. Recently Rijssenbeek et al. [J. Alloys Compd. 454, 233 (2008)] reported that Li(2)Mg(NH)(2) has disordered cation and vacancy arrangements at room temperature and above. We present our first-principles calculations to investigate a series of ordered low-energy configurations for this compound. Specific local orderings are found in the cation-vacancy arrangement, shedding light on the experimental disordered structure models. A possible ordered phase at low temperature is proposed based on these local orderings. Reaction energetics and phase stability are further discussed. (c) 2008 American Institute of Physics. [DOI: 10.1063/1.3003067]

By accurately fitting tight-binding parameters to ab initio band structures from 14 different tetrahedral volumes, tight-binding parametric formulas have been developed for silicon and germanium. The distance dependences for these orthogonal, nearest-neighbor parameters range from r-2.5 to r-3.3. Repulsive potentials are added in order to reproduce the total energies for a number of bulk structures. It is found that the repulsive potential needed has the simple form of a pairwise interaction multiplied by a structure-dependent constant. Transferability is shown with good bulk and cluster results.

We present a tight-binding model for silicon which incorporates two-center intra-atomic parameters. The model is fitted to density-functional theory band structures for silicon in the diamond structure over a number of volumes. It is shown that with only a two-center, orthogonal basis, reasonable total energies can be obtained for many different structures. Thus it eliminates the need to use structure-dependent terms in the total-energy model.

We have performed tight-binding calculations on a model of an amorphous silicon sample generated previously by a molecular-dynamics simulation employing the Stillinger-Weber potential. The sample consists of 588 atoms and contains a high density of floating-bond defects. Two tight-binding calculations are presented, one using the widely accepted Chadi parameters, which include only nearest-neighbor interactions, and the other using the parameters recently proposed by Allen, Broughton, and McMahan (ABM) [Phys. Rev. B 34, 859 (1986)] for a nonorthogonal basis set. Comparison of the densities of states shows similar behavior in the valence band, but the electron density near a defect is less localized with the ABM parameters. It is also found that the projected density of states on the fivefold-coordinated atoms is very close to that on the fourfold-coordinated atoms, while the projected density of states on the threefold-coordinated atoms is distinctly different and has more states in the gap.

Using tight-binding models, the energies of a number of silicon and germanium (111) surfaces are studied. These include reconstructed surfaces with dimers and stacking faults (DS), simple adatom surfaces such as 2x2 and c(2x8), and more complicated cases with dimers, adatoms, and stacking faults (DAS). For reconstructed surfaces containing adatoms, it is found that a simple correction term dependent on the adatom concentration is needed in the present total-energy model to account for the unusual geometry. Similarities between the silicon and germanium reconstructions are seen and compare well with ab initio results. There are also some differences between silicon and germanium, for example, the DS surfaces are lower in energy than the relaxed (1x1) for silicon, but higher for germanium. Si(111) reconstructs into the DAS structure while Ge(111) goes to the simple adatom c(2x8) surface. The c(2x8), 7x7 DAS, (1x1), and 7x7 DS surface reconstructions of Ge(111) were studied with in-plane strain. For these surfaces, a strain of about 2% was sufficient to make the 7x7 DAS/DS surface lower in energy than the c(2x8)/(1x1) surface. An analysis of the energy per atom showed that the dimer-row and associated first-layer atoms played a major part in the differing energy behavior, in agreement with an earlier proposal. An expansive strain was applied to the 2x2, 7x7 DAS, (1x1), and 7x7 DS surface reconstructions of Si(111). With a strain of about 2.5% the adatom surfaces switched relative energies, while the adatom free surfaces required only about 1.5% strain. As for germanium, the dimer-row and associated atoms were of major importance in the differing energy change.

The physical and chemical properties of thin metal films show damped oscillations as a function of film thickness (one-dimensional shell effects). While the oscillation period, determined by subband crossings of the Fermi level, is the same for all properties, the phases can be different. Specifically, oscillations in the work function and surface energy are offset by 1/4 of a period. For Pb(111) films, this offset is similar to 0.18 monolayers, a seemingly very small effect. However, aliasing caused by the discrete atomic layer structure leads to striking out-of-phase beating patterns displayed by these two quantities.