Quantum Theory of Advanced Materials

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Lithium imide (Li2NH) has been considered as a promising medium for hydrogen storage with the following reaction: LiNH(2)+LiH <-> Li(2)NH+H(2). All possible phases involved in the reaction need to be fully characterized in order to understand the right pathway connecting the two end compounds LiNH(2) and Li(2)NH and to further improve its reaction condition to meet the requirements of practical applications. We study from first-principles calculations the possible intermediate compounds Li(2-x)NH(1+x) between Li(2)NH and LiNH(2). Based on the energetics results, possible intermediate phases are identified for 0

The bare and hydrogen-covered diamond (100) surfaces were investigated through pseudopotential density-functional calculations within the local-density approximation. Different hydrogen coverages, ranging from one to two, were considered. These corresponded to different structures (1x1, 2x1, and 3x1) and different hydrogen-carbon arrangements (monohydride, dihydride, and configurations in between). Assuming the system was in equilibrium with a hydrogen reservoir, the formation energy of each phase was expressed as a function of hydrogen chemical potential. As the chemical potential increased, the stable phase successively changed from bare 2x1 to (2x1):H, to (3x1):1.33H, and finally to the canted (1x1):2H. Setting the chemical potential at the energy per hydrogen in H-2 and in a free atom gave the (3x1):1.33H and the canted (1x1):2H phase as the most stable one, respectively. However, after comparing with the formation energy of CH4, only the (2x1):H and (3x1):1.33H phases were stable against spontaneous formation of CH4. The former existed over a chemical potential range ten times wider than the latter, which may explain why the latter, despite having a low energy, has not been observed so far. Finally, the vibrational energies of the C-H stretch mode were calculated for the (2x1):H phase.

It has been suggested that the diffusion of AlH(3) vacancies plays an essential role in the decomposition of NaAlH(4), a prototypical material for hydrogen storage. We find from first-principles calculations that the AlH(3) vacancy induces several isolated vibrational modes that are highly localized in the vacancy region with frequencies within the phonon gaps of pure NaAlH(4) in both the a and g phases. Thus, the proposed existence of AlH(3) vacancies in the dehydrogenation reaction of NaAlH(4) can be possibly confirmed with the experimental detection of these unique vibrational modes associated with the AlH(3) vacancy.

The structural and electronic properties of hydrogen in yttrium are studied using the pseudopotential method within the local-density-functional approximation (LDA). Different concentration regions are considered for the alpha and beta phases. The binding energies associated with different interstitial sites are evaluated as well as the diffusion energy barrier and local vibrational modes. It is found that the occupation of the tetrahedral site is energetically more favorable than that of the octahedral site in the alpha phase. The calculated vibrational frequency is in excellent agreement with the value observed in neutron scattering experiments. Possibility of pairing is also examined from the consideration of energetics.

We have carried out first-principles calculations of Pb (111) films up to 25 monolayers to study the oscillatory quantum size effects exhibited in the surface energy and work function. These oscillations are correlated with the thickness dependence of the energies of confined electrons, which can be properly modeled by an energy-dependent phase shift of the electronic wave function upon reflection at the interface. It is found that a quantitative description of these quantum size effects requires a full consideration of the crystal band structure.

Atomically uniform Pb films are successfully prepared on Si(111), despite a large lattice mismatch. Angle-resolved photoemission measurements of the electronic structure show layer-resolved quantum well states which can be correlated with dramatic variations in thermal stability. The odd film thicknesses N=5, 7, and 9 monolayers show sharp quantum well states. The even film thicknesses N=6 and 8 do not, but are much more stable than the odd film thicknesses. This correlation is discussed in terms of a total energy calculation and Friedel-like oscillations in properties.

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.

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 studied total energies of various ordered structures of PdHx (in which hydrogen occupies the octahedral sites within the fee Pd lattice) using the pseudopotential method and a plane-wave basis within the local-density-functional approximation. The structures considered include the (420)-plane ordering of hydrogen atoms at different concentrations. For x greater than or equal to 1/2 we found that the NiMo- and Ni4Mo (D1(a))-type structures at x=1/2 and x=4/5, respectively, were energetically favored phases, in agreement with the superlattice reflections found in previous neutron-scattering measurements. For the intermediate concentrations, linear variation of the formation energy as a function of x in several (420)-ordered structures explained the observed short-range order. In contrast to an earlier proposal, we did not find the Fermi surface imaging effect responsible in this case. The overall energy variation in different phases indicates the importance of going beyond pairwise interactions between interstitial hydrogen atoms in this system.