We have studied stability of lithium-manganese oxides using density functional theory in the local density and generalized gradient approximation (GGA). In particular, the effect of spin-polarization and magnetic ordering on the relative stability of various structures is investigated. At all lithium compositions the effect of spin polarization is large, although it does not affect different structures to the same extent. At composition ${\mathrm{LiMnO}}_{2},$ globally stable Jahn-Teller distortions could only be obtained in the spin-polarized GGA approximation, and antiferromagnetic spin ordering was critical to reproduce the orthorhombic ${\mathrm{LiMnO}}_{2}$ structure as ground state. We also investigate the effect of magnetism on the Li intercalation potential, an important property for rechargeable Li batteries.

Yttria $({\mathrm{Y}}_{2}{\mathrm{O}}_{3})$ has become a promising gate oxide material to replace silicon dioxide in metal-oxide-semiconductor devices. Using a first-principles approach the electronic structure, defect structure, and formation energy of native point defects in ${\mathrm{Y}}_{2}{\mathrm{O}}_{3}$ are studied. Vacancies, interstitials, and antisites in their relevant charge states are considered. We find that within the band gap of ${\mathrm{Y}}_{2}{\mathrm{O}}_{3}$ oxygen vacancies, oxygen interstitials, yttrium vacancies, and yttrium interstitials can be stable depending on the Fermi level and external chemical potentials. When the Fermi level is constrained to be within the band gap of silicon, oxygen vacancies are the dominant defect type under low oxygen chemical potential condition. A higher oxygen chemical potential leads to oxygen interstitials and ultimately yttrium vacancies.

Abstract not Available.

We have used density functional theory (DFT) to investigate the ternary phase diagram of the Li−Fe−F system and the reactions of Li with iron fluorides. Several novel compounds, not previously identified in the Li−Fe−F system, are predicted to be stable. Electrochemical voltage profiles, derived from the evolution of the Li chemical potential in the calculated phase diagram, are in reasonable agreement with experimental trends. The effect of particle size on the Fe that precipitates when LixFeF3 reacts with Li is also investigated. We find that when 1 nm Fe particles form, the potential for this reaction is considerably reduced from its bulk value and relate this to the experimental observations. Furthermore, we formulate a model for the significant hysteresis that is observed in the lithiation and delithiation of FeF3. Nonequilibrium paths derived by assuming much faster diffusion of Li than Fe are in reasonable agreement with experimental profiles. Our kinetic model predicts that the iron fluoride reaction follows a different path through the phase diagram during conversion (discharge) and reconversion (charge), which results in the voltage profile hysteresis observed during experiment. The proposed kinetic model also explains why upon extraction of Li from a 3/1 mixture of LiF and Fe a rutile FeF2-like structure can form, even when iron should be oxidized to Fe3+ by extraction of three Li+ per Fe.

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We report the first detection of asymmetry in a supernova (SN) photosphere based on SN light echo (LE) spectra of Cas A from the different perspectives of dust concentrations on its LE ellipsoid. New LEs are reported based on difference images, and optical spectra of these LEs are analyzed and compared. After properly accounting for the effects of finite dust-filament extent and inclination, we find one field where the He I and H alpha features are blueshifted by an additional ~4000 km/s relative to other spectra and to the spectra of the Type IIb SN 1993J. That same direction does not show any shift relative to other Cas A LE spectra in the Ca II near-infrared triplet feature. We compare the perspectives of the Cas A LE dust concentrations with recent three-dimensional modeling of the SN remnant (SNR) and note that the location having the blueshifted He I and H alpha features is roughly in the direction of an Fe-rich outflow and in the opposite direction of the motion of the compact object at the center of the SNR. We conclude that Cas A was an intrinsically asymmetric SN. Future LE spectroscopy of this object, and of other historical SNe, will provide additional insight into the connection of explosion mechanism to SN to SNR, as well as give crucial observational evidence regarding how stars explode.

We show that cluster expansions (CE), previously used to model solid-state materials with binary or ternary configurational disorder, can be extended to the protein design problem. We present a generalized CE framework, in which properties such as energy can be unambiguously expanded in the amino-acid sequence space. The CE coarse grains over nonsequence degrees of freedom (e.g., side-chain conformations) and thereby simplifies the problem of designing proteins, or predicting the compatibility of a sequence with a given structure, by many orders of magnitude. The CE is physically transparent, and can be evaluated through linear regression on the energies of training sequences. We show, as example, that good prediction accuracy is obtained with up to pairwise interactions for a coiled-coil backbone, and that triplet interactions are important in the energetics of a more globular zinc-finger backbone.

Fundamental aspects of the cooperative Jahn-Teller effect are investigated using density functional theory in the generalized gradient approximation. ${\mathrm{LiNiO}}_{2},$ ${\mathrm{LiMnO}}_{2},$ and ${\mathrm{LiCuO}}_{2}$ are chosen as candidate materials as they possess small, intermediate, and large cooperative Jahn-Teller distortions, respectively. The cooperative distortion is decomposed into the symmetrized-strain modes and $k=0$ optical phonons, revealing that only the ${E}_{g}$ and ${A}_{1g}$ strain modes and ${E}_{g}$ and ${A}_{1g}$ $k=0$ optical-phonon modes participate in the cooperative distortion. The first-principles results are then used to find values for the cooperative Jahn-Teller stabilization energy and the electron-strain and electron-phonon coupling. It is found that the dominant source of anisotropy arises from the third-order elastic contributions, rather than second-order vibronic contributions. Additionally, the importance of higher-order elastic coupling between the ${E}_{g}$ and ${A}_{1g}$ modes is identified, which effectively causes expansion of ${A}_{1g}$-type modes and allows for a larger ${E}_{g}$ distortion. Finally, the strain anisotropy induced by the antiferromagnetically ordered states is shown to cause a significant difference in the cooperative Jahn-Teller stabilization energy for the different orientations of the cooperative distortion.

Abstract not Available.

A detailed analysis of the formation energies for alkali, earth-alkali, and transition-metal hydrides is presented. The hydriding energies are computed for various crystal structures using density functional theory. The early transition metals are found to have a strong tendency for hydride formation which decreases as one goes to the right in the transition-metal series. A detailed analysis of the changes in band structure and electron density upon hydride formation has allowed us to understand the hydriding energy on the basis of three contributions. The first is the energy to convert the crystal structure of the metal to the structure formed by the metal ions in the hydride (fcc in most cases). In particular, for metals with a strong bcc preference such as V and Cr, this significantly lowers the driving force for hydride formation. A second contribution, which for some materials is dominant, is the loss of cohesive energy when the metal structure is expanded to form the hydride. This expansion lowers the cohesive energy of the metal and is a significant impediment to form stable hydrides for the middle to late transition metals, as they have high cohesive energies. The final contribution to the hydride formation energy is the chemical bonding between the hydrogen and metal in which it is inserted. This is the only contribution that is negative and hence favorable to hydride formation.