Search

Metal borohydrides are intensively studied because of their potential applications as versatile hydrogen storage. The relation between the formation enthalpy and the Pauling electronegativity established for these materials [Phys. Rev. B 74, 045126 (2006)] led to the idea of developing mixed-cation compounds that may provide a route for tuning the thermodynamic stability of metal borohydrides. We report a systematic ab initio investigation of the single-metallic and bimetallic borohydrides, and via an examination of the Born effective charges we provide insight into the physical mechanism determining their stabilities. We show that the decreasing stability of metal borohydrides follows the increasing polarizing ability of the cationic bonding component, expressed as the square root of the cation's dynamical charge divided by its radius in coordinated anion polyhedra around the cation. The charge-to-size ratio thus provides a simple yet physically sound measure of the stabilities of metal borohydrides that can be obtained in relatively simple calculations.

For incident 0.4–7-keV O beams scattered from Cu~001!, we have measured energy and angular distributions of O and O ions in the scattered flux, and performed accurate quantitative measurements of the O/O ratios. The O/O ratios depend on the scattering geometry and, for a given geometry, decrease with decreasing scattered perpendicular velocity. The qualitative differences between the energy and angular spectra for the scattered positive and negative ions indicate that this trend is due to the rapid decrease of the O fraction. These data and general arguments based on the energies of the states of the O-Cu system are used to identify key mechanisms responsible for the presence of O and O in the scattered flux. We conclude that the resonant and Auger charge transfer mechanisms are both important. Furthermore, the data indicate that the O ions are generated in the hard collision between the incident atom and a surface atom. These collisions explain the geometry dependence of the measured O/O ratios. @S0163-1829~98!00640-7#

We report on the results of a joint experimental and theoretical study on the formation of ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ ions in grazing scattering from an Al(111) surface. The ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ fractions in the scattered beam are studied over a wide range of collision energies. This allows us to observe a resonance structure for the ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ formation probability as a function of the collision velocity component parallel to the surface $({\mathbf{v}}_{\ensuremath{\parallel}}).$ Such a characteristic ${\mathbf{v}}_{\ensuremath{\parallel}}$ dependence is the signature of a kinematically induced charge-transfer process between the ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ ion and the Al surface. The theoretical treatment of charge transfer is based on the coupled angular mode method. The multistate aspect of the problem originating from the open-shell structure of the ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ ion is taken into account.

Electron scattering factors of ions are represented in a parametrized form which separates the diverging Coulomb term due to ionic charge from the contribution of the screened atomic field. Using this representation it is shown how dynamical reflection high-energy electron diffraction (RHEED) calculations can be performed for ionic surfaces using conventional numerical techniques. Following the classification of ionic surfaces proposed by Tasker [Surf. Sci. 78, 315 (1979)], we analyze the effect of ionicity on RHEED intensities for three classes of ionic surfaces. In the first case both positive and negative ions are located in the same plane and this leads to the cancellation of long-range Coulomb contribution to the crystal potential. The second class of ionic surfaces consists of layers of ions having net nonzero charge. Ionic layers may be grouped into repeat units of planes of the form (anion)-(cation)-(anion) characteristic of the (111) surface of the fluorite structure so that each repeat unit satisfies the charge neutrality condition and also has zero net dipole moment perpendicular to the surface. For this class of surfaces the Coulomb term leads to unusual features in the distribution of the potential in the crystal. The third type of the termination of an ionic crystal lattice gives rise to a sequence of repeat units characterized by a nonzero net dipole moment resulting in a divergent behavior of the potential. Our calculations show that charge transfer between lattice sites occurring in an ionic crystal affects very substantially the form of RHEED rocking curves. The effect can be interpreted in terms of the change in the effective inner potential. This in turn can be used for quantitative determination of the degree of charge transfer occurring in the surface layer of an ionic crystal.

First-principles electronic structure calculations are carried out for the stage-1 alkali graphite intercalation compounds ${\mathrm{LiC}}_{6}$ and ${\mathrm{XC}}_{8}$ (X=Li, Na, K, Rb, and Cs). We analyze the charge densities and the differences to the reference charge densities of graphite host and intercalant sublattice. For the alkali metals Na, K, Rb, and Cs the computed charge transfer is nearly constant at a value of 0.7 elementary charges (e); values of 0.5e and 0.4e are found for the Li compounds ${\mathrm{LiC}}_{6}$ and ${\mathrm{LiC}}_{8}$, respectively. It is shown that the main fraction, about 0.4e, of the charge transfer is a geometrical consequence of the simple overlap of the charge densities of the graphite and intercalant sublattices.

A calculation method based on the two-component density-functional theory is presented for electron systems with a localized positron. Electron-ion and positron-ion interactions are described by norm-conserving pseudopotentials and full ionic potentials, respectively. The electron and positron densities are solved self-consistently using a plane-wave expansion for electron and a real-space grid method for positron wave functions, respectively. The forces on ions exerted by a positron trapped at an open-volume defect and the ensuing ionic relaxations are determined using first-principles methods. In the case of semiconductors, the self-consistent solution of electron and positron densities as well as the ionic positions are found to depend strongly on the treatment of the electron-positron correlation in constructing the effective potentials. We consider several approximations to the correlation energy while demonstrating the method by calculations for a positron trapped by a Ga vacancy in GaAs. Especially, the effects on the observable positron annihilation characteristics, i.e., positron lifetimes, core annihilation line shapes, and two-dimensional angular correlation maps are discussed.

A quantitative description is suggested for electrode polarization, an ubiquitous phenomenon which takes place at the interface between a metallic and an ionic conductor and results in an increase by many orders of magnitude in the net dielectric response of the sample cell. Based on the fact that due to coulombic interactions, the mobility of charge carriers is drastically slowed down at the metal/ionic conductor interface, this approach quantitatively reproduces the observed scaling laws and opens perspectives in the physics of charge transport at interfaces.

We investigate the ionic Hubbard model on a triangular lattice at three-quarters filling. This model displays a subtle interplay between metallic and insulating phases and between charge and magnetic orders. We find crossovers among Mott, charge transfer and covalent insulators, and magnetic order with large moments that persist even when the charge transfer is weak. Our theoretical results are consistent with experiments on A0.5CoO2 (A=K,Na) and identify these materials as correlated covalent insulators.

The core-level line shapes of Si(111)-(7{times}7), Si(100)-(2{times}1), and Ge(100)-(2{times}1) are examined with synchrotron-radiation photoemission both before and after Cs adsorption. The Cs-induced core-level shifts are determined. The results indicate that the chemisorption bond is partially ionic, and the Si or Ge surface atoms each have an extra charge of about 0.3 electron after Cs adsorption.

We report local-density-approximation-based electronic-structure calculations for the compound LiPdH. As in the case of the recently discovered family of CuO{sub 2}-based superconductors, LiPdH{sub {ital x}} is predicted to be a metal with substantial ionic character. We have used the band-structure results to calculate the McMillan-Hopfield parameters within the rigid muffin-tin approximation. These parameters have values close to those of PdH and lead to the prediction that LiPdH{sub {ital x}} may be a good superconductor.

We investigate the ground-state phase diagram of the half-filled $t\text{\ensuremath{-}}{t}^{\ensuremath{'}}$ repulsive Hubbard model in the presence of a staggered ionic potential $\ensuremath{\Delta}$ using the continuum-limit bosonization approach. We find that with increasing on-site repulsion $U$, depending on the value of the next-nearest-neighbor hopping amplitude ${t}^{\ensuremath{'}}$, the model shows three different versions of the ground-state phase diagram. For ${t}^{\ensuremath{'}}l{t}_{c}^{{*}^{\ensuremath{'}}}$, the ground-state phase diagram consists of the following three insulating phases: band insulator at $Ul{U}_{c}$, ferroelectric insulator at ${U}_{c}lUl{U}_{s}$, and correlated (Mott) insulator at $Ug{U}_{c}$. For ${t}^{\ensuremath{'}}g{t}_{c}^{\ensuremath{'}}$ there is only one transition from a spin-gapped metallic phase at $Ul{U}_{c}$ to a ferroelectric insulator at $Ug{U}_{c}$. Finally, for intermediate values of the next-nearest-neighbor hopping amplitude ${t}_{c}^{{*}^{\ensuremath{'}}}l{t}^{\ensuremath{'}}l{t}_{c}^{\ensuremath{'}}$ we find that with increasing on-site repulsion, at ${U}_{c1}$ the model undergoes a second-order commensurate-incommensurate-type transition from a band insulator into a metallic state and at larger ${U}_{c2}$ there is a Kosterlitz-Thouless-type transition from a metal to a ferroelectric insulator.

Density matrix renormalization group methods are used to investigate the quantum phase diagram of a one-dimensional half-filled ionic Hubbard model with bond-charge attraction, which can be mapped from the Su-Schrieffer-Heeger-type electron-phonon coupling at the antiadiabatic limit. A bond order wave (dimerized) phase which separates the band insulator from the Mott insulator always exists as long as electron-phonon coupling is present. This is qualitatively different from that at the adiabatic limit. Our results indicate that electron-electron interaction, ionic potential and quantum phonon fluctuations combine in the formation of the bond-order wave phase.

Charge ordering in the low-temperature monoclinic structure of iron oxoborate $({\mathrm{Fe}}_{2}{\mathrm{OBO}}_{3})$ is investigated using the local spin density approximation $(\mathrm{LSDA})+U$ method. While the difference between ${t}_{2g}$ minority occupancies of ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ cations is large and gives direct evidence for charge ordering, the static ``screening'' is so effective that the total $3d$ charge separation is rather small. The occupied ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ cations are ordered alternately within the chain which is infinite along the $a$ direction. The charge order obtained by $\mathrm{LSDA}+U$ is consistent with observed enlargement of the $\ensuremath{\beta}$ angle. An analysis of the exchange interaction parameters demonstrates the predominance of the interribbon exchange interactions which determine the whole $L$-type ferrimagnetic spin structure.

We present new efficient (O(N log N)) methods for computing three quantities crucial to electronic structure calculations: the ionic potential, the electron-ion contribution to the Born-Oppenheimer forces, and the electron-ion contribution to the stress tensor. The present methods are applicable to calculations in which the electronic charge density is represented on a uniform grid in real space. They are particularly well-suited for metallic extended systems, where other O(N) methodologies are not readily applicable. Based on a fast algorithm for determining the atomic structure factor, originally developed by Essmann et al. (U. Essmann, L. Perera, et al.,J. Chem. Phys. v.103, p.8577 (1995)) for fast Ewald energy and force computation, the present methods involve approximations that can be systematically improved. The methods are tested on a representative metallic system (bulk Al),and their ability to simultaneously achieve high accuracy and efficiency is demonstrated., Comment: 6 pages, 3 figures

In ionic charge-transfer salts which have segregated stacks of organic donor ($D$) and acceptor ($A$) molecules, the major feature of the classical electrostatic interactions is the strong repulsion between like charges along the stacks. Two ways are described by which the magnitude of this repulsion may be decreased: (i) by the formation of a complex salt (i.e., one whose ratio $D:A\ensuremath{\ne}1:1$); and (ii) by incomplete transfer of charge from $D$ to $A$. In both cases, some of the molecules in the stack are neutral, giving rise to a mixed-valence, partly ionic ground state. The first effect is quantitatively investigated by a Madelung energy calculation of tetrathiafulvalene-${\mathrm{Br}}_{0.79}$ as a function of (theoretical) bromine concentration. The results of this calculation show that the unusual composition of this salt occurs in order to reduce the Coulomb repulsion along the stacks. As an example of the second effect, a similar calculation is described for TTF-TCNQ (tetrathiafulvalene-tetracyanoquinodimethane), as a function of the amount of charge transferred from TTF to TCNQ. Although electrostatic interactions do not give a complete description in this case, they are clearly an important factor in determining the degree of charge transfer in such salts. In fact, in cases where the net electrostatic binding energy is small, incomplete charge transfer may be energetically favored. In this case, the charge distribution is shown to be modulated along the stack and charge density waves are formed, with wave vector "$4{k}_{F}$". These results are compared to the case of Rb-TCNQ, where the Madelung energy is shown to favor complete charge transfer.

A self-consistent pseudopotential approach using a mixed-basis set (plane waves plus localized orbitals) is used to obtain the valence charge density and electronic structure of ${\mathrm{V}}_{3}$Si. The calculated charge density is compared with the results determined from x-ray measurements. Both the experimental and calculated charge densities show a piling up of charge between V atoms along the V-V chain. To determine the bonding of this compound arising from the various interactions, V-V, Si-Si, and V-Si, we have also calculated the charge density for ${\mathrm{V}}_{3}^{*}$ (chain only) and Si (bcc). Taking the difference between the charge density of ${\mathrm{V}}_{3}$Si and that of the ${\mathrm{V}}_{3}^{*}$ and Si combined, we examined the charge transfer coming from the V-Si interaction. The resulting bonding is covalent between V atoms along the chains and metallic between V and Si. The $E$ vs $k$ dispersion from $\ensuremath{\Gamma}$ to $X$ was examined and found to compare well with the recent angle-resolved photoemission data of Aono, Himpsel, and Eastman.

This is the first in a series of papers concerning the electrical and thermal transport properties of dense plasmas. Temperatures and densities considered range approximately from room temperature to ${10}^{4}$ eV and from ${10}^{\mathrm{\ensuremath{-}}4}$ to ${10}^{4}$ times compressed. In the present work we describe theoretical calculations of electrical conductivities using the t-matrix version of the Ziman theory with various self-consistent ionic potential models and realistic structure factors. The theoretical basis is described, and illustrative results are given.

The Shubnikov-de Haas effect has been measured in silicon inversion layers as a function of temperature, carrier density, and oxide charge density. The dependence on oxide charge density provides a critical test of the theory of ionic scattering in the Shubnikov-de Haas effect. The data are found to be in excellent agreement with theory. The temperature dependence permits the study of electron effectie masses. We give a more detailed account of the dependence of the electron effective mass on ionic scattering and the effective mass extrapolated to zero ionic scattering.

In this paper we present ionic thermocurrent (ITC) results obtained for ${\mathrm{Sr}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ce}}_{\mathrm{x}}$${\mathrm{F}}_{2+\mathrm{x}}$ single crystals (0.0001lxl0.4). From the dipolar relaxation peak in the ITC recordings the concentration of ${\mathrm{Ce}}^{3+}$-${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ dipolar complexes has been determined. We conclude that only isolated dipoles contribute significantly to the dipolar relaxation peak and that the dipoles are randomly distributed over the ${\mathrm{SrF}}_{2}$ lattice. We find that in the dilutely doped crystals (xl0.005) interstitial ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ ions dissociated from ${\mathrm{Ce}}^{3+}$-${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ dipolar complexes are responsible for an Arrhenius-type ionic conductivity. The position of the so-called high-temperature peak, observed in the ITC recordings, is directly proportional to the activation energy, from which the association energy of ${R}^{3+}$-${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ dipolar complexes has been obtained (R=La, Ce, Pr, Nd, Eu, Sm, Gd, Er, Dy, Yb, Lu, or Y in ${\mathrm{SrF}}_{2}$ or La, Eu, Gd, or Yb in ${\mathrm{BaF}}_{2}$). At higher doping concentrations non-Arrhenius-type ionic conductivity, caused by hopping of interstitial ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ ions in the random potential energy barrier structure of the heavily doped fluorite lattice, is observed. In addition, results obtained from quenching experiments performed on ${\mathrm{Sr}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{La}}_{\mathrm{x}}$${\mathrm{F}}_{2+\mathrm{x}}$ are presented.

A detailed account of the results obtained for the energetics of a proton in aluminum is reported. The central model used in the calculations is the so-called ''spherical solid model'' in which the part of the ionic potential which is spherically symmetric around the impurity is treated to all orders. Various additional effects, such as the contributions of the nonspherical potential and lattice relaxation are investigated. The variation of the heat of solution of H in Al with position is calculated, and the zero-point vibration energy estimated.

The Hohenberg-Kohn-Sham formalism in local-density approximation is used to calculate self-consistently the effective potential and electron density profile at the surface of a jellium slab in a strong static electric field. In the wide range of induced surface-charge densities accessible to experiments in electrolytic cells, the shape of the calculated electron density profiles is characterized by suitable moments, and the induced charge density by its center of mass and by its width, which are related to static and to optical response properties, respectively. Comparison with previous work reveals the importance of self-consistency.

Ionic conductivity measurements have been made as a function of frequency at a variety of temperatures on single crystals of Sr${\mathrm{F}}_{2}$ varying in thickness from a few millimeters to several centimeters, using several different electrode materials. At high temperatures, uncorrected values for the sample resistivity depend strongly on crystal thickness, electrode material, and measuring frequency. However, a more complete complex-impedance analysis discloses that a single value may be determined for the bulk conductivity, dependent only on temperature, and that all other dependences are clearly ascribable to electrode effects. There is, apparently, no intrinsic frequency dependence resulting from bulk properties, as had been earlier conjectured.