LAPW Calculations Research Articles

Origin of ferroelectricity in LiNbO3 and LiTaO3

The origin of the ferroelectric materials LiNbO3 and LiTaO3 was studied using extensive LAPW calculations. The coupled displacements of the oxygens and lithiums give a deep double well whereas oxygen alone displacements result in a shallower double well as a result of the oxygen-lithium overlap. Density of states calculations show hybridization between the transition metals and the oxygens, and the origin of the ferroelectric instability in these systems is similar to the ferroelectricity in perovskites. The electronic structures of the two materials is compared with the related perovskite ferroelectrics KNbO3 and KTaO3.

First‐principles calculation of the equation‐of‐state, stability, and polar optic modes of CaSiO3 perovskite

We report the results of a first‐principles LAPW calculation of the equation‐of‐state, dynamic stability, and infrared‐active transverse optic vibrational mode frequencies of CaSiO3 perovskite. A Birch‐Murnaghan fit to the computed energy‐volume relation of the cubic phase yields values of Vo=45.62 ų, Ko=227 GPa, and Ko′=4.29 for the thermally‐corrected equation‐of‐state parameters. These values are in excellent agreement with recent quasi‐hydrostatic compression data to 10 GPa, but significantly differ from values derived from higher pressure non‐hydrostatic compression data. We calculate the volume dependence of the infrared‐active TO mode frequencies using a frozen‐phonon approach. The lowest frequency ferroic mode is predicted to occur near 228 cm−1 at ambient pressure and displays classic soft‐mode behavior in the tensile regime, in quantitative agreement with earlier molecular and lattice dynamical calculations, based on empirical potentials. These established a link between the low frequency ferroic mode and the thermally activated crystalline‐amorphous transition in a model CaSiO3 perovskite system. Our present calculations also reveal that the static cubic perovskite structure is unstable with respect to small octahedral rotations, corresponding to Brillouin zone edge dynamical instabilities, at ambient pressure and throughout the pressure range of the lower mantle. We speculate that coupling of the low frequency ferroic mode with octahedral tilting modes and strain lower the activation energy for the crystalline‐amorphous transition.

Insulator-metal transition in solid hydrogen: Implication of electronic-structure calculations for recent experiments.

Electronic structure calculations for compressed molecular hydrogen are performed to provide more insight into the diversity of phenomena recently observed experimentally. We perform full-potential LAPW calculations and analyze them in terms of a molecular tight-binding model. We show that $\sigma _g$ and $\sigma _u$ bands overlap occurs at rather low pressure, while an insulating state persists at specific orientations up to 200 GPa, in accord with previous work, and is due to opening of hybridization gaps at the Fermi level. We also present calculations of electronic properties such as plasma frequencies and electron-vibron coupling.

Optical properties of YBa 2Cu 3O 7− x determined from full potential LAPW calculations

Optical properties of YBa 2Cu 3O 7− x determined from full potential LAPW calculations

Cohesive and electronic properties of ordered Li-Al intermetallic compounds

Self-consistent electronic structure calculations have been performed on ordered lithium-aluminium compounds using the tight-binding linear muffin-tin orbital (TBLMTO) method. The FCC-based ground-state superstructures (namely Ll2 and Ll0 structures) show some systematic trends in their cohesive and electronic properties, which are in reasonably good agreement with the available experimental data. We have also compared the density of states, band structures and total ground-state energies of equiatomic AlLi compounds, between the FCC-based Ll0 structure and the BCC-based B32 structure. While the former shows a two-dimensional metallic behaviour, the latter shows a resemblance to a tetrahedral-bonded covalent solid, and is more stable. After detailed comparison with some recent LAPW calculations, we conclude that the TBLMTO method can be used as an efficient and reasonably accurate first-principles tool for studying the phase stability and chemical bonding in ordered intermetallic compounds.

Chemisorption and vibration of hydrogen on Cu(111)

Ab initio Pseudopotential calculations for hydrogen adsorbed on Cu(111) are performed. The minimum energy adsorption site is found to be the threefold coordinated hollow site. This result contradicts the conclusion of McCash et al., based on EELS and RAIRS experiments. Based on our calculated vibrational frequencies for hydrogen adsorbed at the hollow site and at the bridge site, we suggest a reinterpretation of the main features of the experimental vibrational spectra. Our results are in agreement with the LAPW calculations of Feibelman and Hamann and with effective medium theory (EMT) calculations.

Transferable Total Energy Parametrization for Metals

ABSTRACTWe have investigated total energy expressions that consist of a term describing the eigenvalue sum and a pair potential term. Such expressions can be used to fit the results of first principles total energy calculations at given structures, and then obtain the total energy of another configuration of atoms avoiding the complexity of further ab initio calculations.In this work we present a method of fitting APW total energy results to a non-orthogonal tight-binding Hamiltonian from which the sum of the eigenvalues is derived and to a pair potential represented by a 5th order polynomial. We fit total energies for the fee and bec structures and we then obtain the elastic constants Cij in good agreement with both full potential LAPW calculations and experiment. We present results of this method for Ir.

LAPW calculations of Rh(001) surface relaxation

Linearized Augmented Plane Wave total energy calculations for a Rh(001) surface predict a 5.1% contraction of the separation between the outer two Rh planes, relative to bulk, and virtually no change in the separation of the 2nd and 3rd layers. They also show that an adsorbed monolayer of H reduces the contraction of the outer Rh layer separation to about 1.4%. These results make it plausible that low energy electron diffraction experiments which imply that Rh(001) is unrelaxed were performed in the presence of undetected, adsorbed H.

Electric field gradient in Cu2O from band structure calculations

We have performed self-consistent full potential LAPW calculations of Cu2O. Since the new results agree well with a previous muffin-tin calculation, it can be concluded that non-muffin-tin effects are relatively small in Cu2O. By using the present self-consistent charge density, we, obtain the electric field gradient (EFG) at the Cu site without further approximations. Our theory yields a value of −0.85 e/au3, in good agreement with experiment but in contrast to a previous cluster calculation in which a positive sign of the EFG was obtained. The origin of the EFG and the discrepancies between our band structure and the cluster calculation is analyzed.

Electron-Phonon Coupling Strength of Specific Phonons from First Principles Lapw Calculations

AbstractElectron-phonon matrix elements, phonon linewidths and mode coupling strengths are being calculated for La2-xMxCuO4 (M-divalent cation, for paramagnetic x-0.0 and for x-0.15 in a rigid band picture) from first principles local density calculations. The change in potential due to a particular phonon mode is calculated from the difference of self-consistent one-electron potentials, and appropriate Fermi surface averages are carried out for selected modes, allowing us to obtain the phonon linewidth due to the electron-phonon interaction, and the corresponding coupling strength λQ. Here we establish the numerical accuracy within the dual representation of the potential used in the Linearized Augmented Plane Wave (LAPW) method. Evaluations of phonon linewidths and mode coupling strengths are presented for Al and Nb and compared with previous information on these modes. We present preliminary results for the full matrix elements and coupling of the La2CuO4 oxygen planar X-point breathing mode, and compare with a simpler approximation.

Photoemission Study of Quasi-Two Dimensional Metal; Ag2F

Ultraviolet (UPS) and X-ray (XPS) photoemission spectra of quasi-two dimensional metal Ag 2 F are presented. Three prominent bands are found at about 2.0 eV, 4.6 eV and 6.0 eV from the Fermi level in the UPS spectrum. They are assigned to the F 2 p state, the Ag 4 d state and the mixture of the two states, respectively. The comparison of the photoemission spectra with a recent LAPW band calculation shows that the calculated 4 d band is shifted toward the lower binding energy by about 1 eV relative to that of the experiment. This discrepancy suggests that the 16th band which forms a small ellipsoidal Fermi surface in the LAPW calculation may be above the Fermi level.

Electron density of TiC

Recently Trebin and Bross (1984) obtained valence-electron densities of titanium carbide using the modified augmented plane-wave (MAPW) method. This density differs in symmetry around Ti from previous results obtained by Blaha and Schwarz (1983) who have employed the linearised APW scheme. This discrepancy is important in connection with the chemical bonding in this compound. It is shown that the electron densities obtained by LAPW and APW agree very well with each other and thus should be reliable. New LAPW calculations suggest that the MAPW results may be in error because in the latter work the k-space integration was limited to only high-symmetry directions.

Interaction of helium with a graphite surface

A recently developed microscopic theory for the interaction between helium and a metal surface has shown that the repulsive part of the potential is determined approximately by the unperturbed local density of states of metal electrons at the position of the He nucleus. Using the electronic density of states derived from a self-consistent LAPW calculation for a single graphite layer and representing the attractive part of the helium potential by the Van der Waals interaction, we demonstrate that an excellent fit of the observed bound state energies for the laterally averaged helium-graphite physisorption well is obtained. The amplitude of the lowest non-zero Fourier component of the calculated density of states, however, is found to be too large by about 50%. The inclusion of a corrugated attractive term derived from a superposition of r −6 pair potentials is shown to give virtually no improvement. These results imply that our theory gives an adequate overall description of the helium-graphite interaction, but that the true asymptotic density of states is somewhat smoother than the LAPW calculation suggests.

Electron densities and chemical bonding in TiC, TiN, and TiO derived from energy band calculations

AbstractThe chemical bonding in the interesting class of refractory transition metal compounds is illustrated for TiC, TiN, and TiO. Self‐consistent augmented plane wave (APW) calculations are already available for these compounds. Using the respective potentials we have repeated the band calculations on a finer k grid with the linearized APW method to obtain accurate densities of states (DOS). These DOS can be divided into local partial contributions to characterize the bonding. Further information can be obtained from a decomposition of the metal d DOS into t2g and eg symmetry components. These partial local DOS are compared with the LCAO counterpart and give a first picture of the chemical bonding in these compounds. The electron densities corresponding to the occupied valence states are obtained from the LAPW calculations. They provide further insight into characteristic trends in the series from TiC to TiO: around the nonmetal site the density shows increasing localization; around the metal site the deviation from spherical symmetry changes from eg to t2g. These effects can be traced back to the three types of valence bands. Electron density plots of characteristic band states (all energies of a selected k point in the Brillouin zone) will be shown. These plots can describe the different types of bonding occurring in these systems.