L. L. Boyer

Superconductivity of Metallic Boron in MgB2

Boron in MgB2 forms stacks of honeycomb layers with magnesium as a space filler. Band structure calculations indicate that Mg is substantially ionized, and the bands at the Fermi level derive mainly from B orbitals. Strong bonding with an ionic component and considerable metallic density of states yield a sizable electron-phonon coupling. Together with high phonon frequencies, which we estimate via zone-center frozen phonon calculations to be between 300 and 700 cm(-1), this produces a high critical temperature, consistent with recent experiments. Thus MgB2 can be viewed as an analog of the long sought, but still hypothetical, superconducting metallic hydrogen.

First principles analysis of vibrational modes in KNbO3

Abstract The vibrational modes in KNbO3 are analyzed using first principles total-energy calculations based on local density functional theory. Normal mode frequencies and eigenvectors are determined for the cubic perovskite structure with wave vectors at the center (Γ) and corner (R) of the Brillouin zone using the frozen-phonon technique. The frequencies are in good agreement with available experimental results with the possible exception of the lowest-frequency zone-center (ferroelectric) mode. Our calculations give a ferroelectric mode which becomes unstable with increasing volume. However, at the equilibrium volume it is stable with a frequency of ∼100 cm−1.

THEORETICAL-STUDIES OF CHARGE RELAXATION EFFECTS ON THE STATICS AND DYNAMICS OF OXIDES

Inclusion of spherical charge relaxation in response to the long-range electrostatic potential (potential induced breathing, or PIB) gives improved results for the static and dynamic properties of oxides. PIB is a Gordon-Kim type model, in which the crystal charge density is estimated by overlapping ionic charge densities. No experimental data are used, except for the values of universal constants, and in this sense the results are from first principles. In contrast to earlier models which include some form of charge relaxation, we explicitly include the breathing effects on the self-energy and pair potentials in the model Hamiltonian. PIB is a many-body effect that couples the long-and short-range forces in a way that is not present in any other first principles or empirical models. It leads to the observed violations of the Cauchy relations for the elastic constants whereas central force rigid ion models cannot violate the Cauchy relations. PIB also reduces the predicted LO-TO splitting because the breathing effect introduces dynamical effective charges that are lower in magnitude than the ionic charges. Some results are shown and discussed for MgO (periclase), BeO (bromellite), Al2O3 (corundum), TiO2 (rutile) and SiO2 (quartz and stishovite).

Phase transitions and elasticity in zirconia

Abstract The potential induced breathing (PIB) model is used to calculate equations of state, relative phase stabilities, and elasticity of several different structures of ZrO2. PIB is a non-empirical model for calculation of static and dynamic properties of ionic crystals. The cubic fluorite structure is found to be the stable zero-pressure phase. The monoclinic phase is more stable at low densities and the PbCl2 phase is more stable at high pressures. The rutile structure is found to be relatively unstable, a result in agreement with experiment. Elastic constants are calculated for the monoclinic, tetragonal, and fluorite phases of ZrO2. For the pure monoclinic phase, good agreement with experiment is found for the aggregate shear and Youngs moduli. The elastic constants and pressure derivatives for cubic zirconia are discrepant with experimental data, and it is suggested that disorder greatly affects its elastic behavior. Furthermore, disorder and defects may be important in stabilizing the monoclinic and tetragonal structures.

A self consistent atomic deformation model for total energy calculations: Application to ferroelectrics

An ab initio model for total energy calculations is introduced in which site localized charge densities are determined from the electronic structure of atoms (or ions) in a self consistent overlap potential. In the lowest order expression of the overlap potential the ions are spherical. In this simplest form, polarization can result either from ion displacements or from transfer of charge from one ion to another. Calculations for BaTiO3 and KNbO3 show charge transfer from the O(2p) to the transition metal d states, beginning near the equilibrium volume and increasing with increasing volume. Results are obtained which indicate the transfer effect will be greatly enhanced for transition metal ions on a surface.

Absence of metastable states in strained monatomic cubic crystals

A tetragonal (Bain path) distortion of a metal with an fcc (bcc) ground state will initially cause an increase in energy, but at some point along the Bain path the energy will again decrease until a local minimum is reached. Using a combination of parametrized tight-binding and first-principles LAPW calculations we show that this local minimum is unstable with respect to an elastic distortion, except in the rare case that the minimum is at the bcc (fcc) point on the Bain path. This shows that body-centered tetragonal phases of these materials, which have been seen in epitaxially grown thin films, must be stabilized by the substrate and cannot be free-standing films.

APPLICATIONS OF IONIC MODELS TO THE HIGH-TEMPERATURE SUPERCONDUCTOR LA2CUO4

Abstract The electronic structures of superconducting oxides such as La 2 CuO 4 include significant ionic contributions. Application of ionic models to these materials complements band structure studies; ionic models provide useful reference models for studying the high- T c superconductors. In order to make this comparison useful, it is important to use a non-empirical model that does not involve fitting to experiment. We have applied the Potential Induced Breathing (PIB) Model, a non-empirical model which has shown success in predicting properties of insulating oxides, to La 2 CuO 4 . The PIB overlapping ion change density is compared to a self-consistent LAPW charge density. The out-of-plane oxygen and lanthanum ions are quite ionic, but the in-plane oxygen and copper show significant distortions, primarily displaying movement of charge out of the overlap region. Static minimum energy structures were calculated in tetragonal and orthorhombic symmetries using the PIB model. The ionic model predicts the observed tetragonal to orthorhombic distortion. PIB predicts a lower symmetry ground state. Phonon dispersion curves were calculated for both the tetragonal and orthorhombic structures. Unstable phonon branches are found for both structures using the ionic model. In the tetragonal structure these unstable branches are related to the X-point rotation of the in-plane oxygens, the octahedral tilting associated with the phase transition to the orthorhombic structure, and sliding of the O(Z) and La atoms in the x−y plane throughout the Brillouin zone.

On the relationship between the phonon anomalies and the AB initio calculated Fermi surfaces of TaC and NbC

Abstract First principles augmented plane wave calculations show that the Fermi surfaces of TaC and NbC contain flat parallel sheets, while that of HfC is relatively “normal”. The phonon anomalies in the former two materials are shown to stem from specific transitions across the Fermi surface.

Development of a Kohn-Sham like potential in the self-consistent atomic deformation model

Abstract This is a brief description of how to derive the local ‘atomic’ potentials from the Self-Consistent Atomic Deformation (SCAD) model density functional. Particular attention is paid to the spherically averaged case.

Ab Initio Calculations of the Effect of Non-Rigid Ions on the Statics and Dynamics of Oxides

The potential induced breathing of oxygen ions in a crystal, which plays a major role in explaining the violation of the Cauchy relations and the splitting of the longitudinal and transverse optic frequencies in alkaline earth oxides, is analyzed for BaTiO3 in the perovskite structure and the distorted structure corresponding to the tetragonal phase. The results indicate that nonspherical ions are required to describe the ferroelectric transition.