Search

We show that the energy of substitutional randomization of atoms in zinc-blende compounds is surprisingly small. This suggests the existence of a new class of defects in these materials (“random aggregates”), which consist of regions of ≲ 10 atoms where the sites of the diamond lattice are randomly occupied by A or B atoms in place of the ordered AB crystal occupancy. The structural and electronic properties of these defects are outlined.

Predictions of the equilibrium crystal structure of simple compounds, e.g., within first-principles total energy methods are usually carried out by searching for the lowest energy among a small number, O(10), of intuitively selected candidate structures. We show how calculations of the total energies of O(10) structures can be used instead to define a first-principles, multi-spin Ising Hamiltonian, whose ground state structures on a fixed lattice can be systematically searched using lattice theory methods. This is illustrated for the intermetallic compounds CuAu, CuPd, CuPt, and CuRh, for which the correct ground states are identified out of more than 65,000 possible structures.

We re-examine the validity of the three-dimensional bulk thermodynamic description for the zinc-blende-to-diamond transition observed in (GaAs)1−xGe2x alloys. We use interaction parameters extracted from first-principles local-density total-energy calculations for (GaAs)p/(Ge2)p superlattices and a cluster-variation solution to the corresponding Ising-like hamiltonian. The resulting phase diagram obtained with such a realistic energy model fails to account for the observed critical composition in the relevant temperature range, suggesting that, contrary to a previous view, the transition is not driven by bulk thermodynamics.

First-principles self-consistent total energy calculations for various configurations of atomic and diatomic hydrogen inside fcc palladium have been carried out to clarify some of the solid state aspects of the physics of dense hydrogen in metals. We find that the stability of the system is controlled by the relative position of the hydrogen-induced antibonding level with respect to the Fermi energy. Diatomic H 2 in crystalline palladium is shown to have but metastable local minima whose internuclear separation are yet larger than that of the isolated H 2 molecule.

A new theoretical approach to the multiplet structure of localized states in solids is applied to the Mott insulators NiO, CoO, and MnO. This method circumvents the major approximations underlying the conventional Tanabe-Sugano approach to the multiplet problem in that differential hybridization for e and t 2 states is incorporated directly, the Racah parameter A is allowed to depend on the one-electron configurations, and the crystal-field parameter incorporates both bare-ion effects (as traditionally done) and average multiplet corrections. Analysis of the observed optical spectra of NiO, CoO, and MnO in light of this approach produces (i) an excellent fit to the spectra and new assignments for some of the transitions, (ii) the values of the mean-field parameters for these materials, (iii) a direct measure for the amount by which an ideal, spin-restricted mean-field electronic structure theory would fail to reproduce the excitation energies, and (iv) analysis of the optical transitions above the interband onset.

A self-consistent self-interaction correction to the local spin density approximation is shown to remove many of the anomalies in the predicted exchange, correlation and total energies of atoms, stability of negative ions and band gaps of insulators. It also suggests a new interpretation of the calculated cohesive energies of transition metals.

A precise self-consistent calculation of the electronic structure of neutral substitutional 3d impurities in silicon within the local density formalism reveals striking chemical regularities. The remarkable compression of the energy level ladder corresponding to different charged states in a narrow energy gap is shown to result from a special charge self-regulating mechanism, much like homeostasis in biology. In contrast to free transition atoms, a s→d population cross-over can occur in this system.

Some properties of two-dimensional boron nitride are treated in a truncated crystal approach, employing the iterative extended Huckel method in a charge self-consistent calculation. The values of the band width, band gap, charges, equilibrium distance, and sublimation energy are in satisfactory agreement with experiment.