Computer modelling studies of defects and valence states in La2CuO4

Rational Design of 3d Transition-Metal Compounds for Thermoelectric Properties by Using Periodic Trends in Electron-Correlation Modulation

The electronic structures in solid-state transition-metal compounds can be represented by two parameters: the charge-transfer energy (Δ), which is the energy difference between the p-band of an anion and an upper Hubbard band contributed by transition-metal d-orbitals, and the onsite Coulomb repulsion energy (U), which represents the energy difference between lower and upper Hubbard bands composed of split d-orbitals in transition metals. These parameters can facilitate the classification of various types of electronic structures. In this study, the dependences of anion species (N^3-, P^3-, As^3-, O^2-, S^2-, Se^2-, Te^2-, F^-, Cl^-, Br^-, and I^-) on Δ and U of 566 different binary and ternary 3d transition-metal compounds were investigated using ionic-model calculations. We were able to identify the systematic chemical trends in the variations in Δ and U values with the anion species of 11 different families of 3d transition-metal compounds in a comprehensive manner. The effective use of Δ-U diagrams given here, to facilitate the discovery and development of functional compounds, was demonstrated on thermoelectric compounds by classifying the thermoelectric properties of 3d transition-metal compounds and by predicting unrealized high-performance thermoelectric compounds.

Phonon dispersion and anomalies in one-layer high-temperature superconductors

Phonon dynamics, charge response and the phonon density of states are calculated for the high-temperature superconductors HgBa(2)CuO(4) and Bi(2)Sr(2)CuO(6) within a microscopic model for the electronic density response. The results are compared with previous calculations for La(2)CuO(4) and Nd(2)CuO(4). Our main focus is on the phonon anomalies which are connected with the high-frequency oxygen bond-stretching modes (OBSM) found before in our calculations for p-doped La(2)CuO(4) and n-doped Nd(2)CuO(4). We investigate the question if the characteristic softening of the OBSM and the related strong coupling to the electrons is also present in HgBa(2)CuO(4) and Bi(2)Sr(2)CuO(6). In particular, the importance of the contribution of the more delocalized Cu 4s state besides the localized Cu 3d state on the softening is investigated and the different anticrossing behavior due to the presence of several phonon modes with the same symmetry as the OBSM is studied. This makes the identification of the anomalies quite complicated. Furthermore, the influence of electronic polarization processes at ions out of the CuO plane in the ionic layers on the phonon dynamics is calculated. In this context the qualitative type of charge response, i.e. the presence or not of possible metallic charge fluctuations at the Hg or Bi ion, respectively, linked via the apex oxygen to the CuO plane, proves to be very sensitive for certain phonon modes. All the calculations are compared to the experimental results available so far. The latter, however, are rather incomplete for the Hg and Bi compounds.

Infrared- and Raman-Active Phonons of Magnetite, Maghemite, and Hematite: A Computer Simulation and Spectroscopic Study

Magnetite, maghemite, and hematite have been the subject of numerous studies using vibration spectroscopy to determine their infrared- and Raman-active phonons. However, no complete and unambiguous set of experimentally observed optically active phonons has yet been reported for these iron oxides. The use of atomistic simulation methods with a transferable Buckingham potential provides new data for the phonon densities of states of magnetite and the two associated phases, hematite and maghemite.