C. Haas

Magnetic interactions and covalency effects in mainly ionic compounds

Abstract The effects of covalency on the magnetic interactions like exchange, superexchange and hyperfine interactions are discussed for mainly ionic compounds. Relations are derived using a nearest neighbor cluster model relating the exchange interactions and transferred hyperfine interactions to covalency parameters and to the bond angle. With the aid of some examples we show how information about the superexchange interactions can be obtained from a study of the transferred hyperfine interactions. It is then shown that for compounds where the anion-anion interactions are large but the material is still ionic because of the ionic cation-anion bonding the usually used cluster model for calculating superexchange or the spin density distribution breaks down. This is shown using a model of two strongly interacting anions between two cations. This leads us to a band picture in which the anion outer orbitals are considered to form bands which are covalently mixed with localized states. Some special band structures have been chosen to illustrate the distance dependence of the resulting spin density distribution as a function of the band width.

MAGNETIC-SUSCEPTIBILITY AND ELECTRICAL-PROPERTIES OF VSE2 SINGLE-CRYSTALS

Abstract Magnetic susceptibility, in-plane resistivity and Hall effect data of VSe2 single-crystal plates are reported. These data exhibit anomalies at ∼ 100 K. The occurrence of these anomalies is presumably due to Fermi-surface changes resulting from the onset of a charge density wave instability.

MONOVALENT COPPER IN CHALCOGENIDE SPINEL CUCR2SE4

Abstract X-ray photoelectron spectra of CuCr2Se4 and some other copper and chromium containing compounds were studied to reveal the valence state of the copper atom in CuCr2Se4. Binding energies of the core electrons of the constituting atoms were determined. From these data it is concluded that copper in CuCr2Se4 is monovalent.

Chemical bond model of lattice distortions in hexagonal layers

Abstract The lattice distortions of a hexagonal layer are discussed in terms of a simple model with isotropic interactions. If only nearest-neighbour interactions are considered, the only stable distorted structures are the MnP structure (with zig-zag chains) and the low-temperature NbS structure (with triangles of metal atoms). If also interactions with more distant atoms are considered, more complicated distortions, as observed in crystals with charge density waves, are possible.

MST-Xα Calculations on Molybdenum Disulphide and Some Related Compounds

Self-consistent-field Xα cluster calculations are reported for MoS2, NbS2 and ZrS2. The results are compared with photoemission data. It is concluded that self-consistency and relaxation have a large effect on the relative position of the various energy levels in these compounds.

On the properties of the 1s-forms of TaSe2 and TaS2

Abstract The 1s-forms of TaSe 2 and TaS 2 with octahedral coordination of the metal are diamagnetic; 1s-TaS 2 is a semiconductor at low temperature. The diamagnetism is explained by taking account of spin-orbit coupling which leads to a ground state with zero magnetic moment ( g = 0). This spin-orbit coupling stabilizes the d 1 configuration of the metal with respect to d 2 + d 0 . Thus, it can be understood that 1 s − TaS 2 is semiconducting, while isostructural VSe 2 is metallic. Similarly, BaTaS 2 and BaTaSe 3 are semiconductors, but BaVS 3 is metallic.

LONG-RANGE EXCHANGE INTERACTIONS

Abstract Experiment indicates that rather long-range exchange interactions and supertransferred hyperfine fields are present in some semiconductors. We discuss a model in which a d state interacts with a fully occupied band. Special band structures have been chosen to illustrate the dependence on the various parameters. The resulting exchange interaction is of a longer range than expected on grounds of perturbation theory.

THE VALENCE OF AU IN AUTE2 AND AUSE STUDIED BY X-RAY-ABSORPTION SPECTROSCOPY

Abstract The gold compounds AuTe 2 and AuSe contain Au atoms in two different chemical surroundings. In the literature these different coordinations have been associated with a difference in valency of the Au atoms. In this paper the occupation of the 5 d shell in AuSe and AuTe 2 is deduced from a study of the 4 f → 5 d X-ray absorption edge. A comparison is made with the absorption edges of Au, AuCl and AuCl 3 . The results show that only AuCl 3 has Au atoms in a valence state of III, and that the Au atoms in AuTe 2 and AuSe are all in the same monovalent state I, independent of their chemical environment.

ANISOTROPY OF LATTICE-VIBRATIONS OF LAYERED COMPOUNDS

The cohesive energy of layered ionic compounds consists of contributions of the Madelung energy, favouring symmetric coordinations, a repulsive energy, and a polarization energy stabilizing an asymmetric coordination of the polarizable anions. This explains the occurrence of layer-type structures for compounds with highly polarizable anions and small cations. The anisotropy of the dielectric constant and of the optical lattice vibrations of layered compounds is discussed in terms of a simple model with polarizable ions. The observed anisotropies are attributed to the presence of large static dipoles at the anions. Many layered materials show lattice distortions at low temperatures. The different types of distortions, such as charge density waves, metal clustering due to metal-metal bonding, and ferroelectric distortions, are discussed.

Photoluminescence and thermoluminescence of 4HCdI2: Dedicated to Dr. Franz Jellinek

Abstract Photoluminescence (PL) spectra between 1.8 and 3.5 eV in the temperature range 9–215 K and thermoluminescence (TL) spectra between 9 and 150 K of 4H CdI 2 are reported. At low temperature the PL is due mainly to self-trapped excitons composed of I5 p states and Cd5 s states. At higher temperature, donors and acceptors also are involved in the luminescence process. The TL shows emission peaks at different temperatures, which are assigned to emission from optically generated donor-acceptor pairs.