Structure and bonding parameters in ternary and multinary compounds

Summary A new approach to the study of the structural properties of semiconductors is presented. It is based on a semi-classical approximation which allows us to obtain the structural properties directly from the knowledge of the crystal pseudopotential. Since it leads to analytic expressions, this approach is well suited to elucidate the connections between the crystal structure and the bonding parameters. The relations with previous semi-empirical methods are discussed, with particular attention to the method of the average nuclear effective charge. The metallization trends in multinary compounds are analysed in detail and an interpretation is given in terms of a critical length for the covalent bond. Paper presented at the ≪V International Conference on Ternary and Multinary Compounds≫, held in Cagliari, September 14–16, 1982.     

Alkali metal intercalation into SnS2

The intercalation of sodium and potassium into the layered semiconductor SnS₂ has been investigated by ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and ion scattering spectroscopy (ISS). After deposition of the alkali metals onto (0001) cleavage planes of SnS₂ in ultra high vacuum (UHV), semiconducting intercalation phases were formed. They seem to be homogeneous and disordered under the given experimental conditions. The valence electrons of the alkali metals are transferred into electronic states of the host lattice, whose valence band density of states changes significantly during intercalation. The underlying changes of the binding properties of the host lattice are discussed. The course of intercalation can be separated into three phases. During an induction period the concentration of the alkali metal on the surface remains very small, the electronic states of the substrate are shifted by band bending. During an intercalation period the topotactic reaction proceeds. After reaching saturation compositions of the intercalation phase at the surface, the alkali metal diffuses into the bulk. Crystal or surface defects seem to have a significant influence on the kinetics of intercalation and on the stoichiometry of the intercalation compounds.     

Intercalation reaction kinetics: a TDPAC study of 2H-TaS2 intercalation

Summary We report on thein situ observation of the¹⁸¹Ta nuclear quadrupole interaction during electrointercalation of 2H-TaS₂ with hydrated gadolinium ions by means of time differential perturbed angular correlations (TDPAC). Paper presented at the ?V International Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

1H NMR study of ternary ammonia-alkali metal-graphite intercalation compounds

For the first-stage ternary ammonia-alkali metal-graphite intercalation compounds M(NH₃)_xC₂₄ (x ∼ 4, M = K, Rb, Cs), three sets of triplet ¹H NMR spectral lines have been observed at various temperatures and orientations due to the ¹H-¹H and ¹⁴N-¹H dipolar interactions. We have inferred the structures of these compounds as mobile (liquid-like) intercalant layers of planar M(NH₃)₄ ions in the between the carbon layers. For the intercalated ammonia molecules, the potential barrier is ∼ 0.2 eV and the molecular geometry is very close to the free NH₃ in gas phase.     

Magnetic phases in transition metal chloride intercalation compounds of graphite

Magnetic susceptibility measurements on graphite-FeCl₃ (stage 2), graphite-CoCl₂ (stages 2 and 3), and graphite-NiCl₂ (stages 3 and 5) intercalation compounds are reported, showing anomalies associated with magnetic transitions. The effect of magnetic fields on the transition suggests that the magnetic phases in all these systems are similar and independent of the separation of the planes of magnetic ions. The results for all samples for stage ≥ 2 imply two magnetic phase transitions closely separated in temperature.     

Landau levels in graphite intercalation compounds

The first calculation of the magnetic energy level structure of a graphite intercalation compound is presented. The calculational technique exploits the staging symmetry through the k_z-axis zone folding of the magnetic energy levels of the graphite π-bands. The results are applicable to the interpretation of the magnetoreflection and de Haas-van Alphen type experiments in intercalated graphite.     

Raman scattering in graphite intercalation compounds

Raman scattering results are reported on graphite intercalated with Br₂, ICℓ and IBr. In all of these acceptor compounds, the single E_2g2 Raman peak for pure graphite is replaced by a doublet structure identified with in-plane carbon atom vibrations. In addition, Raman peaks specific to the intercalate species are found at frequencies down-shifted from the stretching modes of the free intercalate molecules.     

Synthesis and characterization of SWNT-heavy alkali metal intercalation compounds,effect of host SWNTs materials

Singlewall carbon nanotubes (SWNTs) produced by electric-arc and laser ablation methods were characterized by X-ray diffraction before and after the reaction with alkali metals (M=K, Rb, and Cs). Reaction with annealed SWNTs gave MC₈ composition at saturation. The alkali metal lattice showed short range order incommensurate with graphene cylinders of SWNTs. X-ray diffractogram simulations have enabled the study of the influence of SWNTs structure on that of intercalation compounds. Chemically-purified bundles, constituted of open SWNTs, can be intercalated inside and between the tubes forming disordered structures. Annealed or pristine bundles were intercalated only between the tubes leading to short or long range ordered structure depending on host crystallinity and alkali metal (K, Rb or Cs). The expansion of the 2D SWNTs lattice after intercalation is comparable to graphite intercalation compounds. Some 2D arrangements of SWNTs and K atoms are proposed and discussed to reproduce XRD results. ¹³C NMR and ESR studies of annealed doped SWNTs emphasize the fact that the intercalation compounds of SWNTs are metallic.     

Island formation in metal halide intercalation compouds

Acceptor action, non-stoichiometry, and island formation of metal halide intercalants in graphite are explained by the tendency toward full six-fold halogen coordination at the boundary of the intercalant. The Fermi level is pinned on the acceptor level split-off from the halogen p-derived valence band.     

Location of hydrogen in intercalation compounds of layered transition metal disulfides, A H NMR study

¹H NMR spectra of the hydrogen bronzes H_xNbS₂, H_xTaS₂ (2H modification), H_xTiS₂ and H_xVS₂ (1T-modif ication) have been measured in order to study location and mobility of hydrogen in layered chalcogenide intercalation compounds. In those materials which crystallize with 2H-modification the large majority of hydrogen atoms is immobile at ambient temperature and appears to be located in intralayer positions with symmetrical hydrogen metal four center bonding. In H_xTiS₂ and H_xVS₂ hydrogen occurs clustered at adjacent tetrahedral sites in the van der Waals gap and diffusion takes place between adjacent tetrahedral sites via S-H…S type hydrogen bonds.     

Magnetic properties of intercalation compounds MxTiS2 (M = 3d transition metal)

Magnetization measurements have been made on 3d transition-metal intercalation compounds of M_xTiS₂ (M = V, Cr, Mn, Fe, Co and Ni; 0 ⩽ x ⩽ 1) at 4.2, 77 and 300 K in static fields up to 20 kOe and in pulsed fields up to 160 kOe. The ac magnetic susceptibility measurements in dc fields have also been made for Fe_xTiS₂. We have identified various types of magnetic phases in M_xTiS₂, such as paramagnetic, spin-glass, cluster-glass or mictomagnetic, and ferromagnetic ones, depending on the kind of guest 3d metals and their concentrations. Magnetic properties of these materials can be understood in terms of an itinerant or band picture rather than a localized or rigid band model.