W. Metz

Charakterisierung von graphit-FeCl3-verbindungen als teilweise geordnete schichtstrukturen

Abstract The stacking of layers in graphite-FeCl3 compounds in general is disordered, but there is a tendency for the formation of periodic structures. The corresponding degree of order is related to the content of FeCl and has a value between 0 (completely random sequence of layers) and 1 (fully periodic stacking). The degree can be evaluated from X-ray diffraction data. Its dependence on the preparation conditions is investigated. Only under special circumstances, nearly periodic compounds are produced, which are compatible with the ideal model of “stages”. However, the saturated compound (69% FeCl3, 1st stage), always has an ordered stacking sequence. Below, in the concentration range down to about 40% FeCl3, the degree of order usually is near 0·5. It decreases for lower FeCl3 concentration. Phase separation is observed above 54% FeCl3, where the samples contain domains of pure graphite, of pure 1st stage and of disordered intercalation compound.

Curved Crystal Transmission Optics for Energy-Dispersive X-ray Absorption Spectroscopy.

The development of a curved crystal monochromator of the Laue type for energy-dispersive X-ray absorption spectroscopy is presented. The quality of the X-ray absorption spectra at high photon energies is compared with spectra measured with silicon crystals in the more frequently used Bragg geometry. In the Bragg case, an asymmetric broadening of the reflectivity profile leads to strong distortions of the near-edge fine structure and to a reduction in spectral resolution. The reflectivity profiles of fiat and curved crystals for Laue and Bragg geometry have been calculated using dynamical theory and are compared with experimental data. The new optics have been used for in situ time-resolved X-ray absorption spectroscopy. An example of the application of the technique for the characterization of a Pd catalyst is given. The X-ray absorption fine structure at the Pd K-edge has been measured during the activation and during the heterogeneous catalytic oxidation of carbon monoxide.

Reactions in the system graphite-CrO3-acetic acid I. Metz

Abstract The conditions of reaction for the intercalation of graphite by chromium oxide from acetic acid solution have been put on a more general basis. Intercalation is possible at all temperatures between 20 and 120 °C. The reaction temperature determines the stage number of the compound formed, for which values have been found between 8 and 2. At temperatures above 70 °C, and inhibition of the reaction has been found, caused by decomposition of the intercalate between the graphitic layers. This irreversible effect is observed most clearly in flakes larger than 100 μm in diameter. Another, reversible inhibition of the reaction has been found which intensifies towards lower temperatures and smaller particles. This effect prevents the intercalation ofpowder graphite at 25 °C. Probably a protective layer of adsorbed molecules is formed around the crystals. These inhibition effects are described quantitatively by the “degree of penetration” which is obtained from X-ray measurements.

Xanes studies on rhenium L absorption edges of Re2O7 graphite intercalation compounds and of other rhenium-oxygen compounds

Abstract The electronic structure as well as the medium-range order of rhenium oxides and Re 2 O 7 graphite intercalation compounds (GICs) were investigated by analyzing the XANES (X-ray absorption near edge structure) of the Re L I–III absorption edges. The Re L I,III edges show for all perrhenates strong white lines whose full widths decrease going from the solid to the highly concentrated to the highly diluted solution. Coincidently the area of the white lines increases. The GICs show the same dependence with the exception that their absorption peaks are always lower, which is caused by a charge transfer from graphite to rhenium. A reduced intensity of the strong pre-edge peak at the Re L I edges in comparison to solid perrhenates and perrhenate solutions indicates a slightly distorted tetrahedral symmetry for the GICs. The analysis of a multiple scattering feature at the Re L I edge reveals for the GIC intensities similar to the solutions but several times lower than those of the solid perrhenates. The loss of the three-dimensional long-range order after the intercalation accompanied by a low density of ReO 4 tetrahedra causes this effect.

Synthesis and characterization of the graphite intercalation compounds of bismuth trichloride

Abstract The intercalation of bismuth trichloride into graphite via the vapor phase has been investigated as a function of the following parameters: temperature of the graphite, vapor pressure of the metal chloride, and pressure of chlorine. Pure-stage compounds with stage index 2, 3, and 4 are easily obtained at high pressures of chlorine (600 mbar). It was not possible to produce a stage 1 compound. With decreasing pressure of chlorine the intercalate content of the compounds becomes smaller until, at the threshold pressure of chlorine (20 mbar), no intercalation is observed anymore. All compounds were characterized by weight uptake and X-ray diffraction. In the case of the pure-stage compounds, the bismuth: chlorine ratio was determined by X-ray fluorescence as 1:3.2 ± 0.1. All compounds that were prepared at a high pressure of chlorine exhibit the same hk0 diffraction pattern. The reflections can be indexed by a rectangular in-plane lattice. The lattice constants indicate a structural relationship between the pure and the intercalated bismuth trichloride. Compounds prepared at low pressures of chlorine exhibit nonreproducible hk0 diffraction patterns.

Structural investigations of the graphite intercalation compounds of the dichlorides of the IIB-elements (Zn, Cd, Hg)

Abstract An account of our structural investigations of the Graphite Intercalation Compounds of ZnCl 2 , CdCl 2 and HgCl 2 is given. In the case of zinc chloride-graphite X-ray absorption spectra show that the Zn atoms in the intercalated layers are coordinated tetrahedrally. For cadmium chloride-graphite hk0 diffraction patterns clearly show that the sandwich structure of a layer as it is observed in pure CdCl 2 is conserved. For mercuric chloride-graphite, monochromatic Laue photographs show an oblique in-plane unit cell of the intercalate.

Some general aspects of the hk0 diffraction patterns of graphite intercalation compounds

Abstract A series of monochromatic Laue photographs (MLPs) showing the hk0 diffraction patterns of some graphite intercalation compounds (GICs) is presented. Due to the sixfold symmetry of the graphene layers and the domain structure of GICs they represent an n-fold superposition (n=1, 2, 3 or 6) of the patterns which is produced by a domain with a single rotational orientation. This “domain twinning” often produces aesthetically attractive patterns. The number n depends on the shape of the unit cell of the intercalate and the rotational orientation between the unit cell axes of the in-plane lattices of the guest and the host.

Graphite intercalation compounds of PdCl2 : structural investigations

Abstract GICs + of Palladium(II)-chloride were investigated by X-ray diffraction and by X-ray absorption spectroscopy at the Cl K edge and the L edges of Pd. These two methods give complementary information on the structure of the intercalate layer. This structure is built up from ∞ 1 [PdCl 2 ] chains as they occur in α-PdCl 2 . Monochromatic Laue photographs reveal that these chains are rotationally disordered with respect to the crystallographic axes of the graphene layers, but show a tendency towards an epitactic arrangement. The XANES regions of all edges also provide evidence of a great similarity between intercalated PdCl 2 and pristine α-PdCl 2 . Angle-dependent XANES measurements at the Cl K and the Pd L III edge show that the angle between the graphene layers and the plane of the chloride chains is about 59°.

Spontan magnetisierte monoatomare Eisenschichten in Graphit

Zur ]3es t immung der Mikrohi~rte H u n t e r L a s t wurde das Silbersehicl~t-Verfahren yon GRODZlNSKI [1, 2] angewendet. Die Versuche w u r d e n m i t d e m I 4 n o o p D i a m a n t e n bei ffin-f Au i l a s t en ]~ (50 bis 500 p) durchgefi~hrt . Die ]3elas t u n g s d a u e r b e t r u g 24 s, die Dieke der Si lberschicht e twa 60-~. Die TabelIe zeigt die n n t e r s u c h t e n Gl~ser (bet GIas Nr. 7 s ind im SiO2-Wert L 0 % B~Oa u n d i m NagO-Wer t t0,5 % K 2 0 en tha l ten) u n d die A u s w e r t n n g der MM3ergebnisse n a c h H = K . P . d -n (n = M eye r -E xponen t , d = E ind ruckd ia gonale in jim, K = 14230). Gleichzeit ig enth/ i l t die Tabel le die W e r t e nach d e m fiblichen Verfahren, bet d e m nach d e m E n f l a s t e n die Diagona le wegen der R i ickfederung im allgeme inen kleiner ist.

X-ray absorption spectroscopy and x-ray diffraction of AuCl3-graphite

Abstract Single crystal samples of a stage 1 compound of AuCl 3 -graphite have been investigated by X-ray diffraction. The 001 patterns show a lattice parameter c (1) of 683 pm. The received hk-pattern can be interpreted by an oblique angled in-plane lattice with lattice parameters: a = 1288 pm, b = 655 pm and γ =112.5°, commensurate with the graphite lattice. This in-plane cell is nearly identical with the a-c lattice plane of pristine Au(III)-chloride. Therefore, we consider that AuCl 3 -graphite is built up of the same planar units Au 2 Cl 6 as they are found in pristine Au(III)-chloride. Angle dependent EXAFS and XANES measurements of HOPG samples indicate the tilting of the planar molecules by an angle of ca. 15° with respect to the graphite layers.