J. Conard

Graphite lamellar compounds EPR studies

We report X-band EPR results in a range of graphite lamellar compounds with: Li (stages I, II, III), K(I, II, III, IX), Rb (I, III) as donors and with: HNO3 (I), SO3(I, II, III) as acceptors. Line shapes are dysonian and g-anisotropy remains low even in stage IX K-compound. From line parameters, relaxation time T2 and c-axis electronic diffusion coefficient D are deduced. n nResults are discussed and compared with metal properties: T2 versus temperature, resistivity, stage and intercalant atomic number for donors, c-diffusion coefficient and mean free path, density of states estimations. Particularly KC8 is shown to exhibit more metallic properties than Li-compounds and low density of states are found in acceptors, contrary to donors, in agreement with 13C NMR.

Structure and transitions in coordinated lithium graphite intercalation compounds

Abstract The LiC n (THF) m ternaries obtained by the direct action of THF on the parent LiC n binaries seem to be quite similar whether the initial binary be 1st stage LiC 6 or 2nd stage LiC 12 or LiC 18 . The basic structure is a slightly flattened [Li(THF) 4 ] + tetrahedral complex with THF molecules probably rapidly exchanging between adjacent complexes. Some excess non complexed Li could induce a coupling of the conduction electrons with the intercalated layer.

Composes ternaires graphite-lithium-tetrahydrofuranne: Synthese et etude par rayons X et resonance magnetique

Abstract Lithium-tetrahydrofuran (THF) graphitides were obtained from the binary LIC n . Whatever the starting binary (LiC 6 , LiC 12 or 2nd stage LiC 18 ), the same 1st stage structure is obtained. The identity period I c = 12.5 A corresponds to the [Li(THF) 4 ] + intercalates with excess Li + depending on the parent binary, and two narrow 7 Li n.m.r. lines are seen. 13 C n.m.r. indicates C planes decoupled from each other with a large-orbital term, as in acceptor compounds. A high density of states at the Fermi level is deduced, but the low asymmetry ratio observed in e.p.r. spectra disagrees with a highly conductive material. A donor compound, without string covalent. coupling between graphite and alkali, has been successfully obtained.

Graphite lamellar compounds 13C NMR studies

Abstract 13 C NMR (natural abundance, room temperature) in a range of graphite lamellar compounds with donors or acceptors is interpreted in terms of charge transfer. The observed shifts and structure are related to anisotropic chemical shifts, or to Knight shifts, or a combination of both effects. Donors shifts - towards high field - agree with an important indirect Knight shift superimposed to a chemical shift. Using Fermi level density of states N ( E F ) deduced from Mizutani and co-workers measurements, large charge transfer are deduced: 0.3 f Acceptors compounds show narrow lines with a 150 ppm axial anisotropy, the H 0 ‖ ( a, b ) component being undisplaced as compared to the graphite one: f appears small or negligible. Donors results are consistent with the occurring of a narrow energy band just above the graphite Fermi level resulting in high N ( E F ). In acceptors, they seem to be characteristic of 2-D compounds without covalent bonding, the main effect of acceptor molecules being to separate the graphitic planes.

X-ray, e.p.r. and n.m.r. studies and structure of some ternary compounds of graphite

The potassium hydrogenographitides, KH2/3C8 and K3H4/3C16, and potassium and rubidium mercurographitides have been extensively studied. In the hydrogenographitide series 1H n.m.r. is not observed, but e.p.r. shows metallic character in stage I and II compounds. 13C n.m.r. suggests charge transfer, in agreement with a negative hydrogenated ion in the lattice. A structure is proposed using the stability of the H2− ion when the bond is sufficiently lengthened. With mercurographitides, e.p.r. cannot be seen because of the Hg spin-orbit coupling. 13C n.m.r. agrees with a model having negatively charged Hg atoms between two K or Rb layers. The possible structure for this layer is compared with the structure of alloys of the same stoichiometry.

Structure and dynamics of intercalated water in clay minerals

Abstract At the clay-water interface, the adsorbed water has its own structure. Proton dynamics is described by three motions: two uniaxial reorientational motions and one jump diffusional motion. The lifetime of protons in the first layer is given by the residence time τ 0 of the jump diffusion process (10 -10 s). During this time the water molecules spin around their spin c 2 axis with a correlation time τ 1f of the order of 4–5 ps, whereas they rotate with the cation hydrate, with a correlation time τ 1s of the order of 30 ps.

Nuclear relaxation times of Li in Li-graphitides

Abstract In stage I Li-graphitide, we try to determine the nature of interactions between Li nuclear spins and their neighborhood by variable temperature measurements of the Li nuclear spin lattice relaxation time T1. Two methods are used: 7Li pulsed NMR; and β-radiation detection of 8Li-nuclear polarization produced by polarized neutron capture. Both methods are in qualitative agreement, but a quantitative discrepancy appears in the low-temperature range. Results are discussed in relation with several mechanisms: contact interaction and Korringa relation, quadrupolar mechanism and diffusion modulated dipolar interaction.

Deregistration of the Cs lattice from the graphitic one: Influence on electronic properties in CsC24

Abstract New results have been obtained from 133 Cs N.M.R. in H.O.P.G.-derived Cs 24 between 100 and 400K. At T > T u , the structural transition temperature, the Cs atoms occupy 5 (or 7) well defined hexagonal sites, characterized by their T-increasing Knight shifts. When T increases further, the lines split even more and broaden till they successively disappear. Broadening is due to Cs deregistration from graphitic lattice and K.S. increase reveals an increase of the Fermi wave function s-character. This latter evolution is confirmed by X.P.S. and U.P.S. spectra. Correlation with structural results is attempted.

Mobility of tetrahydrofuran in Cs(THF)xC24 and Li(THF)1.4C6: A neutron and N.M.R. study

Abstract Q.N.S. and N.M.R. study on first and second stage Cs(THF) x C 24 ternary compound has been performed at various temperatures. In the 1st stage Cs(THF) 1.7 C 24 Q.N.S. and 1 H N.M.R. show that a fraction of THF molecules are mobile at 300K. 133 Cs N.M.R. indicates that caesium site looks axial at this temperature and inelastic neutron scattering presents one vibrational peak. When the temperature decreases, mobile molecules freeze as caesium site loses its symmetry; simultaneously, a low energy mode grows. In the 2 nd stage Cs(THF) 1.05 C 24 , only a weak mobility of THF is observed until 318K; this motion can be interpreted as a pseudorotation of the molecule. No particular symmetry of Cs site is observed but neutron inelastic scattering becomes strongly structured indicating definite site for the molecule. We interpret those results by considering the 2 nd stage with well defined site for THF that get mobile when new molecules are intercalated to obtain the 1 st stage. Comparing with the Li(THF) x C 6 results with similar experiments, THF appears more mobile as no particular site seems present in this compound.

Alkali-hydrides graphite intercalation compounds: Some electronic studies

Abstract Magnetic resonances of 4 species (1H, 13C, 23Na and e−) and photoelectron spectroscopy studies have been performed on graphite-NaH and graphite-KH intercalated compounds of various stages (1 to 7). A differentiation between low stage (i.e. 1 to 3) NaH-GICs and high stage ones is clearly evidenced from E.P.R.. A first insight is given on charge sharing, possible occurrence of a small amount of localized paramagnetic centers and nature of the H-state.