Superconductivity of the potassium graphite intercalation compounds

The stratified compound C₄K has been found to pass to a superconducting state with the onset of transition at T_c = 1.3 K.     

Magnetic properties of first- and second-stage MnCl2 graphite intercalation compounds

The magnetic properties of graphite-MnCl₂ intercalation compounds (stages 1 and 2) were investigated from 3 to 300K by EPR and static susceptibility experiments. The behaviour of the magnetic susceptibility anisotropy, EPR linewidth and g-factor gives evidence for short range order effects increasing sharply below 50K. The data are analyzed in connection with the two-dimensionality of the compounds.     

Variable charge transfer and band structure of graphite intercalation compounds: C8nAsF5

Using measurements of oscillatory magnetoresistance, the charge transfer in stage 2 AsF₅-graphite is found to vary over the range f ? 30–45%. By following the change of Fermi surface areas with f, a more precise determination of the band structure is possible. The band parameters are nearly identical to those of pristine graphite.     

Superconductivity in the graphite-potassium intercalation compound C8K

Superconducting properties of pseudo-single crystals of C₈K were investigated by low frequency a.c. magnetic susceptibility and electrical resistivity measurements. The measured values of T_c were between 128 and 198 mK for 13 samples. Measurements of the superconducting transition in a magnetic field revealed a remarkable anisotropy, such that if θ is defined as the angle between the applied magnetic field and the layer plane, type II superconductivity was observed for 0° ≦ ∣ θ ∣ ? 25° and type I superconductivity for 25° ? ∣ θ ∣≦ 90°. The angular dependences of H_c2 and H_c3 were fairly well-explained by the effective mass model.     

Superconductivity in graphite-potassium intercalation compound C8K

The critical temperature T_c, and the angular and temperature dependences of the upper critical field H_c2 in highly oriented pseudo-single crystals of C₈K have been studied by using a magnetic susceptibility measurement. The magnetic susceptibility abruptly changes around 139 mK at zero field. The angular dependence of H_c2 shows a very large anisotropy.     

C-axis thermoelectric transport in low stage SbCl5 graphite intercalation compounds

c-axis thermal conductivity, electrical resistivity and thermopower measurements performed on stages-2, 3 and 4 SbCl₅-graphite intercalation compounds in the temperature range 3 < T < 300 K are reported. Contrary to the electrical resistivity and thermopower data, the temperature variation of the thermal conductivity is qualitatively different from that previously observed on other intercalation compounds.     

The de Haas-van Alphen effect of graphite intercalation compounds with SbCl5 and HNO3

The de Haas-van Alphen (dHvA) effect of SbCl₅-graphite intercalation compounds of stage 2, 3 and 4, and residual HNO₃-compound of stage 3 has been studied. The dHvA spectra are stage dependent, and no combination frequency relations are found, which are in disagreement with Batallan et al.'s report. The amount of charge transfer per intercalant estimated on the basis of the rigid band model is 0.44, 0.49 and 0.43 for stage 4, 3 and 2 SbCl₅-compounds and is 0.14 for stage 3 HNO₃-compound.     

Different electronic structures of C6Li and C8K graphite intercalation compounds studied by positron annihilation

The 2λ-angular correlation distributions of the stage 1 Graphite Intercalation Compounds C₆Li and C₈K are found to be very different. For C₈K, the distributions are dominated by a free electron contribution similar to those in pure alkali metals. In contrast, the distributions of C₆Li remain similar to those of graphite. The two materials are classified into s(+ π)-band metals (C₈K) and π-band metals (C₆Li).     

Superconductivity in the Th-doped Y NiC2 compounds

We have observed superconductivity in the two-phase samples with nominal compositions (Y _1−xTh_x)NiC₂ (x=0.1,0.2,0.3,0.4,0.5). As determined by magnetic, electrical and heat capacity measurements, the superconductivity phase transition temperature T_c increases with Th concentration to . Powder X-ray diffraction data provide the evidence for bulk superconductivity in an orthorhombic CeNiC₂-type majority fraction of the sample volume while the minor impurity phase ThC₂ has no effect on the superconductivity. The variation of room temperature lattice parameters, a, b, c and v of these substitutional intermetallic samples indicates the systematic substitution of Y ³⁺ ions by Th⁴⁺ ions irrespective of the existence of the 2nd impurity phase.     

Superconductivity in the RexMo6S8

The rare earth molybdenum sulfides RE_xMo₆S₈ (RE = rare earth) have been synthesized and are found to be superconducting with a few exceptions. This result, which is surprising in view of the high concentration of magnetic ions, is correlated with the particular structure of these compounds based on on units of Mo₆S₈.     

Infrared studies of lithium intercalation in the FePS3 and NiPS3 layer-type compounds

Infrared spectra (700-30 cm^-1) of several lithium intercalates (chemically prepared) Li_xMPS₃, with M=Fe, Ni and 0<x<1.5, have been recorded and compared with those known for the corresponding host lattices. These lithium intercalates are mainly characterized by new absorption bands at 336 cm^-1 and 310 cm^-1 for the iron and nickel compounds, respectively. These bands assigned to lithium vibrations increase progressively with lithium content : it is concluded that Li⁺ ions are more likely to occupy the 2d and 4h “octahedral” sites in the gaps. In addition, the spectra of the nickel derivatives reveal some geometric distortion within the layers and a progressive strengthening of the Ni-S interactions. These results are correlated to the best energy yields obtained in NiPS₃/lithium batteries.     

X-ray compressibility measurements of the graphite intercalates KC8 and KC24

The isothermal compressibilities of pristine graphite and stages 1 and 2 potassium-graphite have been measured at room temperature. Diamond anvil X-ray diffraction techniques were employed to determine the c-axis lattice constant as a function of hydrostatic pressure up to 12 kbar. The compressibilities $\text{k}{}_{\mathrm{c}}\text{?}\text{1}\text{C}{}_{33}$ were found to be (2.73±0.09)×10^-12, (2.13±0.09)×10^-12, (5.3±0.8)×10^-12 and $\text{(1.6±0.2)×10}{}^{-12}\text{cm}{}^2\text{dyn}$ for graphite, KC₈, stage 2 KC₂₄ and stage 3 KC₂₄, respectively. The compressibility of KC₈ was comparable to that of RbC₈ deduced from neutron scattering experiments.     

X-ray characterization of graphite intercalation compounds

The understanding of electronic and lattice properties of graphite intercalation compounds depends critically on the model describing the structural properties. We report here results showing that well-staged as-grown samples do not exhibit the expected in-plane intercalant density, and that careful analysis of the 00? x-ray diffractograms reveals important information on the in-plane occupation probability.     

Surface composition of graphite intercalation compounds

We present Auger spectroscopy measurements on graphite intercalation compounds, for donor (cesium) and for acceptor (MnCl₂) intercalants. The surface composition of both compounds is steeply different from their bulk composition. In the case of high stage cesium compounds, a segregation of the intercalant towards the surface layers is observed, leading to an increased concentration between the first two graphite layers (CsC₈ or CsC₆). On the opposite, an intercalant depletion between these first layers is observed for the first stage (MnCl₂)C₆ compounds.Both effects are explained by the screening of surface states, which involves an electrostatic energy of the same order of magnitude as the intercalant chemical potential. This is thus sufficient to allow the local intercalant concentration to change.     

Lattice dynamics of graphite intercalation compounds

We apply a k_z-axis zone folding model that accounts for the staging symmetry to calculate the phonon dispersion curves for graphite intercalation compounds of arbitrary stage. The results are applied to calculate the phonon density of states, velocity of sound, and elastic constants for intercalated graphite.     

Magnetoreflection studies of graphite intercalation compounds

Magnetoreflection spectra are presented for the first time for donor graphite intercalation compounds and for acceptor compounds of low stage. Analysis of these spectra yields values for the K-point effective masses for the conduction and valence bands. Shifts in Fermi level are determined and a breakdown in selection rules for K-point transitions is reported.     

Low temperature thermal conductivity of graphite-FeCl3 intercalation compounds

The low-temperature variation of the in-plane thermal conductivity of graphite- FeCl₃ intercalation compounds is reported. Around room temperature, though holes are participating, there is an important lattice contribution, but much smaller than in pristine graphite. The electronic contribution dominates in the liquid helium range. These preliminary results suggest lattice thermal conductivity measurements as a new tool to investigate defects introduced by intercalation.     

Salting-out in intercalation compounds: Removing copper from Cu3Mo6S8 by intercalating lithium

We study the removal of the copper in Cu₃Mo₆S₈ by the intercalation of lithium. For 0<×<1, Li_xCu₃Mo₆S₈ is a single phase compound. For x?1, the copper atoms are forced out of the Mo₆S₈ host. We present a lattice-gas model which shows how the intercalation of Li can force the Cu out of the host.