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Polycrystalline GaN fibers have been produced by the polymer-derived-ceramic (PDC) technique. The wurtzite-polymorphic fibers appear to emerge from complex nucleation and grain growth mechanisms, being mostly unconstrained during initial polymer to ceramic conversion. The importance of carrier polymer architecture and alignment is highlighted towards controlled microstructure.

Carbon-based cryosorbers are generally synthesized in the form of powders, which compromises the volumetric capacity in gas storage applications. Here we report the synthesis of monolithic carbide-derived carbon (CDC) by chlorination of fully-dense ceramic titanium carbide plates. Volume change is minimal, consistent with conformal transformation from TiC to CDC, and the weight loss is consistent with nearly 100% conversion. The resulting materials have a microporous carbon structure with little or no macroporosity and exhibit enhanced volumetric gas storage capacity compared to powder equivalents. Optimized volume uptakes are 35 g L−1 at −196 °C and 60 bar for H2, 193 V(STP) v−1 at 35 bar and 219 V(STP) v−1 at 60 bar (25 °C) for CH4. Monolithic CDCs thus offer potential as gas storage media for on-board fuel-cells and other applications.

Physisorption of hydrogen in microporous molybdenum carbide (Mo 2 C)-derived carbons has been studied as a function of synthesis conditions. Changes in local structure induced by varying the chlorination temperature afford controllable variations in average pore size and specific surface area. Optimal hydrogen storage capacity of 4.3 wt%, measured at −196 °C and 35 bar pressure, is obtained from a sample chlorinated at 660 °C for 3 h. This optimum correlates with the largest fraction of total pore volume having average pore sizes in the 0.6–0.8 nm range.

Hydrogen storage measurements on peels formed from powdered carbide-derived carbons (CDCs) mixed with 3–5 wt% PTFE binder are presented. Polymer interactions with adsorption sites are explored by examining CDCs derived from different precursors. The observed trends in loss of gravimetric capacity suggest that the detrimental effect of the polymer is greatest for large (>1.5 nm) pores and small ( 2 /L, respectively, at 77 K and 4 MPa hydrogen pressure.

Laser flash and self-heating 3 ω techniques were employed to determine the anisotropic thermal conductivity and thermal diffusivity of a highly oriented, free-standing multiwalled carbon nanotube (MWCNT) sheet and a yarn drawn from a sidewall of the MWCNT forest grown by chemical-vapor deposition. Normalized to ideal high density structure the thermal conductivity and the thermal diffusivity along the alignment are 50 ± 5 W/m K and 45 ± 5 mm 2 /s, respectively, and are mostly limited by dangling terminals of bundles, intrinsic defects of individual nanotubes and phonon scattering within the bundles, which form the supporting matrix of the MWCNT sheet. The high degree of tube–tube overlap substantially decreases the electrical and thermal interconnection resistance, which usually dominates in randomly deposited mat-like nanotube assemblies. The extremely high surface area of the MWCNT sheet leads to excessive radial heat radiation that does not allow transferring the heat energy along the MWCNT sheet by means of phonons to distances >2 mm. On the other hand, the high surface area and negligible heat capacitance make it a perfect material for bolometric sensing ( r = 38 V/W) and heat dissipation.

High surface area nanoporous carbon has been prepared by thermo-chemical etching of titanium carbide TiC in chlorine in the temperature range 200–1200 °C. Structural analysis showed that this carbide-derived carbon (CDC) was highly disordered at all synthesis temperatures. Higher temperature resulted in increasing ordering and formation of bent graphene sheets or thin graphitic ribbons. Soft X-ray absorption near-edge structure spectroscopy demonstrated that CDC consisted mostly of sp 2 bonded carbon. Small-angle X-ray scattering and argon sorption measurements showed that the uniform carbon-carbon distance in cubic TiC resulted in the formation of small pores with a narrow size distribution at low synthesis temperatures; synthesis temperatures above 800 °C resulted in larger pores. CDC produced at 600–800 °C show great potential for energy-related applications. Hydrogen sorption experiments at −195.8 °C and atmospheric pressure showed a maximum gravimetric capacity of ∼330 cm 3 /g (3.0 wt.%). Methane sorption at 25 °C demonstrated a maximum capacity above 46 cm 3 /g (45 vol/vol or 3.1 wt.%) at atmospheric pressure. When tested as electrodes for supercapacitors with an organic electrolyte, the hydrogen-treated CDC showed specific capacitance up to 130 F/g with no degradation after 10 000 cycles.

We review our current progress on semiconductor nanowires of β-Ga2O3, Si and GaN. These nanowires were grown using both vapor–solid (VS) and vapor–liquid–solid (VLS) mechanisms. Using transmission electron microscopy (TEM) we studied their morphological, compositional and structural characteristics. Here we survey the general morphologies, growth directions and a variety of defect structures found in our samples. We also outline a method to determine the nanowire growth direction using TEM, and present an overview of device fabrication and assembly methods developed using these nanowires.

We have investigated the dispersion of single-walled carbon nanotubes (SWNTs) in heavy water with the surfactant octyl-phenol-ethoxylate (Triton X-100) using small angle neutron scattering. The results indicate an optimal surfactant concentration for dispersion, which we suggest results from competition between maximization of surfactant adsorption onto SWNT surfaces and a depletion interaction between SWNT bundles mediated by surfactant micelles. The latter effect drives SWNT reaggregation above a critical volume fraction of micelles. These behaviors could be general in dispersing SWNTs using amphiphilic surfactant. The data also reveal significant incoherent scattering from hydrogen in SWNTs, most likely due to acid and water residues from the purification process.

Porous solids are of great technological importance due to their ability to interact with gases and liquids not only at the surface, but throughout their bulk. Although large pores can be produced and well controlled in a variety of materials, nanopores in the range of 2 nm and below (micropores, according to IUPAC classification) are usually achieved only in carbons or zeolites. To date, major efforts in the field of porous materials have been directed towards control of the size, shape and uniformity of the pores. Here we demonstrate that porosity of carbide-derived carbons (CDCs) can be tuned with subångström accuracy in a wide range by controlling the chlorination temperature. CDC produced from Ti3SiC2 has a narrower pore-size distribution than single-wall carbon nanotubes or activated carbons; its pore-size distribution is comparable to that of zeolites. CDCs are produced at temperatures from 200-1,200 degrees C as a powder, a coating, a membrane or parts with near-final shapes, with or without mesopores. They can find applications in molecular sieves, gas storage, catalysts, adsorbents, battery electrodes, supercapacitors, water/air filters and medical devices.

Gallium phosphide nanowires with a most probable diameter of approximately 20.0 nm and more than 10 microns in length have been synthesized by pulsed laser vaporization of a heated GaP/5% Au target. The morphology and microstructure of GaP nanowires have been investigated by scanning electron microscopy and transmission electron microscopy. Twins have been observed along the crystalline nanowires, which have a growth direction of [111]. Raman scattering shows a 4 cm-1 downshift of the longitudinal optical phonon peak in the nanowire with respect to the bulk; the transverse optical phonon frequency and line width are, however, the same as in the bulk. The quantum confinement model first proposed by Richter et al. cannot explain the observed behavior of the Raman modes.

Single-wall carbon nanotubes can be doped, or intercalated, with electron donors or acceptors, similar to graphite and some conjugated polymers. The resulting materials show many of the same features: enhanced electrical conductivity, conduction electron paramagnetism, partial or complete reversibility, and cyclability. Reactions may be carried out in vapor or liquid phase, or electrochemically. Structural information is sketchy at best, due to the limited quality of currently available materials and solvent-related effects. Recent developments in coagulation-based fiber extrusion and partially aligned materials offer new opportunities for novel material modifications by chemical doping.

We describe, in detail, a readily scalable purification process capable of handling single-wall carbon nanotube (SWNT) material in large batches. Characterization of the resulting material by SEM, TEM, XRD, Raman scattering, and TGA shows it to be highly pure. Resistivity measurements on freestanding mats of the purified tubes are also reported. We also report progress in scaling up SWNT production by the dual pulsed laser vaporization process. These successes enable the production of gram per day quantities of highly pure SWNT, which should greatly facilitate investigation of material properties intrinsic to the nanotubes.

The electronic structure of single-wall nanotubes in "buckypaper" has been studied by using soft X-ray emission and resonant inelastic soft X-ray scattering. We observe electronic states derived from the K point and located close to E-F in the nanotubes

Co-evaporation of carbon, in the presence of helium with some other element as catalyst, yields interesting new molecules of the fullerene family which have tubular morphologies with a cross-section at the nanoscale but lengths at the macroscale. In particular, using Ni or Co based mixtures as catalyst with the electric arc technique, one obtains single wall carbon nanotubes in large quantities, with an average diameter of 1.4 nm and which assemble themselves in highly crystalline bundles containing a few tens of elements. These nanotubes have been observed, characterized and studied using high resolution microscopy techniques. X-ray and electron diffraction have been used to characterize the crystalline nature of individual bundles. The diameter dispersion has been studied in detail using Raman spectroscopy which shows impressively well-resolved structures of the observed modes. A majority of (10, 10) armchair tubes is observed, but some other configurations are also present with slightly different diameters.

Inter- and intra-layer disorder in a representative hard carbon derived from epoxy novolac resin at 1000°C have been investigated by comparing both the elastic structure factor S(Q) and radial distribution function obtained from pulsed neutron time-of-flight measurements with calculations from simple hypothetic systems with specific disorder characteristics. The spatial relationship between these building blocks, planar graphene fragments with length scale of 1 nm, is probed in terms of inter-layer alignment and intra-layer connectivity. Coupled with the density of dangling bonds obtained from ESR and residual hydrogen concentration, these simulations provide sufficient information to construct an overall model of the carbon structure. Of special interest is the physical origin of the ubiquitous broad (002)-like reflection frequently interpreted as that from layers with a uniform interlay spacing (d002) It is found to be directly related to the misalignment between layers with an average interlayer spacing as that in crystalline graphite (3.35 A).

The superconducting properties of fullerene intercalation compounds can be rationalized fairly well in the context of standard theories. In contrast, our understanding of correlations between properties and chemical composition, crystal structure, molecular orientations and valency remains rudimentary. Key features which were believed to be well understood have been called into question by recent experiments while others remain unexplained.

We report the result of 13 C nuclear magnetic resonance (NMR) measurements on A 6 C 60 ( A =K, Rb, Cs) and Ba 3 C 60 . By using high-resolution magic angle spinning, we were able to identify an isotropic line around 156 ppm for all investigated compounds. NMR spectra of the saturated alkali compounds are quite similar. The corresponding isotropic lines show three narrow components consistent with orientationally ordered C 60 molecules leading to three non-equivalent carbon sites in these compounds as reported by x-ray studies. No line splitting was observed for the Ba 3 C 60 isotropic line.

The splitting of the tetrahedral Rb - and K - N M R lines in Rb 3 C 60 and K 3 C 60 is analyzed in terms of the mechanism of alkali-cation vacancies. The calculations indicate the presence of vacancies only in the tetrahedral positions, and the inter-atomic potentials between the alkali cations and the fullerene molecules account satisfactorily for the temperature dependence of the position of the distorted line.

Synchrotron-based X-ray powder diffraction has proved indispensable in the study of fullerene solids, their alkali metal intercalation compounds and chemical derivatives. Here we review some recent developments to which David E. Cox has made important material and intellectual contributions, not only in advancing the field of fullerenes but in helping to train a new generation of young scientists. Topics covered include finite vacancy concentrations in superconducting phases, estimates of the vacancy formation energies from temperature-dependent phase analysis, and the effects of alkali metal intercalation on orientational states, order-disorder transitions and superconductivity.

We have measured the time decay of the magnetization for a well characterized superconducting fullerene Rb 3 C 60 . The temperature dependence of the magnetic relaxation rate dM(t)/dln(t) exhibits a maximum at a characteristic temperature T max , which decreases with increasing field. At low temperatures a nonlinear logarithmic time decay of the magnetization is observable only after several hours, but is clearly observable after much shorter times at high temperatures. This nonlinear behavior suggests the existence of a vortex glass state in Rb 3 C 60 and is consistent with the collective creep model. PACS #: 74.30.Ci, 74.60.Ge, 74.70.Wz

In order to better understand the properties and composition of solids of polyfluorinated C[sub 60], X-ray photoelectron spectroscopy (XPS) was used to characterize several different C[sub 60]F[sub x] samples synthesized in two different laboratories. Quantitative measurements of the F[sub 1s], C[sub 1s], and O[sub 1s] XPS signals established the elemental stoichiometry of each sample. The F/C ratios for the different samples ranged from 28/60 to 53/60 in general agreement with previous mass spectroscopy studies of each sample. C[sub 15] energies and shifts as well as solid-state nuclear magnetic resonance studies provided additional information about the degree of fluorination and level of C-C bond breaking. 22 refs., 3 figs., 3 tabs.

A review is given of crystal structures and order/disorder phenomena in the fullerene solids C 60 , C 60 O and C 70 and in the alkali metal intercalation compounds of C 60 . Recent results on orientational melting and quenched orientational disorder in the pristine solids are summarized, and the effect of small perturbations on the quasi-spherical molecular shape are described. The intercalation properties of C 60 are discussed in the context of other host lattices such as graphite. The variety of orientational correlations in fullerenes and fullerides is rationalized in terms of competing contributions to the orientational potentials. Structural properties of alkali fullerides are correlated with electronic properties and superconductivity, and an updated binary phase diagram is described.

AC impedance and x-ray diffraction measurements versus temperature and pressure on several ternary fullerene intercalation compounds are reported. The results support our previously established empirical correlation between superconducting onset temperature and 300K fcc lattice constant. Compounds which do not follow this correlation either phase separate or are otherwise unstable at low T and/or high P.

Rietveld refinements of x-ray powder profiles show that the C 60 molecules in Na 2 RbC 60 are orientationally ordered in a manner essentially the same as in pure C 60 , the only fullerene-based superconductor thus far to do so. Fits to data at both 300 K and 27 K are significantly better in space group Pa 3 (simple cubic) than in Fm 3 m (merohedral disorder) or by assuming uniform spherical shells of charge (complete disorder). The anomalously low T c of this compound cannot be attributed to excess orientational disorder, and therefore remains a mystery.

Solid C[sub 60] reacts with low-pressure fluorine at 300 K, yielding yellow-to-white solids which are air-stable for long periods, with average compositions in the range C[sub 60]F[sub 35-44]. Mass spectrometry reveals broad distributions of F/C ratios, with only trace amounts of perfluorofullerene C[sub 60]F[sub 60]. Infrared and other data indicate the presence of C-F bonds, from which we deduce a [open quotes]fuzzyball[close quotes] structure with fluorines externally attached to the fullerene skeleton. Most of the material sublimes in vacuum at 300[degrees]C; molecules with F/C ratios > 48 probably decompose. The sublimed product is highly crystalline; X-ray analysis yields an average C-F bond length 1.49 A. Thin films may be prepared by sublimation or spin-coating from solution. Contact angle measurements using water droplets suggest potentially interesting lubricating properties. 22 refs., 6 figs., 1 tab.

Synthesis of carbon by extraction of metals from carbides has been successfully used to produce a variety of micro-porous carbide-derived carbons (CDCs) with narrow pore size distributions and tunable sorption properties. This approach is of limited use when larger mesopores are targeted, however, because the relevant synthesis conditions yield broad pore size distributions. Here we demonstrate the porosity control in the 3–10 nm range by employing preceramic polymer-derived silicon carbonitride (SiCN) precursors. Polymer pyrolysis in the temperature range 600–1400 °C prior to chlorine etching yields disordered or graphitic CDC materials with surface area in the range 800–2400 m2 g−1. In the hierarchical pore structure formed by etching SiCN ceramics, the mesopores originate from etching silicon nitride (Si3N4) nano-sized crystals or amorphous Si–N domains, while the micropores come from SiC domains. The etching of polymer-derived ceramics allows synthesis of porous materials with a very high specific surface area and a large volume of mesopores with well controlled size, which are suitable for applications as sorbents for proteins or large drug molecules, and supports for metal catalyst nanoparticles.

The growth of thin C 60 films on GaSe(0001) has been studied by infrared High Resolution Electron Energy Loss Spectroscopy (HREELS) in specular reflection geometry. In contrast to previously studied disordered films on Si(100), it was found that deposition on a GaSe substrate heated to 120°C leads to well-ordered, epitaxial C 60 films. The structural order, manifested by a sharp LEED pattern, was confirmed by the clear detection in the specular beam of all four T lu dipole-active vibrations of the C 60 molecule. Several other infrared inactive modes, which were prominent in the C 60 /Si HREELS spectrum, are still seen in C 60 /GaSe but with reduced intensities. Application of the dielectric theory of reflection EELS allows a quantitative determination of the contribution of the dipole-active modes to the total spectrum. Although the relative oscillator strengths of the four infrared modes are in good agreement with optical absorption measurements, their absolute values, as determined by theoretical simulation, are found to be three times larger than the optical results.

The evolution with hydrostatic pressure (0 < P < 8.1 kbar) of the interchain ordering in trans-polyacetylene was measured using 4-circle synchrotron x-ray diffractometry and a Merrill-Bassett diamond anvil cell. The P-dependence of (HK0) reflections signify a lowering of the two-dimensional space group symmetry from pgg to p2 with a threshold P < 0.5 kbar, with an angle γ between the a and b axes of about 92° throughout the entire P range. The interchain compressibility is large and highly anisotropic. Below 6 kbar, Δa/a and Δb/b are 8.3 × 10−3/kbar and 3.8 × 10−3/kbar respectively. Both a and b stiffen above 6 kbar as the system approaches true close-packing.

We present new x-ray diffraction data on trans-(CH)x which shows that the blond alteration is out of phase on adjacent chains, and therefore the correct space group is P2 1 n . Previous attempts to resolve this question by analyzing only the (00L) intensities have yielded ambiguous results; all experiments show some (001) intensity which is not allowed P2 1 n . By analyzing indidual off-axis peak intensities, we show unambiguously that the bulk 3-D structure is inconsistent with P2 1 a (in-phase bond alternation), and therefore the weak, sample-dependent (001) intensity probably results from uncorrelated local defect regions with in-phase bond alternation on adjacent chains.

The electrical resistance of single-wall carbon nanotubes (SWNT) produced by condensation of a laser- vaporized graphite=Ni=Co mixture at 1200C was studied under quasihydrostatic pressures up to 90 kbar. The resistance exhibits a positive temperature coefficient, characteristic of a metal, up to 10 kbar, whereas the absolute value decreases abruptly by a factor 10. From 10- 30 kbar R increases with pressure and the TCR becomes negative. At still higher pres- sure, up to 90 kbar, R decreases gradually with pressure, similar to the case of graphite. Raman scattering and electron microscopy performed after 25-kbar pressurizations indicate that the SWNT and its lattice are preserved. We propose that the sequential behavior of R.P/ reveals in turn the processes of compaction, defect formation by kinking, and finally the van der Waals compression of the inter-tube spacing in the triangular lattice.

The diameter-dependent Young's modulus, E, and quality factor, Q, of GaN nanowires were measured using electromechanical resonance analysis in a transmission electron microscope. E is close to the theoretical bulk value ( approximately 300 GPa) for a large diameter nanowire (d=84 nm) but is significantly smaller for smaller diameters. At room temperature, Q is as high as 2,800 for d=84 nm, significantly greater than what is obtained from micromachined Si resonators of comparable surface-to-volume ratio. This implies significant advantages of smooth-surfaced GaN nanowire resonators for nanoelectromechanical system (NEMS) applications. Two closely spaced resonances are observed and attributed to the low-symmetry triangular cross section of the nanowires.

High-pressure reaction of Li metal and graphite yields LiC 2 , three times as dense in Li as the usual LiC 6 . LiC 6 decomposes rapidly but incompletely into LiC 6 + Li upon releasing pressure. A dramatic enhancement is observed in the metastability of the high Li density when the graphite contains 0.5 at.% substitutional boron. Average compositions from X-ray (OOL) intensities, determined several months after releasing the pressure, are LiC 2.2 and LiC 3.8 with and without boron respectively. The Li “layer” is split along c by 0.5 A suggesting a puckering which reduces the repulsive interactions.

We have measured the bulk modulus K for C61D2 up to 1 GPa in the temperature range 175–343 K. For face-centered cubic C61D2 above 290 K, we find an anomalously low value for K below about 0.15 GPa, ...

We report the production and characterization of polymer nanocomposites with single-walled carbon nanotubes having improved mechanical properties and exceptional nanotube alignment. High-pressure carbon monoxide nanotubes (HiPco) were efficiently distributed in polystyrene (PS) and polyethylene (PE) with a twin-screw compounder. Nanotube concentrations were 1, 5, 10, and 20 wt% in PE composites and 0.7 wt% in PS composites. PE composites were melt-spun into fibers to achieve highly aligned nanotubes. Polarized Raman spectroscopy shows that the degree of alignment increases with decreasing fiber diameter and decreases with increasing nanotube loading. The orientation distribution function of a 1 wt% HiPco/PE composite had a full width at half-maximum of approximately 5 degrees. The elastic modulus increases up to 450% relative to PE fibers for 20 wt% nanotube loading at an intermediate fiber diameter of 100 microns.

Powder profiles have been obtained from unoriented films of (CH) x and (CD) x solution-doped with lithium by using x-ray and neutron diffraction respectively. Detailed intensity analyses show improved fits with a lower symmetry R3 model for the two-dimensional interchain packing, as compared to the previously proposed high-symmetry R3C space group. The R3-based structure is derived from the R3C one by a local distortion of the interchain packing motif, in a very similar way to the symmetry breaking observed in the tetragonal phases for larger alkali metals.

The authors succeeded in intercalating fullerite electrodes with lithium using all-solid-state electrochemical cells and polymer electrolytes. This method offers several advantages over previous studies of fullerenes in solution. The first redox cycle for pure C{sub 60} is shown as a linear sweep voltammogram. Three well defined peaks: A, B, and C, are apparent at 2.3, 1.9, and 1.5 V respectively vs Li. At each potential within the C peak, a slow relaxation of the current was observed. The absence of trailing current after the A, B, and C peaks proves that all the available sites have been occupied in the C{sub 60} solid at each of these reduction steps. Reduction beyond the fourth wave implies extensive reorganization of the C{sub 60} solid, as corroborated by the markedly different shape of the first oxidation following the deep reduction to 0.2 V. Only three oxidation waves were observed, even on extending the sweep to 3.5 V, and no peaks were observed on the second reducing potential sweep. It is interesting to note that the roughly constant 0.5 V separation between waves A and B, B and C, and C and D corresponds approximately to the electron-by-electron reduction in solution. This could bemore » interpreted as evidence for weakly perturbed molecular orbitals in the solid state which continue to dominate electroactivity.« less

The poor performance of hydrogen storage materials continues to hinder development of fuel cell-powered automobiles. Nanoscale carbons, in particular (activated carbon, exfoliated graphite, fullerenes, nanotubes, nanofibers, and nanohorns), have not fulfilled their initial promise. Here we show that carbon materials can be rationally designed for H2 storage. Carbide-derived carbons (CDC), a largely unknown class of porous carbons, are produced by high-temperature chlorination of carbides. Metals and metalloids are removed as chlorides, leaving behind a collapsed noncrystalline carbon with up to 80% open pore volume. The detailed nature of the porosity-average size and size distribution, shape, and total specific surface area (SSA)-can be tuned with high sensitivity by selection of precursor carbide (composition, lattice type) and chlorination temperature. The optimum temperature is bounded from below by thermodynamics and kinetics of chlorination reactions and from above by graphitization, which decreases SSA and introduces H2-sorbing surfaces with binding energies too low to be useful. Intuitively, pores of different size and shape should not contribute equally to hydrogen storage. By correlating pore properties with 77 K H2 isotherms from a wide variety of CDCs, we experimentally confirm that gravimetric hydrogen storage capacity normalized to total pore volume is optimized in materials with primarily micropores ( approximately 1 nm) rather than mesopores. Thus, in agreement with theoretical predictions, a narrow size distribution of small pores is desirable for storing hydrogen, while large pores merely degrade the volumetric storage capacity.

Liquid anhydrous sulfuric acid forms molecular “shells” wrapped around single-walled carbon nanotubes (SWNTs). Temperature-dependent X-ray scattering from aligned acid-swollen fibers shows that crystallization of the bulklike acid surrounding the structured shells is templated by the aligned SWNTs, while the structured shells remain partly ordered, at least for temperatures from 100 to 500 K. The (2 0 0) or (2 0 2) planes of the templated H2SO4 crystallites are parallel to the nanotube axes. This provides solid evidence for the direct protonation of SWNT since the molecules are terminated by hydrogen bonds.

We have measured the reflectivity spectra of stages 1 and 2 graphite intercalated with KHg amalgam (KHgC4 and KHgC8) from 0.5–6eV, and compared them to the corresponding stages 1 and 2 binary compounds, KC8 and KC24. Plasma edges for the ternaries appear at about the same energies as in the binaries, but differences are observed in the reflectivity values at the minima. Kramers-Kroenig analysis yields thresholds for the onset of interband transitions at 1.3eV and 1.2eV for KHgC4 and KHgC8, compared to 2.7eV and 1.7eV for KC8 and KC24. The thresholds are explained in terms of interband transitions in the mercurographitides involving the Hg levels. A model for these transitions is presented based on the Mott metal-semiconductor transition in mercury. This model is shown to be consistent with the hypothesis that the mercury layers are negatively charged.

Room temperature measurements of the 0.07–2.0 eV optical reflectance of carefully prepared stage 1–4 AsF5-graphite intercalation compounds have been performed. Stages 1–3 show simple metallic behavior with a well-defined plasma edge. Curve fits to the data give good agreement between dc and optical conductivities for stage 1 and 2. Comparison between stage 2 data for AsF5 and HNO3 compounds suggests that the higher conductivity of the former arises from a longer carrier relaxation time rather than from a greater carrier density.

We have observed dHvA frequencies in several samples of stage 2 graphite-AsF 5 intercalation compound. A consistent interpretation is given in terms of two nested hole surfaces located at each of the 6 corners of the 2D graphite zone. A fractional ionization f = 0.42 is obtained, consistent with spin susceptibility, with the Blinowski-Rigaux analysis of reflectivity, and with recent chemical results.

NMR and static magnetic susceptibility (χ) measurements on stage 1 AsF5 intercalated graphite (C8AsF5) are presented. The relaxation times, Ti and T2, of the F19 nuclear magnetic resonance were measured over the temperature range 136K – 295K; χ was measured over the range 80K – 295K. The NMR results indicate a motionally narrowed line with gradual ordering of the intercalant as the temperature is decreased. The magnetic susceptibility is independent of temperature with no observable Curie law controbution. The absence of localized moment behavior for the intercalant is interpreted either in terms of chemical disproportionation of the AsF5 to closed shell ions or in terms of the Anderson model of localized moments in metals.

An initial survey of the conduction electron spin resonance is presented for a series of graphite compounds intercalated with acceptor molecules: stages 1–3 AsF 5 , stages 2–5 HNO 3 , and stage 2 Br 2 and ICl. The g-values and lineshapes were studied as functions of temperature and concentration. The results suggest metallic behavior but with very small density of states at the Fermi energy: N(E F ) ∼10 20 /cm 3 eV. The temperature dependence of the linewidth is dominated by an order-disorder transition of the intercalant layers, implying that the conduction electrons are not entirely confined to the graphite portion of the crystal. The decrease in g-value anisotropy upon intercalation can be understood in terms of Elliott's theory.

Temperature-dependent c-axis resistivity measurements on stages 1–7 potassium-graphite compounds show a crossover from metallic to activated behaviour between stages 4 and 5, as well as the onset of a new type of anomaly for stage ≥4. KC 8 shows in addition a tendency toward a metal-insulator transition below 40K. The high-stage transport process is discussed in terms of weak depletion of the interior layers by interlayer screening. We suggest that the high stage anomalies reflect an instability in the c-axis charge distribution.

Raman measurements on low stage compounds of graphite intercalated with AsF 5 , HNO 3 and SbCl 5 are reported. The spectrum of the stage 1 C 8 AsF 5 acceptor compound is found to be in sharp contrast with that reported for the donor stage 1 C 8 M ( M = K , Rb , Cs ) compounds. Whereas the stage 1 donor compounds have been found to exhibit a characteristic broad, asymmetric Breit-Wigner line shape, the spectrum of C 8 AsF 5 contains a single Lorentzian line in this frequency region. Temperature studies of the bounding layer mode lineshape parameters in C 8 AsF 5 showed no evidence of the order-disorder transition in the adjacent intercalate layer. The bounding layer mode frequencies of AsF 5 , HNO 3 and SbCl 5 graphite are reported, but no intercalate layer modes were observed.

Basal resistivity ϱa has been measured in situ on MC8 compounds (M=K, Rb, Cs) from 5–300K. We find ϱa(T) = A+BT+CT2, in agreement with Suematsu et. al., except our value of A is ∼200 times smaller implying fewer defects. In MC8 compounds RH at 5K is positive and increases linearly with magnetic field, suggesting a complex Fermi surface. On the other hand, RH for RbC24 is negative and field-independent but the magnitude is inconsistent with a simple one-carrier model (assuming one free electron per Rb).