The thermodynamics of intercalation of bromine in highly oriented pyrolytic graphite had been studied by determining the pressure-temperature equilibrium diagram for stages 2–4. The standard heat and entropy of reaction for the transformation from stage n to stage n−l (n=5, 4, 3) were found to be roughly the same, though the enthalpy of reaction became slightly more negative as the stage number increased. The heat and entropy of formation from pure graphite were thus found to be −10.9 kcal mol-1 Br2 and −30.4 cal mol−1 Br2 K−1 respectively for stage 2, −11.32 kcal mol−1 Br2 and −30.6 cal mol−1 Br2 K−1 respectively for stage 3, and −11.5 kcal mol−1 Br2 and −30.6 cal mol−1 Br2 K−1 respectively for stage 4.

A new model for intercalation kinetics is presented whick takes into account both nucleation of the interlaminar spaces and intercalant layer growth. For nitric acid intercalation, the nucleation constant, β, ranges from 1.25×10−3 to 10−4 sec−1 depending on graphite type (HOPG > powder > fibers); the corresponding range of effective diffusivities, D, is 2.5×10−5 to 10−10 cm2/sec. It is emphasized that effective diffusivities obtained from sorption kinetics are reaction enhanced.

Stepwise isothermal desorption experiments show that the diffusivities of HNO3 neutrals are lower than the effective diffusivities on sorption. At desorptive equilibrium, intercalant retention is in the order HOPG > fibers > powder. In HOPG, resistance is constant over the desorptive transition stage II → III → IV; during the stage IV → V → VI regime (dynamic vacuum), resistance doubles. A similar resistance gain is noted on desorption of neturals from nitrated graphite fibers.

We have studied stage transformations of binary potassium-graphite intercalation compounds by in-situ x-ray (00ℓ) scans. In the transition region of stages one and two, both stages coexist and all Bragg reflections exhibit resolution limited widths. The characteristics of the stage transformation are identical in intercalated HOPG and single crystals. This implies that the stage transformation is a direct process with no need of forming intermediate structures, favoring electrostatic interaction as the dominant stabilizer for pure stage donor compounds. We have also studied the elastic distortion due to potassium atoms substitutionally dissolved in rubidium intercalant layers. From the interlayer spacing variation we find a double force tensor Pzz = 0.36 eV for potassium defects in RbC8 in the small defect concentration region.

We have studied the diffusive motion of alkali atoms in thermodynamic equilibrium in RbC24 by quasielastic neutron scattering. The neutron spectra show Lorentzian line profiles. The widths increase with increasing temperature. In addition a ‘de Gennes’ narrowing is observed at Q=1.2 Å-1, which coincides with the first peak in the intercalant structure factor. The narrowing becomes more pronounced at higher temperatures in contrast to a normal liquid. The data can be interpreted by the motion of clusters of alkali atoms rather than by a liquid type motion. This adds a dynamical aspect to the “floating solid’ as described by Nelson and Halperin for the melting of a 2-dimensional solid.

The c-axis sound velocities and attenuations for HOPG, stage 3 and stage 4 SbCl5-graphite were determined by ultrasonic techniques in the temperature range between 4 and 325 *K. The temperature variation of C33 for HOPG agrees closely with the theoretical prediction based on the Lennard-Jones interlayer potential. In SbCl5-graphite, the longitudinal sound attenuation increases sharply and the transverse sound cannot propagate for temperatures above 200 °K, indicating a commensurate-to-incommensurate in-plane phase transition. The stage dependence of C33 at low temperatures indicates that the interlayer forces beyond the nearest layers can not be neglected.

A Born-von Kármán lattice dynamical model for the graphite Br2 intercalation compounds is presented. The low frequency bromine branches are calculated using a commensurate (√3 × √13)R(30°, 13.9°) unit cell with two Br2 molecules/unit cell. In-plane zone folding is used to calculate the high frequency graphitic modes at the Brillouin zone center.

The [001] L-mode phonon excitations of C24Rb were studied by neutron inelastic scattering in the region where the phonon dispersion curve of the acoustic branch anticrosses the transverse Einstein oscillation level. The interaction between the longitudinal collective mode and the transverse Einstein mode could be observed only in the temperature region where the rubidium atoms form an ordered structure. The mechanism of the interactions between the transverse and longitudinal modes is discussed in relation to the symmetry of the atomic position of rubidium.

Dispersion curves for low-frequency transverse modes propagating in the basal plane have been measured in the compounds KC8, KC24 and RbC24 by means of neutron inelastic scattering. The acoustic branches show at low q a behavior ω2= Aq2+Bq4 characteristic of layered materials. The optical branches are derived qualitatively from graphite-like optical branches hybridized with new alkali-like branches. In stage 2 compounds, the shear constant C44, which can be obtained by extrapolating the acoustic branch towards q=0, is appreciably smaller than in stage-1 compounds or in pure graphite. At low temperatures, it was noted in KC24 that a frequency gap in the acoustic branch opens up near q=0.6 A-1, corresponding to the Brillouin zone boundary of the low temperature alkali superstructure.

Neutron scattering methods have been used to determine the partial phonon densities of states gM(ν) for inplane vibrations of M atoms in MCx compounds, where M = K, Rb or Cs, and x = 8, 24, 36. Only modes with both phonon wave vectors and displacements in the basal plane are represented in gM(ν). Detailed measurements of the temperature dependence were made for RbC24 and KC24, in order to study the influence of the order-disorder transformations in these compounds. The results are interpreted in terms of different pictures of the disordered state.

We report results from a Raman scattering study of stage 2 Graphite-Rb and Graphite-K in the low frequency region (ω < 150 cm-1) as a function of temperature (80K < T < 300K). Four features are seen in the 80K spectra and are interpreted in terms of a Born– von Kármán lattice dynamics model. The temperature-dependence of the Raman spectra is discussed in connection with reported phase transitions in stage 2 alkalimetal graphite compounds.

The stage dependence of various second-order Raman features observed for acceptor (FeCl3, SbCl5) and donor (Rb) Graphite Intercalation Compounds is reported and discussed in terms of recent Born-von Kármán lattice dynamics calculations of Al-Jishi and Dresselhaus.

Coherent inelastic neutron scattering results on the second-stage graphite intercalation compound C8Br are presented. Phonons polarized along the c-axis and in the basal plane were observed. A phenomenological Born-von Kármán force constant model with non-axial symmetry is used with parameters adjusted in agreement with the results. Several specific points of the phonon spectrum which are outside of the limits of the model are discussed.

Ion implantation of highly oriented pyrolytic graphite (HOPG) is studied using various characterization techniques, including Raman spectroscopy and Secondary Ion Mass Spectroscopy (SIMS). Particular attention is given to the annealing of the implantation-induced lattice damage using both hot substrate implantation (200 < T1 < 1000°C) and postimplantation annealing. The Raman spectra provide detailed information on the implantation-induced structural disorder by analysis of the disorder-induced and Raman-allowed features in the first- and second-order spectra. SIMS measurements show that the implanted profile is essentially the same for hot substrate and room temperature implantation for the case of HOPG. It is shown that implantation at elevated temperatures prevents amorphization more effectively than implantation at room temperature and subsequent annealing at the same elevated temperature. The annealing results show that fundamentally different defects are created during room temperature and hot substrate implantation.

Hydrogen/deuterium gas was absorbed in the potassium graphite intercalation compound, C24K, at 77K and then the low temperature specific heats were measured in situ as a function of the hydrogen concentration in the temperature range 1.5–6K. Enhancement of the electronic specific heats upon hydrogen sorption indicates formation of bound states in the vicinity of the Fermi level. Increase of the Debye temperatures by introduction of hydrogen shows the hardening of the lattice. An observed specific heat anomaly was ascribed to a dispersion-less optical phonon with characteristic temperatures of about 17K.

NMR techniques have been used in graphite intercalation systems for chemical analysis and for determinations of structure, molecular dynamics, exchange kinetics, conductivity and electron concentration. Salient results will be presented. We have been most interested in understanding the reaction

3AsF5 + 2Cn ⇋ 2CnAsF6 + AsF3 (1)

for which a single, extremely narrow 19F NMR line is observed when AsF5 reacts with graphite. In contrast, when the reaction

NO2AsF6(in CH3NO2) + Cn→NO2 ↑ + CnAsF6 (2)

is carried out, followed by addition of the stoichicmetric amount of AsF3, separate spectra for the AsF6 (a broad doublet) and lor AsF3 (a sharp 1:2:1 triplet) are observed, indicating that these two species do not exchange fluorine atoms, and suggesting that eq. (1) is not reversible. The AsF3 triplet gives an order parameter for the molecular thrae fold axis with respect to the graphite c-axis. The carbon-13 spectrum for a fifth stage intercalate of HNO3 shows the 2:2:1 triplet expected if each kind of layer Is resolved.

The 13C NMR lines of the stage I cesium graphite compound are broad (ca. 600 Hz) at all orientations, reflecting immobile Cs intercalation; in terms of an axial pattern the principal values are δ11 = 95 ± 5 and δ┴6 = 130 ± 6 ppm. For third and fourth stage compounds the bounding layers give a relatively sharp (60–100 Hz) pair of lines at all orientations, thus demonstrating axially symmetric shift tensors: δ11 = 66 and 56 ppm; δ┴ = 159 and 149 ppm, respectively. These well defined axial tensors reflect atomic motions in the incommensurate Cs layer; the pair of lines may arise from the two classes of carbons - those above carbons in a subsequent layer and those above hexagon centers. Lines of inner layers are sharpest for special orientations →c II →B0 and →c ┴ →B0.

By 133Cs and 13C NMR, EPR, susceptibility anisotropy and X-Ray diffraction,we studied the thermal evolution of Cs24 from 100 to 500 K. A smooth transition is observed near 300 K which can be interpreted in structural and electronic terms: at lower temperatures, the Cs atoms are located in a disordered manner on graphitic hexagonal sites and are almost completely ionized; in the high temperature state, the Cs atoms are completely disordered relatively to the graphitic lattice and their electrons are at least partly relocalized on the metallic s-function.

Stage 2 samples of graphite-SbF5 intercalVtion compounds were prepared by reacting SbF5 with HOPG at 200°C for 5 days. 121Sb Mössbauer spectra taken at 4.2K reveal an intense and narrow feature in the Sb(5+) region and a weak and broad feature in the Sb(3+) region. Interpretation of these data in terms of possible Sb(5+) and Sb(3+) intercalant species is discussed in connection with chemical aspects of intercalation.

19F nuclear magnetic resonance and As X-ray absorption results on AsF5-graphite intercalation compounds are discussed in terms of species of the commonly used oxidation reaction of the graphitic planes. Three fluorine populations and two oxidation states of the arsenic are clearly present, but they cannot be easily related to the oxidation reaction of the graphite planes.

We have examined graphite salts of the composition C23nSbF6(CH3NO2)1,7 n = 1,2 by121Sb Mössbauer spectroscopy at 4.2K. The spectra reveal a strong and narrow feature in the Sb5+ region and no evidence of any features in the Sb3+ region. This result is in general accord with the expectation of only Sb5+ in the chemical form of SbF6− anion in the intercalate layers.