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Resume Nous avons mesure le spectre de phonons de C 7 Br par diffusion coherente inelastique de neutrons. Une attention particuliere a ete portee aux faibles vecteurs d'onde afin de determiner les constantes elastiques. Nous avons elabore un modele de constantes de force de Born-von Karman (BvK) qui explique bien l'ensemble des resultats experimentaux, et donne des expressions theoriques pour les constantes elastiques C 11 , C 33 , C 44 . Les C ii ont ete obtenus aussi bien directement a partir de la pente initiale des branches de phonons acoustiques, que par lissage de l'ensemble des donnees au modele de BvK. Nous avons observe experimentalement un dedoublement des branches de phonons acoustiques dans le plan des couches du a l'anisotropie planaire de la maille de C 7 Br.

We have investigated the magnetic properties of first and second stage intercalated FeCl3 and CoCl2 graphite by neutron diffraction at low temperatures. Each case is very different: in FeCl3 we have found that in both stages strong 2-D magnetic correlations develop below 30 K. In the first stage compound we observe a transition from the 2-D phase to a 3-D order at 3.8 K. The in-plane order remains the same and corresponds to an incommensurate magnetic modulation of the propagation vector 0.394 a . By contrast, in both stages of the CoCl2 compounds, we observe a 3-D antiferromagnetism along the c-direction without any 2-D behaviour.

We have studied by inelastic neutron scattering the 2DXY system CoCl2 intercalated in graphite of first stage using a single crystal and a H.O.P.G.. We have observed both spin waves and localized spin diffusion which gives a wavevector dependent quasielastic broadening. The spin waves are only observed for wavelength higher than the correlation length which varies with the temperature. The fit of the magnon branches confirm the 2DXY nature of the system and gives a value J=21.5 K. The spin diffusion in small areas is due to the mobility in the vortex cores and the diffusive behavior should derive from the interaction of these movements with the magnons.

We have studied by incoherent quasi-elastic neutron scattering the behavior of the diffusion constant and the molecular movements in the second-stage of nitric acid intercalated in graphite near the melting transition. The transition is continuous and the high temperature phase is a lattice liquid. The diffusion constant follows the critical behavior predicted in a defect mediated phase transition. However, the X-ray diffraction pattern is closer to that of a lattice liquid than that of a hexatic phase.

Results for magnetothermal oscillations and the in situ deHaas-van Alphen effect in acceptor and donor graphite intercalation compounds are presented. The following intercalates have been investigated: C 8 s (NO 3 H) s = 2, 3 and 4, C 7 s (SO 3 HF) s = 2, 3, C 4 s Br s = 2, C 6 s Li s = 2, C 4 s HgK s = 1, where s is the stage. The observed frequencies range between 40 and 1500 Tesla and the effective masses between 0.05 m 0 and 1 m 0 . In acceptor compounds characteristic combinations of frequencies are present which are explained by the magnetic breakdown of the primitive orbits on the FS. A 2D FS model is proposed which includes the electronic structure proposed by Blinowski-Rigaux and the folding due to the periodicity of the intercalate layer. The results for the C 16 (NO 3 H) are analyzed in terms of a detailed FS. The charge transfer coefficient found is ƒ = 0.6 and the Fermi level shift E F = −1.08 eV.

We present in-situ neutron diffraction studies on graphite intercalated with bromine. We have made kinetics studies and determined the phase diagram of the system versus temperature and bromine pressure. We have observed in high stages the commensurate-incommensurate transition and the melting of the bromine layer. Even in these stages, the stacking of the bromine layers is three-dimensional so the two-dimensional theory cannot be applied. We also present some inelastic results which confirm the registered nature of the incommensurate and liquid phases.

We have observed by neutron inelastic spectroscopy the vibrational spectrum of α-HNO 3 intercalated in graphite, between 200 cm −1 and 2000 cm −1 at 4.5 K, 180 K and 230 K. We have observed strong anisotropic effects, showing that the HNO 3 molecules plane make an angle of about 25° with the c-axis at 4.5 K. The NO'H stretch mode of the HNO 3 molecule is strongly attenuated when one crosses the commensurate-incommensurate transition at 210 K.

We confirm by neutron diffraction experiments that the magnetic phases of the two kinds of graphite-FeCl3 are very different : the α compound orders at 1.7 K into an in-plane antiferromagnetism and the s one, obtained by reaction with the atmosphere, presents an in-plane incommensurate modulation. We present here the results of specific heat measurements which show the 3D behavior of the first stage compound and a Log T behaviour for the second stage one. The phase diagram is determined by magnetic susceptibility which shows also the behavior of disordered systems.