H. Lauter

Magnetic order of FeCl3 and CoCl2-graphite intercalation compounds by neutron diffraction

Abstract 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.

Neutron diffraction on bromine intercalated in graphite

Abstract 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.

The anisotropy of the C44 shear constant in bromine-graphite intercalation compound: A precursor effect of the anisotropic melting

We have measured by inelastic neutron scattering the anisotropy of the out-of-plane shear constant in the second stage bromine intercalated graphite in the commensurate phase. The constants are very weak, C44 = 2 ± 3 × 107 N/m2 and C55 = 18 ± 4 × 107 N/m2, mainly in the y direction in which the incommensurability of the striped domain phase and the anisotropic melting appear at higher temperature.

The magnetic phases of FeCl3 intercalated in graphite

Abstract 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.

Magnetic order of FeCl3 and CoCl2 intercalated graphite

We have investigated by neutron diffraction the magnetic properties of first‐ and second‐stage FeCl3 and CoCl2 graphite intercalation compounds. They can be considered as quasi‐two‐dimensional magnetic systems in which one can vary both the 3D magnetic coupling and the intralayer magnetic interactions. The FeCl3 compounds are Heisenberg systems in which we have found that strong 2D magnetic correlations develop below 40 K; in the first‐stage compound, the 3D coupling is strong enough to allow a transition to a 3D order below 3.8 K. The in‐plane order is always an incommensurate modulation. In the CoCl2 compounds we have observed on the contrary an antiferromagnetism along c axis with the moments in the layer plane. They are 2D X‐Y systems with a Kosterlitz–Thouless transition in which however the 3D coupling plays an important role.

Lattice dynamics of graphite intercalation compounds

We have studied by coherent inelastic neutron scattering the phonon dispersion relation of the second stage bromine graphite intercalation compound. We have observed the almost complete low energy spectrum (<12 THz) with longitudinal and transverse phonons propagating along the direction perpendicular to the layers (c axis) as well as in the layers plane. The modes polarized along the c axis present energy gaps in the c direction branches and the acoustic branch polarized in the layers plane, opposite to the optic ones, is very sensitive to the intercalate nature. The main character of the modes polarized in the layers plane is a dispersionless optic branch along the c direction at 1 THz. A phenomenological Born-von Karman force constant model with non axial symmetry is developed in order to interpret our results.