F. Batallan

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.

Observation of a Critical Behaviour of the Diffusion Constant for Two-Dimensional Melting: HNO3 Intercalated in Graphite

We have studied the behaviour of the diffusion constant by incoherent quasi-elastic neutron scattering at the disorder transition of the second-stage α-HNO3 intercalated in graphite. The transition is continuous. This disordered phase is a lattice liquid phase. It shows simultaneously strong orientational correlations as previously observed by X-ray diffraction and translational diffusion. The diffusion constant follows an exp [- 2b/(T - Tc)1/2] law, as predicted for 2D defect-mediated phase transitions.

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.

Magnetic specific heat of a quasi 2D system: The first and second stages of CoCl2 intercalated in graphite

Abstract We have observed in the specific heat measurements on the first and second stage of CoCl2 intercalated in graphite a magnetic part with a singularity at 9.7 and 7.8 K respectively. The tail corresponds to 2D fluctuations, and not to the singularities due to the appearance of the c-axis antiferromagnetism.

Study of the melting of HNO3 intercalated in graphite by quasi-elastic neutron scattering

We have studied by incoherent quasi-elastic neutron scattering the molecular dynamics of HNO 3 intercalated in graphite at several temperatures. In the semi-incommensurate phase we have observed a rotational motion in which the number of rotating molecules varies with the temperature. In the liquid phase 2D jump translational motion appears in addition to the rotational one. The jump vector is equal to the in-plane lattice vector of graphite. The translational diffusion constant may be fitted by a power law in reduced temperature.

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.