No abstract

The high-pressure phase diagram of CsC 8 graphite intercalation compound has been investigated at ambient temperature up to 32 GPa. Combining x-ray and neutron diffraction and Raman and x-ray absorption spectroscopies, we report that CsC 8 when pressurized undergoes phase transitions around 2.0, 4.8, and 8 GPa. Possible candidate lattice structures and the transition mechanism involved are proposed. We show that the observed transitions involve structural rearrangements in the Cs subnetwork, while the distance between the graphitic layers is continuously reduced at least up to 8.9 GPa. Around 8 GPa, important modifications of signatures of the electronic structure measured by Raman and x-ray absorption spectroscopies evidence the onset of a transition which could imply a change of stage. Finally, we also provide the c-axis compressibility of CsC 8 between 0 and 8.9 GPa.

A heat-treatment device has been developed in order to analyse the dynamics of structural transformation of powdered materials. This furnace is able to reach 1123 K, with a lowtemperature gradient along the quartz capillary and is designed to work on a curved INEL CPS 120 detector with a classical X-ray generator (molybdenum tube). The device permits the collection of a set of recordings at different temperatures during a short period of time for moderately diffractive materials.

The three‐dimensional (3D) structure of the second‐stage CSC24 graphitide was investigated by recording hk.l monochromatic rotating‐crystal diffractograms. At low temperature, the two‐dimensional (2D) structure is formed of an incommensurate (2.53 x 2.53 R 14.5°) and a commensurate (2 x 2 R 0°) lattice. Below 140 K, whereas the Cs incommensurate layers become 3D ordered, the 2 x 2 R 0° commensurate layers are randomly distributed in the structure. The commensurate structure, with composition CsC16 (stage two), probably equilibrates the lack of Cs induced by the incommensurate structure (composition CSC26), in order to reach the global composition CSC24 of the compound. Depending on the temperature, two kinds of stackings appear. Below 140 K, the incommensurate Cs layers are arranged in a 2D‐modulated initial stacking, transformed at a lower temperature into a rhombohedral stacking in which Cs atoms are modulated along the c axis. Simulation of the selection rules allowed the determination of models of the incommensurate layers stacking along the c axis, with translations from one Cs layer to another layer commensurate with the graphite lattice. The initial stacking is described by a pseudo‐two‐layer model with successive random translations and a short‐range order. The structure appearing at a lower temperature is described by a pseudorhombohedral stacking with definite translations. The transition from the initial stacking with a first‐neighbour interaction to the pseudorhombohedral stacking with a second‐neighbour interaction could be a result of the change in interactions between the Cs atoms and the graphitic host, due to the dominating influence of the graphite periodic potential at low temperature.