Structural Study of Novel Graphite−Lithium−Calcium Intercalation Compounds

Three new layered compounds were synthesised by immersing a pyrographite platelet in a molten Ca−Li alloy creating a new graphite intercalation compound family. The samples were studied by X-ray and neutron diffraction, revealing that the intercalated sheets are polylayered. The study of the 00l reflections allowed us to establish the c-axis stacking of these three phases. The α-phase exhibits a five-layered intercalated sheet which has something in common with a Li−Ca−Li−Ca−Li slice cut in the CaLi2 structure (ThMn2 Laves phase type). The β-phase, which is richer in metallic elements and with a greater repeat distance, possesses seven-layered intercalated sheets due to the splitting of the middle lithium plane in three. The third phase is a pseudo-binary compound, containing monolayered sheets and with a formula close to CaC6. Electron microdiffraction allowed us to determine the 2D unit cell for each compound, all of which were commensurate with that of graphite. Charge transfer from the intercalate to the host lattice was evaluated for the three phases from hk0 data, leading to high values. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)