A structural and lattice vibrational study of graphite-alkali metal ternary compounds.

Abstract

We study the structure and lattice vibrations of one member of the Alkali-metal/Graphite ternary compound family, namely $C\sb8K\sb{1-x}Rb\sb{x}.$ The relative concentration (x) is varied in a series of samples so as to encompass the entire range available (0 to 1). The in-plane structure is monitored by conducting a series of $h\sb{o}k\sb{o}$0 x-ray scans and appears to remain unchanged. The lattice parameter in the out-of-plane direction, and certain vibration modes, exhibit a non-linear behavior in the concentration range x = 0.5 to 0.7 for samples based on disordered artificial graphite matrices, but is otherwise linear for structurally "clean" samples. We find that the anomalous softening of phonon modes previously reported in highly oriented pyrolitic graphite (HOPG) samples does not occur in naturally occurring single-crystal (SC) samples. X-ray and Raman scattering experiments show that the anomaly is an artifact due to the inhomogeneity of synthetic polycrystalline graphite.