Graphite and graphite intercalation compounds under pressure: Raman modes, optical reflectivity, and phase changes

Abstract

We have investigated the high pressure behavior of pristine graphite and graphite intercalation compounds (GICs) by Raman spectroscopy and by optical reflectivity measurements in the spectral range 0.5 eV < < ω < 4 eV. Results for pressure coefficients and mode Grüneisen parameters of the two Raman-active E 2g modes of pristine graphite are presented and compared to published theoretical calculations. The optical response of stage-2 acceptor GICs with SbCl 5, FeCl 3, and Br 2 intercalants provides information on the pressure dependence of the Fermi level and overlap interaction between paired graphite layers. The latter results are compared to the overlap interaction in pristine graphite under pressure as determinde from reflectivity measurements. Stage-1 alkali metal GICs have been prepared by in-situ solid state reaction in a diamond window pressure cell. In the presence of excess alkali metal we observe discontinuous blue shifts of the visible plasma reflection edges by, e.g., 0.35 eV in K-GIC at 3.5 kbar and ∼ 1.2 eV in Li-GIC at 30 kbar. These reversible changes in the optical response are attributed to the formation of dense phases with alkali metal concentrations significantly higher than those attainable at ambient pressure. For the system Na-graphite, we find phases with plasma edges in the visible and near-UV spectral range, indicating the formation of two different stage-1 compounds at 25 kbar and above ∼ 27 kbar.