A Study on Change in Structure of Graphite under High Pressure by Raman Scattering

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In order to elucidate the change in the crystalline structure of graphite caused by pressure, Raman scattering of graphite under pressure was investigated. A highly oriented pyrolytic graphite (HOPG) sample was loaded into the diamond anvil cell using 4 : 1 methanol-ethanol pressure medium. Raman scattering from the HOPG basal plane at pressures up to 3.3 GPa was measured with 457.9-, 488.0- and 514.5-nm excitation by using microscope. Although several Raman bands due to features in the density of state in the higher-order region could not be observed because of scattering from diamond anvil and the thin layr of pressure-transmitting medium between diamond window and sample, the E_<2g> band and overtone (2D) of the -1360-cm^<-1> band (D band) were observed. Both the frequencies of the E_<2g> and 2D modes increase sublinearly under pressure. The pressure coefficients of the peak position of the 2D band was about three times larger than that of the E_<2g> band. The lambda dependence of 2D band was almost unchanged by pressure, while the magnitude of the 2D band shift with increasing lambda seems to be slightly increased by pressure. In addition the peak position of the E_<2g> band was insensitive to lambda under pressure. The pressure dependence of the 2D band is the first observation. The experimental results suggest that the 2D band is the key to the clarification of the change in the structure of graphite induced by high pressure.