Experimental and DFT high pressure study of fluorinated graphite (C2F)n

Abstract

We studied the high pressure behavior of poly(dicarbon monofluoride) (C2F)n up to 37 GPa both theoretically, within the first-principle framework of the density functional theory, taking into account van der Waals corrections to the total energy, and experimentally, combining in situ Raman spectroscopy, XRD and X-ray photoelectron spectroscopy (XPS) measurements. The (C2F)n structure has a complex mixed sp2 sp3 character where graphene layers are intercalated among sp3 carbon hybridized structural domains. We observed three phase transitions in the high pressure range explored. The first transition is observed around 12 GPa and is dominated by strain effects. Above 16 GPa the intercalated graphene layers corrugate and change from the sp2 to sp3 hybridization state. Above 25–27 GPa the new pressure-induced phase becomes unstable. We find that, by varying applied pressure or by changing the hybridization due to the fluorination of graphite, the band structure of (C2F)n can be modified, resulting in valence and conduction bands touching at the K point, as in graphene, thus turning (C2F)n into a semimetal. This could provide a new pathway to reversibly engineer the band structure and conductivity for applications in high energy density batteries.