Pressure Response of Crystalline Fluoranthene Probed by Raman Spectroscopy

The pressure response and structural stability of fluoranthene crystals up to 8 GPa are investigated using Raman spectroscopy. The vast majority of the Raman peaks upshift with pressure, either sublinearly (intermolecular modes) or quasilinearly (intramolecular modes), reflecting the bond hardening upon volume contraction. The frequency shifts, accompanied by intensity redistribution among the Raman peaks, are by far larger for the former than those for the latter vibrations, compatible with their nature: weak intermolecular van der Waals interactions and strong intramolecular covalent bonds. For pressures higher than 2 GPa, changes in the linear pressure coefficients of the Raman peak frequencies, mainly towards lower values, are observed. These are more pronounced for intermolecular and C–H stretching vibrations. For P > 4.7 GPa, the pressure coefficients are further reduced, while all the observed pressure-induced changes are fully reversible upon pressure release. These changes may be interpreted either as two structural transitions at ~2 and ~4.7 GPa or as a single, but sluggish, structural phase transition in the pressure range 2–4.7 GPa, featuring the reorientation and different stacking of the molecules. From the high-pressure Raman data in the low-pressure phase, a bulk modulus of ~7 GPa at ambient pressure is estimated for solid fluoranthene.