The effect of high pressure on the lattice structure and dynamics of phenacenes

We studied the effect of high pressure on three phenacenes, aromatic molecules with a zig-zag configuration of the benzene rings. The lattice structure and vibrational dynamics of crystalline phenanthrene (C₁₄H₁₀, three benzene rings), chrysene (C₁₈H₁₂, four), and picene (C₂₂H₁₄, five) were investigated by means of X-ray diffraction and Raman measurements. Raman spectra were compared with theoretical ones obtained from ab-initio Density Functional Theory calculations. Experimental and theoretical results allowed to identify the onset of a structural transition in phenanthrene at 7.8 GPa under hydrostatic conditions and at 5.7 GPa under non-hydrostatic conditions. We found that this transition is related to a reorientantion of the molecules in the ab plane. On the contrary, chrysene and picene do not undergo any phase transition in the investigated pressure range, thus suggesting that molecular size plays an important role in the occurence of pressure induced structural modifications in aromatic compounds.