Structural transformations and stability of benzo[a]pyrene under high pressure

Benzo[a]pyrene (BaP), C20H12, is a representative of polycyclic aromatic hydro­carbons (PAHs), which are ubiquitous in nature and the universe, where they are subjected to extreme conditions. This paper reports the results of investigations of the high-pressure behavior of BaP up to 28 GPa using in situ synchrotron single-crystal X-ray diffraction. We identified two previously unknown polymorphs, BaP-II (P21/c) at 4.8 GPa and BaP-III (P1) at 7.1 GPa. The structural transformation from BaP-I (P21/c) to BaP-II (P21/c) manifests as an abrupt change in the intermolecular angle and the unit-cell parameters a and b, whereas the transformation from BaP-II (P21/c) to BaP-III (P1) is characterized by a decrease in symmetry. According to density functional theory calculations, BaP-III is the most stable phase above 3.5 GPa. These studies advance our understanding of the structural dynamics and stability of PAHs under high pressure.