High-Pressure Elastic Properties of Polycyclic Aromatic Hydrocarbons Obtained by First-Principles Calculations

Abstract

The crystal structures and compressibility parameters of benzene and a number of polycyclic aromatic hydrocarbons (PAHs) were calculated by the methods of the first-principles density functional theory with the gradient approximation of exchange and correlation potentials in the PBE form accounting for van der Waals interactions (optPBE–vdWB) at pressures of 0–20 GPa. A comparison with experimental data for benzene, naphthalene, tetracene, and pentacene demonstrated the high accuracy of our calculations. All the compounds have similar compressibilities, bulk moduli (8–12 GPa), and their pressure derivatives (6.9–7.5). The similarity of the calculated parameters indicates the main role of a decrease in interatomic distances during PAH compression and weak deformation of the molecules and benzene rings. The compressibility is weakly dependent on the number of atoms (benzene rings) in the molecule or crystal structure type (most of the PAHs have the space group P2₁/a). Compounds with many benzene rings and with a denser structure of rings (cyclic pyrene and coronene) are less compressible than less dense PAHs (tetracene and hexacene). Some PAHs (benzene, phenanthrene, pyrene, coronene) have high-pressure modifications, but a correct description of their structures allowing calculation of their elastic moduli has not been made so far. The obtained data on PAH compressibility can be used to develop high-temperature equations of state and calculation of the equilibrium composition of liquid and solid components of the C–O–H system.