High-pressure elastic properties of polycyclic aromatic hydrocarbons obtained by first-principles calculations

Crystal structure and compressibility parameters of benzene and a number of polycyclic aromatic hydrocarbons (PAHs) were calculated by first-principles methods of the density functional theory with a gradient approximation of the exchange-correlation potential in the form of PBE, taking into account the van der Waals interactions optPBE-vdW in a pressure interval of 0–20 GPa. A comparison with the experimental data for benzene, naphthalene, tetracene, and pentacene shows a high accuracy of the calculations. All studied materials have a close compressibility with the bulk modulus from 8 to 12 GPa and its pressure derivative 6.9–7.5, which consistent with a decrease in the intermolecular distances and a weak deformation of the molecules and benzene rings themselves. There is a weak dependence of the compressibility on the number of atoms (benzene rings) in the molecule or on the type of crystal structure (most PAHs have a space group P 21/a). Compounds with a large number of benzene rings, as well as a denser ring structure – cyclic (pyrene, coronene) have lower compressibility with respect to less dense PAHs (tetracene, hexacene). Some PAHs, benzene, phenanthrene, pyrene and coronene, have high-pressure modifications, but a correct description of their structures, which allows to obtain elastic modules, has not yet been made. The obtained data on PAH compressibility can be used in the development of high-temperature equations of state and calculation of the equilibrium composition of the liquid and solid components of the C-O-H system.