Dielectric properties and the role of grain boundaries in polycrystalline tetracene at high pressures

The dielectric properties and the role of grain boundaries in polycrystalline tetracene were studied at pressures of up to 21.3 GPa using in situ impedance spectroscopy measurements. Two anomalous changes in some of the electrical parameters (resistance, relaxation frequency and relative permittivity) were found at 6.0 and 19.0 GPa, which were related to the structural rearrangement and condensation reaction processes, respectively. Within this pressure range, grain boundaries play an important role in polycrystalline tetracene's conduction mechanism. Based on first-principles calculations, it was found that the variation in bulk resistance under pressure was mainly caused by a change of band gap. Intermolecular interactions are more sensitive to pressure than intramolecular interactions. The decrease in the relative permittivity under pressure indicates the existence of space charge polarization of the interface layer in addition to electronic polarization.