Structural Evolution of Solid Phenanthrene at High Pressures

The effect of hydrostatic pressure on the structure of solid phenanthrene (C14H10) was investigated up to 25.7 GPa through synchrotron X-ray powder diffraction and an evolutionary algorithm for crystal structure prediction based on van der Waals density functional calculations. We observed the onset of a phase transition around 8 GPa from the monoclinic P2(1) low-pressure phase with two molecules per unit cell arranged in a herringbone fashion to a new high-pressure phase. The best candidate structure for this phase exhibits three molecules in a P1 triclinic unit cell in a parallel arrangement, stabilized by dominant pi-pi intermolecular interactions. The P2(1) and P1 phases coexist in the pressure range from 8 to 13 GPa, whereas above 13 GPa only the P1 high-pressure phase is present. At higher pressures (P > 20 GPa), experiments and first-principles calculations suggest a tendency toward amorphization.