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 <i>P</i>2₁ 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 <i>P</i>1 triclinic unit cell in a parallel arrangement, stabilized by dominant π–π intermolecular interactions. The <i>P</i>2₁ and <i>P</i>1 phases coexist in the pressure range from 8 to 13 GPa, whereas above 13 GPa only the <i>P</i>1 high-pressure phase is present. At higher pressures (<i>P</i> > 20 GPa), experiments and first-principles calculations suggest a tendency toward amorphization