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 P21 low-pressure phase with two molecules per
unit cell arranged in a herringbone fashion to a new highpressure
phase. The best candidate structure for this phase
exhibits three molecules in a P1 triclinic unit cell in a parallel
arrangement, stabilized by dominant ?-? intermolecular
interactions. The P21 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