Structural Evolution of D_5h(1)-C_90 under High Pressure: A Mediate Allotrope of Nanocarbon from Zero-Dimensional Fullerene to One-Dimensional Nanotube

The hybridization of fullerene and nanotube structures in newly isolated C_90 with the D_5h symmetric group (D_5h(1)-C_90) provides an ideal model as a mediating allotrope of nanocarbon from zero-dimensional (0D) fullerene to one-dimensional nanotube. Raman and infrared spectroscopy combined with classical molecular dynamics simulation were used to investigate the structural evolution of D_5h(1)-C_90 at ambient and high pressure up to 35.1 GPa. Interestingly, the high-pressure transformations of D_5h(1)-C_90 exhibit the features of both fullerene and nanotube. At around 2.5 GPa, the D_5h(1)-C_90 molecule in the crystal undergoes an orientational transition to a restricted rotation. At 6.6 GPa, the tubular hexagonal part occurs and transforms into a dumbbell-like structure at higher pressure. The material starts to amorphize above 13.9 GPa, and the transition is reversible until the pressure exceeds 25 GPa. The amorphization is probably correlated with both the intermolecular bonding and the morphology change. Our results enrich our understanding of structural changes in nanocarbon from 0D to 1D.