Equation of state of an anisotropic three-dimensional ${\mathrm{C}}_{60}$ polymer: The most stable form of fullerene

Fullerite is a solid composed of pseudospherical ${\mathrm{C}}_{60}$ molecules with strong covalent intramolecular C-C bonds and weak intermolecular van der Waals interactions. The discovery of their one-dimensional- (1D), 2D-, and 3D-polymerized forms has renewed the interest of compressibility studies of clathrate phases composed of a continuous network of mainly ${\mathrm{sp}}^3$ carbon atoms. Using accurate x-ray-diffraction measurements performed under high pressure, we report the room-temperature pressure-volume equation of state of anisotropic 3D ${\mathrm{C}}_{60}$ polymer up to 35 GPa. A very high bulk modulus of 288 GPa has been extracted from the fitting of the unit-cell volume to a Vinet equation of state and confirms the very high stiffness of this low-density material. Two different linear compressibilities (231 and 320 GPa) were obtained, respectively, along the axial direction and the radial direction of the elliptical Debye-Scherrer diffraction patterns indicating the presence of two types of intermolecular bonds. No phase transition or collapse of the ${\mathrm{C}}_{60}$ cages was observed during compression. The pseudo-fcc 3D ${\mathrm{C}}_{60}$ polymer remains stable up to 35 GPa and is thus, through multiple intermolecular ${\mathrm{sp}}^3$ bonding, the most stable form of fullerite.