Pressure and temperature diagram of polymerized fullerite

We have systematically studied the temperature-pressure diagram of ${\mathrm{C}}_{60}$ at pressures between 37 kbar and 80 kbar and from room temperature to 900 \ifmmode^\circ\else\textdegree\fi{}C. We show here the specific role played by the temperature in the phase transformations observed in the $P\ensuremath{-}T$ region covered by this study. As temperature is increased, we follow the continuous transformation of fcc fullerite to an orthorhombic phase of polymerized ${\mathrm{C}}_{60}$ chains that, at higher temperature, polymerize to two-dimensional ${\mathrm{C}}_{60}$ polymers. Above 800 -900 \ifmmode^\circ\else\textdegree\fi{}C the ${\mathrm{C}}_{60}$ cage collapses with formation of an $s{p}^{2}$ amorphous phase. The continuous evolution of the x-ray patterns during these phase transformations is explained on the basis of a homogeneous solid-state polymerization.