Orientational phase transition in methane-intercalated fullerite C60

The known first-order orientational phase transition in pure ${\mathrm{C}}₆₀$ occurs at 260 K and is accompanied by the change from face-centered cubic $(Fm3m)$ to simple cubic unit cell $(Pa\overline{3})$. While ${\mathrm{C}}₆₀$ molecules rotate freely above 260 K, they are fixed in inversion centers below 260 K. The phase transition in methane-intercalated ${\mathrm{C}}₆₀$, ${(\mathrm{C}{\mathrm{H}}₄)}_x({\mathrm{C}}₆₀)$, occurs at $236\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. It was studied by DSC and single crystal x-ray diffraction. Above 236 K, the intercalate adopts the face-centered cubic structure which is characterized by freely rotating ${\mathrm{C}}₆₀$ and disordered ${\mathrm{CH}}₄$ molecules. The ordered low-temperature crystal structure possesses simple cubic unit cell in the space group $Pa\overline{3}$ in which both ${\mathrm{C}}₆₀$ and ${\mathrm{CH}}₄$ molecules are fixed on threefold inversion axes. In the structure of ${(\mathrm{C}{\mathrm{H}}₄)}_0.72{\mathrm{C}}₆₀$ at 100 K, a ${\mathrm{C}}₆₀$ molecule is fixed by its 12 ${\mathrm{C}}₆₀$ neighbors due to donor-acceptor van der Waals interactions between electron-rich 6:6 C-C bonds and electron-deficient cage pentagons. At 200 K, the second orientation of a ${\mathrm{C}}₆₀$ molecule is present (with 7.5% occupancy) in which 6:6 C-C bonds are directed to cage hexagons. The methane molecules are situated in octahedral holes and statistically disordered over two orientations around inversion centers. The phase transition temperature of $(\mathrm{C}{\mathrm{H}}₄)\phantom{\rule{0.16em}{0ex}}{\mathrm{C}}₆₀$ intercalate is lower than in ${\mathrm{C}}₆₀$ by $\ensuremath{\sim}24\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ due to the increased ${\mathrm{C}}₆₀\ensuremath{\cdots}{\mathrm{C}}₆₀$ separation. This study provides a reliable structural information about the low-temperature structure of the ${(\mathrm{C}{\mathrm{H}}₄)}_0.7({\mathrm{C}}₆₀)$ intercalate thus demonstrating significant differences between orientational phase transitions in pure fullerite ${\mathrm{C}}₆₀$ and ${\mathrm{C}}₆₀$ intercalated with methane. In addition, the molecular structure of ${\mathrm{C}}₆₀$ fullerene in the ${(\mathrm{C}{\mathrm{H}}₄)}_0.7{\mathrm{C}}₆₀$ intercalate is distinguished by a high precision which is superior to the former experimental data for ${\mathrm{C}}₆₀$ and its adducts reported in the literature.