Phase transitions at high pressure in the fullerene C$_{60}$ molecular donor-acceptor complex $\{$Hg(dedtc)$_2\}_2$ $\cdot$ C$_{60}$

Raman spectra of crystals of the molecular fullerene C$_{60}$ donor-acceptor complex $\{$Hg(dedtc)$_2\}_2$ $\cdot$ C$_{60}$ (fullerene with mercury diethyldithiocarbamate) have been measured at a pressure up to 8.4 GPa and at room temperature. A phase transition has been revealed in the pressure range of 1.2–2.0 GPa, which is accompanied by a splitting of the degenerate intramolecular phonon modes $H_g$ (1)–$H_g$ (4) and $H_g$ (7)–$H_g$ (8), as well as by a softening of the $H_g$ (2) mode of the fullerene C$_{60}$. As the pressure further increases to the maximum value, the intensity of the bands varies smoothly. A decrease in the pressure leads to the reverse transition to the initial state at 1.2 GPa. The splitting of the degenerate modes $H_g$ (1)–$H_g$ (8) and the softening of the $H_g$ (2) modes resemble their behavior in the formation of dimers in fullerite crystals and indicate the possible formation of dimers in two-dimensional fullerene layers under hydrostatic compression of the $\{$Hg(dedtc)$_2\}_2$ $\cdot$ C$_{60}$ complex.