Pressure-induced transformations and optical properties of the two-dimensional tetragonal polymer of C-60 at pressures up to 30 GPa

The Raman spectra of the two-dimensional tetragonal (2D(T)) polymeric phase of C-60 have been studied in situ at pressures up to 30 GPa and room temperature. The pressure dependence of the phonon modes shows an irreversible transformation of the material near 20 GPa into a new phase, most probably associated with the covalent bonding between the 2D polymeric sheets. The Raman spectrum of the high-pressure phase is intense and well resolved, and the majority of modes are related to the fullerene molecular cage. The sample recovered at ambient conditions is in a metastable phase and transforms violently under laser irradiation: the transformed material contains mainly dimers and monomers of C-60 and small inclusions of the diamond-like carbon phase. The photoluminescence spectra of the 2D(T) polymer of C-60 were measured at room temperature and pressure up to 4 GPa. The intensity distribution and the pressure-induced shift of the photoluminescence spectrum drastically differ from those of the C-60 monomer. The deformation potential and the Gruneisen parameters of the 2D(T) polymeric phase of C-60 have been determined and compared with those of the pristine material. (C) 2002 MAIK Nauka/Interperiodica.