Energy spectrum and phase transitions in C70 fullerite crystals at high pressure

Abstract

Measurements have been made of the Raman, optical absorption, and luminescence spectra of single crystals and pellets of the fullerite C70 at T=300 K and at pressures up to 12 GPa. The baric shift dω/dP and the Gruneisen parameters of the Raman-active intramolecular phonon modes have been determined. It has been established that the dω/dP value for certain phonon modes abruptly changes at pressures of P1≈2 GPa and P2≈5.5 GPa, as do the half-widths of the Raman lines. These features in the Raman spectrum are associated with phase transitions at high pressure. The baric shifts of the absorption and luminescence edges of C70 crystals have been determined and are −0.12 eV/GPa and −0.11 eV/GPa, respectively, for absorption and luminescence. The baric shift of the absorption edge decreases significantly with increasing pressure and is −0.03 eV/GPa at 10 GPa. These data have been used to determine the deformation potential of the fullerite C70, which is about 2.1±0.1 eV.