ROOM-TEMPERATURE RAMAN-SCATTERING STUDIES OF EXTERNAL AND INTERNAL-MODES OF SOLID NITROGEN AT PRESSURES 8-LESS-THAN-OR-EQUAL-TO-P-LESS-THAN-OR-EQUAL-TO-54 GPA

Raman spectra of solid nitrogen were studied from 8 to 54 GPa at room temperature. Between 8 and 16 GPa three out of four possible lattice modes of the delta-N2 phase were observed. Above 16 GPa up to eight lattice modes can be distinguished, seven of which correspond to lattice modes observed at 15 K in the epsilon-N2 phase. Branching of existing lattice and vibronic modes above 20 GPa and above 40 GPa indicate additional slight structural modifications. The observed pressure shift of vibrons of the N-14-N-15 isotopic species appears to favor factor group effects instead of bond weakening to be responsible for the downward shift of one vibronic mode at higher pressures.