Infrared and Raman studies on high pressure phases of solid N2:: An intermediate structural modification between ε and δ phases

The vibrational properties of high pressure phases of solid nitrogen have been studied along several isobars between 30 and 300 K by infrared and Raman spectroscopy. The existence of an intermediate phase (delta(loc)) between the high temperature delta and the low temperature epsilon phases has been confirmed. The analysis of the infrared spectrum in the fundamental region, combined to the knowledge of the Raman spectrum both in the fundamental and in the lattice region, furnishes direct information on the structural properties of the delta(loc) phase. In the delta(loc) phase the disklike molecules are ordered in fixed positions while the spherelike units behave as hindered rotors. An insight on the localization dynamics of the nitrogen molecules is given. A different structure from that of the delta phase is obtained from a group theoretical analysis of the observed Raman and infrared components. As to the epsilon phase, a temperature dependent reorganization of the molecules located on the faces of the rhombohedral cell is suggested. (C) 1998 American Institute of Physics.