Orientational behavior of solid nitrogen at high pressures investigated by vibrational Raman spectroscopy

Abstract

In this paper the results of an experimental study of the vibrational Raman shift of solid δ-, ε-, and β-nitrogen will be presented. The measurements have been performed in a diamond anvil cell, from 0.5 to 14 GPa and from 120 to 400 K. The accuracy of the spectroscopic measurements has been optimized in order to determine the Raman shift as a function of temperature on an isobar. As will be shown, measurements along an isobar are appropriate for obtaining information about the orientational behavior of the molecules. In this study the results of β-nitrogen will be compared with those of δ- and ε-nitrogen. In β-nitrogen the Raman shift as a function of pressure shows a remarkable curvature, as compared to the results in δ- and ε-nitrogen. Also in contrast to the behavior in the other phases, in β-nitrogen the slope of the shift versus the temperature on an isobar changes from negative to positive when the pressure is increased. It will be argued that these results might be due to a short range orientational ordering of the molecules, as predicted by recent computer simulations. This ordering might cause a resonance coupling effect, resulting in a negative effect on the Raman shift in β-nitrogen.