  The phase diagram and stability limits of diatomic solid nitrogen have been
explored in a wide pressure--temperature range by several optical spectroscopic
techniques. A newly characterized narrow-gap semiconducting phase $\eta$ has
been found to exist in a range of 80--270 GPa and 10--510 K. The vibrational
and optical properties of the $\eta$ phase produced under these conditions
indicate that it is largely amorphous and back transforms to a new molecular
phase. The band gap of the $\eta$ phase is found to decrease with pressure
indicating possible metallization by band overlap above 280 GPa.
