  We report the discovery of a new class of molecular phases of solid nitrogen
at high pressures and temperatures by Raman and infrared spectroscopy and
powder x-ray diffraction. Unlike the molecular phases consisting of disk- and
sphere-like molecular disorder ($\delta$-, $\epsilon$- and $\zeta$-N$_2$) and
reportedly stable over a wide P-T range, one of the new phases ($\iota$) is
diatomic with disk-like molecules. A second new phase (the higher pressure
$\theta$-phase) is characterized by strong intermolecular interactions and
infrared vibron absorption. Both phases exhibit wide P-T ranges of stability
and metastability.

Materials Science