Structures of Molecular Nitrogen at High Pressures.

We have studied structural properties of the high pressure phases of molecular nitrogen at room temperature using synchrotron radiation and angle dispersive powder diffraction with image plates as detectors. The cubic δ-phase was found to be stable between 5 and 11 GPa. At 11 GPa a phase transition to a new tetragonal phase was observed. Structural refinements suggest, that the transition is due to an orientational ordering of the disklike disordered molecules of the δ-phase. At 16. 5 GPa the transition to the rhombohedral ε-phase occurred.