Solid Nitrogen at Extreme Conditions of High Pressure and Temperature

Abstract

We review the phase diagram of nitrogen in a wide pressure and temperature range. Recent optical and x-ray diffraction studies at pressures up to 300 GPa and temperatures in excess of 1000 K have provided a wealth of information on the transformation of molecular nitrogen to a nonmolecular (polymeric) semiconducting and two new molecular phases. These newly found phases have very large stability (metastability) range. Moreover, two new molecular phases have considerably different orientational order from the previously known phases. In the iota phase (unlike most of other known molecular phases), N 2 molecules are orientationally equivalent. The nitrogen molecules in the theta phase might be associated into larger aggregates, which is in line with theoretical predictions on polyatomic nitrogen.

A Novel High-Density Phase and Amorphization of Nitrogen-Rich 1H-Tetrazole (CH2N4) under High Pressure

“…All these splittings in internal modes are believed to arise from interactions between crystallographically inequivalent molecules, as Davydov splitting 43 . In many cases, Davydov splitting is related to a possible increase in the number of molecules in a unit cell as well as associated symmetry lowering 44 45 46 . Thus, our Raman spectra indirectly identify the phase transition forming a pair of molecules in a unit cell of phase II.…”