High P-T transformations of nitrogen to 170 GPa

Abstract : X-ray diffraction and optical spectroscopy techniques are used to characterize stable and metastable transformations of nitrogen compressed up to 170GPa and heated above 2500K. X-ray diffraction data show that ɛ-N₂ undergoes two successive structural changes to complex molecular phases ζ at 62GPa and a newly discovered κ at 110GPa. The latter becomes an amorphous narrow gap semiconductor on further compression and if subjected to very high temperatures (∼2000K ) crystallizes to the crystalline cubic-gauche-N structure (cg-N) above 150GPa. The diffraction data show that the transition to cg-N is accompanied by 15% volume reduction.