Structural and electronic properties of sodium azide at high pressure: A first principles study

The structural and electronic properties of NaN3 at high pressures were studied through ab initio calculations. Three new phases with I4/mcm, P6/m and C2/m structure were found to be stable at pressures of 6.5, 58 and 152 GPa, respectively. Similarity of the Raman spectra revealed that the experimental post-α phase should adopt the I4/mcm structure. The calculated insulator–metal transition at 58 GPa directly explained the observed darkening of NaN3 sample at above 50 GPa. The three proposed structures contain azide, N6 hexagon and polymeric nitrogen, respectively. Our finding of the novel N6 hexagon in NaN3 at moderate pressures provides a new view of the pressure-induced polymerization process of metal azides.