Phase diagram of nitrous oxide: Analogy with carbon dioxide

We present the phase diagram for nitrous oxide (N₂O) derived from in situ high pressure and temperature Raman and x-ray diffraction studies. Two new phases (II and IV) are discovered above 600 K between 18 and 50 GPa; both are quenchable to ambient temperature. The crystal structures and stability fields of N₂O phases are similar to those of CO₂ below 50 GPa and 800 K. However, we found subtle differences in their physical properties and crystal structures, indicating an increased disparity (or ionicity) between N—N and N—O bonds in bent N₂O-IV (Pbcn). The present results thus explain the divergence observed at higher pressures and temperatures; N₂O disproportionates into ionic NO⁺NO₃^- and N₂, whereas CO₂ polymerizes into an extended covalent solid.