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.