Melting curve and chemical stability of ammonia at high pressure: Combined x-ray diffraction and Raman study

The melting curve and stability of ammonia (NH₃) is investigated up to 40 GPa and 3500 K by x-ray diffraction and Raman spectroscopy in the laser-heated diamond anvil cell. The NH₃ samples were directly heated by the 10.60.16em0exμm radiation of a CO₂ laser to reduce the risks of chemical reactions. Melting was unambiguously detected by the appearance of the liquid diffraction signal upon temperature increase. The melting temperature of NH₃ is found to steadily increase with pressure up to 40 GPa, and the previously reported turnover is not observed. As a result, the melting line of NH₃ is expected to cross the isentropes of Neptune and Uranus in the pressure range 55--65 GPa, implying the possible presence of superionic solid NH₃ in these planets. Our x-ray and Raman measurements confirm the appearance of N₂ and H₂ upon heating the liquid phase from 6 to 40 GPa. But while the equilibrium 2NH₃N₂+3H₂ balances towards the dissociated elements at low pressure and high temperature, ammonia is found to the more stable species in the range 20--40 GPa, 300--3000 K.