Abstract
The melting curve and stability of ammonia () is investigated up to 40 GPa and 3500 K by x-ray diffraction and Raman spectroscopy in the laser-heated diamond anvil cell. The samples were directly heated by the radiation of a 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 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 is expected to cross the isentropes of Neptune and Uranus in the pressure range 55–65 GPa, implying the possible presence of superionic solid in these planets. Our x-ray and Raman measurements confirm the appearance of and upon heating the liquid phase from 6 to 40 GPa. But while the equilibrium 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.
1 More- Received 20 August 2018
- Revised 28 February 2019
DOI:https://doi.org/10.1103/PhysRevB.99.134107
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