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

The melting curve and stability of ammonia (${\mathrm{NH}}_{3}$) is investigated up to 40 GPa and 3500 K by x-ray diffraction and Raman spectroscopy in the laser-heated diamond anvil cell. The ${\mathrm{NH}}_{3}$ samples were directly heated by the $10.6\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$ radiation of a ${\mathrm{CO}}_{2}$ 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 ${\mathrm{NH}}_{3}$ 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 ${\mathrm{NH}}_{3}$ is expected to cross the isentropes of Neptune and Uranus in the pressure range 55--65 GPa, implying the possible presence of superionic solid ${\mathrm{NH}}_{3}$ in these planets. Our x-ray and Raman measurements confirm the appearance of ${\mathrm{N}}_{2}$ and ${\mathrm{H}}_{2}$ upon heating the liquid phase from 6 to 40 GPa. But while the equilibrium $2{\mathrm{NH}}_{3}\ensuremath{\rightleftharpoons}{\mathrm{N}}_{2}+3{\mathrm{H}}_{2}$ 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.