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

Abstract

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 10.6 µm 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 2N H3 N2 + 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.

Transformation of Ammonium Azide at High Pressure and Temperature

“…At 13 GPa–300 K, N and NH were solid and phase separated (hence, the inhomogeneous aspect of the sample shown in panel (a4) of Figure S3 ), and a stronger Raman signal from the NH solid was recorded in sample regions, where NH crystallized. Although not tested, we consider it unlikely that H was formed in the decomposition of AA, since (1) none were formed at higher pressures (see below) and (2) NH is more stable than N + H at this pressure [ 4 , 30 , 31 ]. Thus, we conclude that AA is not stable and decompose into N and NH in the presence of liquid N at 15 GPa.…”

Fluid-like elastic response of superionic NH 3 in Uranus and Neptune

“…S2A). These temperatures correspond to the melting temperatures of NH 3 proposed by the latest X-ray diffraction experiments (23). Given that the longitudinal wave velocity (V p ) is expressed as the following relation:…”

“…The broadening of the N-H stretching band is associated with the increase in the proton diffusion constant, as discussed in Raman measurements for H 2 O (26). The full width at half maximum (FWHM) of the N-H stretching band at 30 GPa moderately increased with increasing temperature and most likely shows no significant change due to the melting in this pressure region, given that the N-H stretching bands of the phase III and the liquid are very similar in shape (23). Meanwhile, the FWHM in the run at 55 GPa discontinuously increased at ∼1,800 K (Fig.…”

Compressive behavior and electronic properties of ammonia ice: a first-principles study

“…Alternatively, the formation of ionic phase Pma2 was also experimentally evidenced at 125 GPa and 300 K by Palasyuk et al 25 A recent melting curve study by X-ray diffraction and Raman spectroscopy also implied the possible presence of superionic solid NH 3 on Neptune and Uranus. 26 As described above, most studies have been devoted to exploring the high-pressure and high-temperature phase diagrams of ammonia ice. However, some atomic structures and electronic properties of ammonia ice under high pressure are still insufficiently investigated.…”