The sound velocity, melting curve, and phase diagram of ammonia have been investigated from 15 to 55 GPa and up to 3300 K using Brillouin spectroscopy of ${\mathrm{CO}}_2$ laser-heated ammonia samples in a diamond anvil cell. The melting transition was unambiguously identified from the appearance of the Brillouin peaks of fluid ammonia upon temperature increase, the latter being well resolved from that of the solid phase. Our melting temperatures are in good agreement with previous measurements and theoretical predictions. Within experimental uncertainties, we do not observe any discontinuous variation of the sound velocity or of the slope of the melting curve at the passage from plastic phase III to the superionic (SI) phase $\alpha$-SI, which is consistent with most previous experimental and theoretical results. We also find no evidence of the superionic $\gamma$-SI phase with fluidlike elastic behavior reported in a recent work and suggest that it was mistaken with the fluid phase.

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• High-pressure studies
• Liquid crystal phase transitions
• Mechanical & acoustical properties
• Phase diagrams
• Hydrogenated materials
• Molecular solids
• Supercritical fluids
• Brillouin scattering & spectroscopy
• Pressure techniques
• Raman spectroscopy