Melting and dissociation of ammonia at high pressure and high temperature

Raman spectroscopy and synchrotron x-ray diffraction measurements of ammonia (NH ₃) in laser-heated diamond anvil cells, at pressures up to 60 GPa and temperatures up to 2500 K, reveal that the melting line exhibits a maximum near 37 GPa and intermolecular proton fluctuations substantially increase in the fluid with pressure. We find that NH ₃ is chemically unstable at high pressures, partially dissociating into N ₂ and H ₂. Ab initio calculations performed in this work show that this process is thermodynamically driven. The chemical reactivity dramatically increases at high temperature (in the fluid phase at T > 1700 K) almost independent of pressure. Quenched from these high temperature conditions, NH ₃ exhibits structural differences from known solid phases. We argue that chemical reactivity of NH ₃ competes with the theoretically predicted dynamic dissociation and ionization.