Metallization of Shock-Compressed Liquid Ammonia

TL;DR

This study explores the behavior of liquid ammonia under extreme pressures and temperatures, revealing a transition to a conductive plasma state and providing data relevant to planetary interior models.

Contribution

It presents the first experimental evidence of electronic conduction in high-pressure ammonia and extends the phase diagram to unprecedented conditions up to 350 GPa and 40000 K.

Findings

Ammonia transitions from molecular liquid to plasma around 90 GPa and 7000 K.

Reflectivity measurements indicate increasing electrical conductivity with pressure.

Conductivity in ammonia exceeds that of water at similar pressures.

Abstract

Ammonia is predicted to be one of the major components in the depths of the ice giant planets Uranus and Neptune. Their dynamics, evolution, and interior structure are insufficiently understood and models rely imperatively on data for equation of state and transport properties. Despite its great significance, the experimentally accessed region of the ammonia phase diagram today is still very limited in pressure and temperature. Here we push the probed regime to unprecedented conditions, up to $\sim$350 GPa and $\sim$40000 K. Along the Hugoniot, the temperature measured as a function of pressure shows a subtle change in slope at $\sim$7000 K and $\sim$90 GPa, in agreement with ab initio simulations we have performed. This feature coincides with the gradual transition from a molecular liquid to a plasma state. Additionally, we performed reflectivity measurements, providing the first experimental evidence of electronic conduction in high-pressure ammonia. Shock reflectance continuously rises with pressure above 50 GPa and reaches saturation values above 120 GPa. Corresponding electrical conductivity values are up to 1 order of magnitude higher than in water in the 100 GPa regime, with possible significant contributions of the predicted ammonia-rich layers to the generation of magnetic dynamos in ice giant interiors.