Prediction of a reservoir of N-rich high-energy density material at the Earth's mantle.

Water and nitrogen are abundant in the Earth's interior and atmosphere, and their mixture under high pressure is a fundamental and valuable scientific issue for physics and chemical science. Based on structural prediction and first-principles simulations, we propose that the two nitrogen-rich stoichiometries, H 2 ON 6 and H 2 ON 10 , become energetically stable above ∼70 GPa. Further ab initio molecular dynamics calculations indicate that H 2 ON 6 and H 2 ON 10 undergo phase transitions from solid to superionic and finally to fluid phase with increasing temperature. The superionic regions in their phase diagram correspond to the extreme conditions of Earth's mantle, implying that the Earth's interior might be a possible reservoir of the N-rich hydrates. In addition, H 2 ON 6 remains dynamically stable under ambient conditions with an estimated energy density of 6.53 kJ g -1 , indicating that it might be a high-energy density material. These results not only provide essential information for the understanding of the Earth's interior, but also provide guidance for the design of high-energy density materials.