Chemical Potential of Nitrogen at High Pressure and High Temperature: Application to Phase Diagram Calculations of Nitrogen-Rich Compounds

High-pressure research of nitrogen-rich compounds is driven by a quest for hard materials and compounds with high energy density, but also has implications for planetary science. To predict the formation of nitrogen-rich compounds at high pressure and high temperature, knowledge of the chemical potential of nitrogen at extreme conditions is an indispensible ingredient. Here, we provide models together with intelligible data for the chemical potential of molecular nitrogen at temperature and pressure conditions relevant for experiments in the diamond anvil cell. In combination with first-principles calculations, we derive pressure−temperature phase diagrams readily accessible to guide experimental efforts. We demonstrate the validity of our approach for characteristic systems, including pure nitrogen and nitrogen-rich metal nitrogen phases that have already been discovered at pressures above 50 GPa. We then provide predictions for new metal-nitrogen compounds awaiting their synthesis at high pressure and high temperature.