The chemical potential change of nitrogen at high pressure/ high temperature is a crucial ingredient for predicting the formation of nitrogen-rich compounds. Here, we provide intelligible data for the chemical potential change 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 show the validity of our approach for three characteristic systems: pure nitrogen and nitrogen-rich Si−N and Ti−N phases appearing at high pressure/high temperature.
We investigate the pressure−temperature phase diagram of the tantalum− nitrogen system through a combination of density functional theory computations and thermodynamic calculations. Accounting for the chemical potential of nitrogen at high pressure and high temperature, we obtain a Gibbs energy surface for every Ta−N phase in the pressure−temperature space. Combining the data yields a coherent stability map that identifies the one most favorable Ta−N structure at given p, T conditions in the presence of excess nitrogen. Slices through the phase diagram at constant pressure or constant temperature are used to examine the manifold of competing structures and locate optimum conditions with maximum driving force for successful syntheses. We rationalize high-pressure experiments that synthesized new Ta−N polymorphs and predict the temperature and pressure conditions necessary to synthesize Ta 2 N 2 (N 2 ) and Ta 2 N 8 .
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