Computing the Tantalum-Nitrogen Phase Diagram at High Pressure and 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 Ta2N2(N-2) and Ta2N8.