Exploration of crystal structures and phase transitions in Hf3N4
Using the adaptive genetic algorithm (AGA) crystal structure prediction method, we explored the stable/meta-stable phases of Hf3N4. A new ground-stage phase (C2/m) of Hf3N4 at ambient pressure was predicted. Calculated phonon dispersion and elastic constants of this phase showed it to be dynamically and mechanically stable. We also produced a pressure–temperature phase diagram of Hf3N4 that included the C2/m, orthorhombic Zr3N4-type (Pnam) and I3d phases. Our results revealed that increasing the pressure and temperature would transform the C2/m phase of Hf3N4 to the I3d and Pnam phases, respectively. Although the orthorhombic Zr3N4-type phase was indicated to be metastable over all pressures, the contributions from its larger vibrational entropies than for C2/m indicated it to be stable only between temperatures of ∼414 and ∼686 K at low pressure. The I3d structure was calculated to be stable at a wide range of temperatures at high pressure and at a wide range of pressures at high temperature, with this result being in good agreement with the experimental results.