Prediction of novel hard phases of Si3N4: First-principles calculations

• We explored three metastable phases of Si3N4 — t-Si3N4, m-Si3N4, and o-Si3N4.
• The enthalpies of t and m- are much lower than that of γ at ambient pressure.
• ois one further high pressure phase than γ.
• o-Si3N4 is the most hardest phase in Si3N4 system.
• Increasing pressure will change them from brittle to ductile state.

Exploration of novel hard metastable phases of silicon nitride was performed using a recently developed particle-swarm optimization method within the CALYPSO software package. Three potential hard metastable phases of t-Si3N4, m-Si3N4, and o-Si3N4 were predicted. These phases are mechanically and dynamically stable at ambient pressure based on their elastic constants and phonon dispersions. t-Si3N4 and m-Si3N4 exhibit lower energies than γ-Si3N4 at pressures below 2.5 GPa and 2.9 GPa, respectively, which promise that the formers could be obtained by quenching from γ-Si3N4. o-Si3N4 is a better high-pressure metastable phase than CaTi2O4-type Si3N4 proposed by Tatsumi et al. and it can come from the transition of γ-Si3N4 under 198 GPa. The theoretical band gaps of t-Si3N4, m-Si3N4, and o-Si3N4 at ambient pressure were 3.15 eV, 3.90 eV, and 3.36 eV, respectively. At ambient pressure, the Vickers hardness values of t-Si3N4 (32.6 GPa), m-Si3N4 (31.5 GPa), and o-Si3N4 (36.1 GPa) are comparable to β-Si3N4 and γ-Si3N4. With the pressure increasing, t-Si3N4, m-Si3N4, and o-Si3N4 will change from the brittle to ductile state at about 15.7 GPa, 7.3 GPa and 28.9 GPa, respectively.

This figure shows the crystal structures of three Si3N4 predicted in this manuscript, and left to right: t-Si3N4, m-Si3N4 and o-Si3N4.Figure optionsDownload as PowerPoint slide