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

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 gamma-Si3N4 at pressures below 2.5 GPa and 2.9 GPa, respectively, which promise that the formers could be obtained by quenching from gamma-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 gamma-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 beta-Si3N4 and gamma-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. (C) 2015 Elsevier Inc. All rights reserved.