First-principles investigation of the structural, electronic and optical properties of olivine-Si₃N₄ and olivine-Ge ₃N₄

Si₃N₄ and Ge₃N₄ are important structural ceramics with many applications because of their outstanding high-temperature and oxidation-resistant properties. Two stable phases of them, α and β, have thus far been synthesized. The high-pressure and high-temperature spinel phases of these two materials were noticed to have wide, direct electronic band gaps that are comparable to those of the promising newly developed solid-state optoelectronic materials such as lasers and LEDs. Another high-pressure and high-temperature phase, the olivine phase, has also attracted attention recently. In the present work, the structural and electronic properties of the new olivine nitrides Si₃N₄ and Ge ₃N₄ are studied by the FLAPW method with PBE-GGA exchange and correlation potential. The stability of the two materials and the transition pressure are investigated. It is found that olivine-Ge₃N₄ is not stable and is difficult to be observed, while olivine-Si ₃N₄ can be synthesized under appropriate conditions. The atomic sites have been optimized and the ground-state properties such as equilibrium lattice constant, bulk modulus, band structure and density of states have been obtained. Furthermore, the dielectric function has been calculated based on the random phase approximation.