Four Si3N4 crystal structures were predicted using an ab initio evolutionary methodology. The mechanical and dynamic stabilities were confirmed by the density functional theory assuming zero-pressure conditions. Energetic stability calculations indicated that the structures are metastable phases at ambient pressure, but their formation is more favorable at high pressures. At zero pressure, the densities of the hp-Si3N4, cp-Si3N4, oc-Si3N4, and ti-Si3N4 phases were 3.21, 3.28, 3.70, and 3.24 g/cm3, respectively. The calculated band structures and densities of states indicated that they have semiconductive properties, with gaps ranging from 0.754 to 3.968 eV. Mechanical property calculations revealed that the hardness of the Si3N4 compounds ranged between 11.2 and 23.3 GPa, which were higher than the corresponding values for the synthesized Si3N4 phases. These four Si3N4 structures are potentially valuable candidates for the synthesis of Si3N4 compounds.

In this paper, a N-rich B–N polymorph named as B20N24 is proposed through first-principles calculations. The stability of the B20N24 polymorph at ambient conditions is confirmed using the phonon dispersion spectra and the Born stability criteria. Electronic properties calculations show that B20N24 exhibits a semiconducting feature, with a 0.87 eV direct band gap derived from HSE06 functions, which is much lower than many other B–N polymorphs. Specifically, owing to its cage-like framework, B20N24 may be used in hydrogen storage at a capacity of ~6.8 wt.%. The B20N24 polymorph enriches the B–N system theoretically, and this polymorph is promising for use in electronic devices and hydrogen storage.

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