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We used the first - principles method based on density functional theory to calculate the crystal structures, electronic structures, optical properties and phonon spectra of α - Si3N4 and β - Si3N4• The obtained results have been theoretically analyzed. The calculated results show that both phases contain strong covalent bonds and are insulators. The band gap of α - Si3N4 is slightly larger than that of β - Si3N4• The calculated optical absorption coefficients show that α - Si3N4 and β - Si3N4 mainly absorb ultraviolet light, and their energy loss peaks are around 24. 4 eV. In the phonon spectra, α - Si3N4 and β - Si3N4 have no imaginary frequency, indicating that they are stable. In the range of 0 - 1000 K, the heat capacity of α - Si3N4 is about twice that of β - Si3N4 . The calculated results in this paper will provide a theoretical basis for the related applications of Si3N4. [ABSTRACT FROM AUTHOR]

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According to the experimental synthesis of ZnTiO3 ferroelectric compound with LiNbO3 ( LN) structure, the characteristics of the LN-ZnTiS3 compound were designed and studied based on the first-principles method. The calculated results show that the LN-ZnTiS3 compound meets the conditions of mechanical stability. According to the analysis of the chemical potential equilibrium phase diagram, LN-ZnTiS3 will not form a stable structure under normal pressure, but can form a stable structure under external pressure. The calculated density of states and energy band indicate that the valence band maximum (VBM) of LN-ZnTiS3 is mainly composed of S-p orbitals, while the conduction band minimum (CBM) is composed of Ti-d orbitals. The substitution of sulfur atoms can promote the electron state above Fermi energy level of the system to drop significantly to the lower energy level, thus reducing the band gap of LN-ZnTiS3 . The calculated band gap of LN-ZnTiS3 is 1. 04 eV, which can promote the absorption of visible light. It shows that LN-ZnTiS3 is a potential high-efficiency photovoltaic material. [ABSTRACT FROM AUTHOR]