Bi_xBa_(1-x)TiO_3电子及能带结构的第一性原理研究

采用基于第一性原理的赝势平面波方法,研究了ABO_3钙钛矿复合氧化物BaTiO_3中A位离子被Bi原子取代后对其构型、电子及能带结构的影响.计算结果表明,Bi取代Ba之后会降低BaTiO_3的对称性,空间点群随着取代量的变化而变化,结合能逐渐降低.通过能带结构的计算发现Bi_xBa_(1-x)TiO_3为直接带隙型半导体.Bi的取代可调节Bi_xBa_(1-x)TiO_3的禁带宽度,从x=0.125到x=0.625时,Bi的取代量越大,其带隙越宽,吸收光谱蓝移.x0.625时,禁带宽度又逐渐减小,吸收光谱红移.由态密度图可看出,其价带顶主要是O-2p与Bi-6s态杂化而成,导带底主要由Ti-3d态构成.

First principle study of electron and band structure of BixBa1-xTiO3

Some perovskite structured catalysts have narrower forbidden band widths than pure TiO₂, and they have been widely used in a number of photo-catalytic reactions. The ions in the perovskite may be replaced by other ions while maintaining the structure unchanged for its tailorable character. Bi–Ti–O can form into the typical perovskite composite oxide BiTiO₃ under specific preparation conditions. The regulation of the energy gap of the perovskite BaTiO₃ can be realized by substituting Bi for Ba to form the Bi_xBa_1-xTiO₃ perovskite structure to improve its photo-catalytic activity. But the improvement mechanism and the electron and band structures of Bi_xBa_1-xTiO₃ are still not very clear. In this study, we exhibit a detailed theoretical investigation to predict the electronic structure, band gap and optical absorption properties of Bi_xBa_1-xTiO₃ structures based on the first-principles plane-wave ultrasoft pseudopotential method. The exchange and correlation interactions are modeled using the generalized gradient approximation and the Perdew-Burke-Ernzerhof exchange-correlation functional. The cutoff kinetic energy of the electron wave function is 340 eV, and the k-point sampling sets 3×3×3 division of the reciprocal unit cell based on the Monkhorst-Pack scheme. In the geometrical optimization, all forces on atoms are converged into less than 1×10^-5 eV/atom, the maximum ionic displacement is within 0.001 Å and the total stress tensor decreases to the order of 0.05 GPa. The DFT calculation results reveal that the symmetry and binding energy decline in the Bi_xBa_1-xTiO₃ structure, and the bond lengths of Ba–O and Ti–O decrease a little after Ba has been substituted by Bi atom, except for the structure of Bi_0.5Ba_0.5TiO₃. The photo-catalysts of Bi_xBa_1-xTiO₃ are direct band gap semiconductors, and the substitution Bi can regulate the band gaps of Bi_xBa_1-xTiO₃. The band gaps become wider from x=0.125 to x=0.750 with the carrier concentration decreasing, and then decreases with the higher carrier concentration increasing when x=0.875. It is predicted that the band width of Bi-based perovskite will be much lower than that of Ba-based perovskite. In the case of the density of states we reveal that the top of the valence band is hybrided by O-2p and Bi-6s and the bottom of the conduction band state is mainly constituted by the Ti-3d state. The electron transport properties and carrier types are mainly determined by Ti-3d, O-2p state and Ba-5p electronic states in BaTiO₃ and Ti-3d, O-2p, Bi-6s and Bi-6p electronic states in Bi_xBa_1-xTiO₃ respectively. The absorption spectra indicate that the ultraviolet absorption performance can be improved in Bi_xBa_1-xTiO₃ system, which may effectively improve the photo-catalytic activity of BaTiO₃.