The effect of oxygen vacancy on structures and optical properties of the lead-free Ba0.5Sr0.5TiO3 by first-principles calculation

The structural and optical properties of lead-free Barium strontium titanate Ba0.5Sr0.5TiO3 with oxygen vacancies (BST-O) were investigated by first-principles calculations of density functional theory. The results show that the O vacancy has significant influence on the electronic structure and optical performance of the materials. The structures generate a phase transition from cubic to tetragonal phase, when an O atom has removed from the pure Ba0.5Sr0.5TiO3 (BST). The electron density conformation is distributed, which induce conspicuous distortion of the oxygen octahedron. The calculated optical properties, complex dielectric function, refractive index, absorption coefficient, show that the absorption spectra of the BST-O have multimodality and smaller amplitude by comparing with ideal BST. And the oxygen vacancy lead the increase of refractive index and the decrease of absorption coefficient in visible light frequency region (1.59~3.11 eV), the optical anisotropism is also found in the BST-O. Introduction Barium strontium titanate is one of the well known perovskite materials with its excellent ferroelectric and dielectric performance, which is considered as a candidate to replace lead based materials due to the environment issue. Its desirable high dielectric constant, low dielectric losses, strong remnant polarization and low current leakage has been attractive for its fundamental research, and the material could be used for various applications such as dynamic random, access memory, ceramic capacitors, pyroelectric sensors, infrared detectors, chemical sensor, biosensor, microwave devices and optoelectronic applications in recent years . With the intensive study on BST material, a large quantity of tentative exploration on ceramic, film, composite and doping vario-property of BST have been completed . However, most of the experiments of BST focused on the properties in electrical and microwave frequency . The optical response of BST is not very clear. The first principle method is an effective and efficient method for material research, which is applied to the study of perovskite type materials in the last decades . Many typical perovskite materials were studied by this method, such as tantalum niobate (KTN), strontium titanate (SrTiO3) and lead zirconate titanate (PZT) , which has succeed in predicting electronic properties, band structure and optical properties of perovskite materials. The related properties of ideal BST are also investigated though first principle method, but defect state of BST is scarcely researched [12, . As is well known, the oxygen defects are unavoidable during experimental process, F. M. Pontes et al verified that the postannealing temperature and oxygen atmosphere have important influence on the dielectric properties of BST . In this paper, the possible defects in the process of the experiment were considered. The two models of the ideal bulk of BST and BST with O vacancy were designed and the effect of oxygen vacancy on structures and optical properties of the lead-free Ba0.5Sr0.5TiO3 were investigated by first-principles in detail. 5th International Conference on Information Engineering for Mechanics and Materials (ICIMM 2015) © 2015. The authors Published by Atlantis Press 830 Calculation details The structures and optical properties of the lead-free oxygen vacancy Ba0.5Sr0.5TiO3 were calculated by a widely used plane-wave pseudopotential total energy program CASTEP [15] in the framework DFT. The local density approximation (LDA-CAPZ) is used to describe the electronic-correlation function, which have be seen as a typical and accurate mode for the perovskite materials [16, . The Broyden–Fletcher–Goldfarb–Shannon (BFGS) algorithm and Vanderbilt-type ultrasoft pseudopotential (USP) were utilized for geometry optimization. In geometry optimization and optical properties computing, the energy cutoff was set as 380 eV and a 3×3×3 k-point Monkhorst-Park mesh in the Brillouin zone was used. All atoms were not fixed and were relaxed until the total energy variation of each atom was below 5.0×10 eV/atom and the displacement of each atom was below 5.0×10 A. Ba (5s2, 5p6, 6s2), Sr (4s2, 4p6, 5s2), Ti (3s2, 3p6, 3d2, 4s2) and O (2s2, 2p4) were considered as valence electrons configurations. The number of k-point and the cut-off energy were tested by the plane wave energy convergence. The cubic phase Ba0.5Sr0.5TiO3 is a structure with the space group of PM-3M. The experimental lattice parameter of BST is a=b=c=3.930 A , so empirical value 4.0 A is chosen as a initial lattice length. A 2×2×2 supercell of pure BST and a 2×2×2 supercell of BST with oxygen vacancy were designed as computational models, respectively, which were showed in Fig. 1. The calculations of electronic structure and optical properties were based on the previously optimized models. Fig. 1 Supercells of pure BST and BST with oxygen vacancy Geometry and electrons structures of the BST-O The structure properties of materials are very important and fundamental for practical applications. In our calculations, the stable ground states of pure BST and BST-O were found after geometry optimization. Computational structure parameters are presented in Table 1. The lattice parameters of pure BST are a=b=c=7.8036 A, α=β=γ=90° and the cell volume is 475.217 A, which is consistent with the experimental data and testifies the structure is classified as cubic phase, PM-3M group . In contrast, when an O atom point defect is introduced into the system, the lattice parameters are a=7.8202 A, b=7.8107 A, c=7.8101 A and the volume of BST-O is 477.053 A. The whole calculated lattice parameters almost approach to experimental values (error less than 1%). From these data, the phase structure of BST-O belong to tetragonal phase which is different from that of BST. The BST-O’ s cell volume is slight larger than that of pure BST, which suggests that O defect lead to the structure distortion of BST and this result is also in accordance with the previous experiment . Surprisingly, the comparison of bulk modulus for pure BST and BST-O calculated by LDA indicates the pure BST’ s bulk modulus is much larger than the BST-O’ s, from the Table 1, the difference between two models’ bulk modulus is 155.49798 GPa, therefore it can be inferred that O vacancy play an important role on mechanical character of BST.