| Pyrochlore oxides have a general formula A2B2O7, where A=Y or lanthanide or actinide elements, and B=transition metal elements. Consequently, pyrochlores have remarkable properties and applications in superconductivity, thermal barrier coatings, luminescence, ferromagnetism. Besides, pyrochlores have been proposed as important immobilization hosts for high-level radioactive waste. According to previous ion irradiation and chemical leaching studies, Gd2Zr2O7and La2Zr2O7pyrochlores exhibit good resistance to irradiation and chemical durability, while there is a lack of theoretical investigation of actinides solubility in pyrochlores. On the other hand, it is controversial on the valence states of actinide cations in pyrochlores due to the difference in experimental conditions. Thus, it is important to study the electronic and energetic properties of actinides incorporation in pyrochlores. Thanks to the radioactivity of actinides, it is difficult to study them in experiments. Based on the similarities on thermodynamic properties between Ce and Pu, we chose Ce as a surrogate for Pu.With the improvement of computer abilities and calculation methods, ab initio calculation based on density function theory makes progress tremendously. Nowadays, this method is applied extensively in material science, biology technology and molecular chemistry. It becomes an effective tool to calculate the electronic and energetic properties and a bridge which link the theoretical and experimental research. In this paper, all the calculations were performed by density function theory plus Hubbard U correction (DFT+U) method. Using generalized gradient approximation (GGA), we studied the solubility, electronic feature and structural properties of Ce incorporation in Gd2Zr207and La2Zr207pyrochlores. The main researches are as follows:1. We have investigated the structural and electronic properties as well as Ce solubility in Gd2-yCeyZr2O7(y=0,0.5,1.0,1.5,2.0) compositions. We firstly calculated the ground state properties of Gd2Zr2O7and Ce2Zr2O7, such as lattice parameters, elastic properties and band gap values. Our calculations are in good agreement with experimental and theoretical results, meaning that our method can reproduce the ground state properties of these pyrochlores. Based on the optimal structure, we studied Ce solution behaviors in different contents. Our calculations showed that Gd2Zr2O7and Ce2Zr2O7form a solid solution over the entire range of cerium content. Furthermore, we found that Gd2Zr2O7with Ce dopant at the Gd site is less radiation resistant than Gd2Zr2O7.2. We have studied the structural and electronic properties and solution behaviors of Ce in La2Zr2O7pyrochlore.To obtain a suitable Ce Ueff value, we first determined the relationship between Ueff and the ground state properties of Ce2Zr2O7, including lattice constants, Xo48f parameter, band gap values. We found that the ground state properties of Ce2Zr2O7can be reproduced at Ueff=5.0eV and the Ce4f electrons are completely localized. The La2Zr2O7-Ce2Zr2O7solid solution exists over the entire range of Ce content and can form stable compositions. We also found that Ce incorporation in La2Zr2O7can increase the radiation resistance, which is consistent with previous experimental measurements by Lian et al. We also found that Ce prefers to occupy the A site rather than B site and exhibits a trivalent state, agreeing well with the previous molecular dynamics simulation results.In conculsion, we analyzed the charge states of Ce, the relationship between Xo48f and Ce content, Ce solubility in La2Zr2O7and Gd2Zr2O7and predicted their response behavior to irradiation. It turns out that that the Ce solubility in pyrochlore depends on the host. We also suggested that La2Zr2O7may be a more favorable material for immobilizing actinide wastes than Gd2Zr2O7. |
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