A Study Of Electron Radiation Responses Of Zirconate Pyrochlores:an Ab Initio Molecular Dynamics Study

Nuclear energy,as a new type of energy with its high efficiency,safety,cleanliness,and sustainable development,has recived extensive attention from all over the world.The widespread application of nuclear energy is also accompanied by the generation of nuclear waste,which restricts the sustainable and healthy development of nuclear energy.The problem of how to treat nuclear waste safely and effectively has become extremely important.Zirconate pyrochlores,due to high thermal stability,high chemical durability,and remarkable resistance to radiation-induced amorphization,have been proposed as excellent candidates for immobilizing nuclear waste.As the product for immobilization of Pu in zirconate pyrochlores,how the Pu₂Zr₂O₇ pyrochlore responds to electronic excitation still remains unclear thus far.In order to safely dispose of nuclear waste,it is necessary to understand the structural and electronic properties of zirconte pyrochlores and their microstructural evolution under electronic excitation.The main work of this paper is as follows:1.We have carried out density functional theory plus Hubbard U（DFT+U）method to study the geometric structure,band structure and electronic properties of zirconate pyrochlores（A₂Zr₂O₇,A=Pu,La and Gd）.It reveals that the lattice constants,oxygen position parameters x _O48f and bond lengths of zirconate pyrochlores are all in good agreement with the experimental and theoretical values.It is noted that the x _O48f value for zirconate pyrochlores are different,suggesting that zirconate pyrochlores may have different responses to ion-irradiation.It is also found that all the zirconate pyrochlores are of direct band gaps.The band gap of Pu₂Zr₂O₇ is determined to be 2.12 e V,which is smaller than that of La₂Zr₂O₇ and Gd₂Zr₂O₇,suggesting that the valence electrons in Pu₂Zr₂O₇are more easily to be excited to the conduction bands if enough energy is provided.We have also analyzed the density of state distribution of the zirconate pyrochlores.The different electronic structures may result in different responses to electronic excitation.2.The response of zirconate pyrochlores（A₂Zr₂O₇,A=La,Pu,Gd）to electronic radiation are simulated by an ab initio molecular dynamics method.It is shown that the Pu₂Zr₂O₇ undergoes a crystalline-to-amorphous structural transition with 0.3%electronic excitation,while for La₂Zr₂O₇ and Gd₂Zr₂O₇ the structural amorphization occurs with1.6%electronic excitation.During the microstructural evolution,the anion disorder further drives cation disorder and eventually results in structural amorphization of zirconate pyrochlores.The difference in responses to electron radiation among zirconate pyrochlores mainly results from the strong correlation effects between Pu 5f electrons and the smaller band gap of Pu₂Zr₂O₇.These results suggest that Pu₂Zr₂O₇ is less resistant to amorphization under local ionization rates that produce low level of electronic excitation,since the level of the concentration of excited electrons is relatively low in Pu₂Zr₂O₇.In summary,the structural and electronic properties of zirconte pyrochlores and their microstructural evolution under electronic excitation are systemically investigated.It is shown that the Pu₂Zr₂O₇ pyrochlore is less resistant to structural amorphization under electronic excitation than La₂Zr₂O₇ and Gd₂Zr₂O₇.The presented results may advance the fundamental understanding of the radiation response of zirconate pyrochlores to electronic excitation and provide important information for related research on immobilization of nuclear wastes by zirconate pyrochlores.