| With the rapid development of global nuclear energy,a large amount of highly radioactive waste(also known as High-Level Nuclear Waste, referred HLW) has been produced. The HLW contain highly radioactive nuclide, such as actinides(237Np,238 U,239Pu,241 Am,243Am,244Cm), fission nuclide(129I,90 Sr,137Cs,99Tc). The radioactive waste will do a great harm to the environment if it has been improper disposed. The deep geological disposal is considered as the most effective and reasonable way to dispose the high-level radioactive wastes. It is the priority for all to choose appropriate immobilization matrixes to solidify actinides which are separated from high level radioactive waste.Pyrochlore with the general formula A2B2O6O′,the larger cation(A) generally is lanthanide or actinide with the valence of trivalence or bivalence,the smaller cation(B) is the second sub-group ions or other lanthanide ion with valence tetravalence or pentavalence. Pyrochlore has been extensively studied as one of the most ideal candidate immobilization matrixes of the deep geological disposal due to its good durability properties, irradiation properties, physical properties and chemical properties. Pu, U and Ce share many common chemical and physical properties. So Ce is often used as a nonradioactive surrogate for Pu, U in many researches. There are a number of reports about the solubility of Ce4+ in the pyrochlore, it show that the solubility of Ce4+ in pyrochlore is low. This consequence can be explained by the big difference between cations radii of Ce4+(0.97?) and Ti4+(0.606?). The Ce4+ will occupy the B position of the ceramic instead of Ti4+ when doped in pyrochlore, but as the lanthanide cation, the Ce4+ has significant difference with the transition cation Ti4+ on the cations radius, physical and chemical properties. In this study,we focus on the compounds that Ce3+-doped in the A-site(16d) of Gd2Ti2O7 pyrochlore. Solution combustion followed by high-temperature calcining was used to obtain the(Gd2-xCex) Ti2O7 oxides. The results of this study were shown below:1. As a substitution for Gd, 39.62 mol% of Ce3+ is doped in the A-site(16d) of(Gd2-xCex)Ti2O7 with x=0.8.2. The lattice parameter increases almost linearly from 10.166 ? for Gd2Ti2O7 to 10.2297 ? for Gd1.2Ce0.8Ti2O7 with the increasing of the amount of doped Ce. The O48 f positional parameter, xO48 f, generally decreases as the amount of doped Ce increases.3. The calculation results based on density functional theory(DFT) are quite consistent with the experimental result. The lengths of <Ce–O48f> and <Gd–O48f> bonds increases approximately linearly with the replacement of Ce for Gd, whereas the lengths of <Ce–O8b> and <Gd–O8b> gradually decreases with increase of Ce content. However, the change of <Ce–O48f> and <Gd–O48f> bonds are appreciably larger than the change of <Ce–O8b>, <Gd–O8b> and <Ti–O48f> bonds. It is mainly because of the shift of the O48 f and the degree of order of pyrohlore structure increase as the Ce content increasing.4. The Gd2Ti2O7 still maintain pyrochlore structure under high-pressure until 46 GPa. The pyrochlore structure transforms to defect-fluorite initiated 15.7 GPa for Gd1.8Ce0.2Ti2O7, and complete transformation to defect-fluorite under 38.5GPa. The monoclinic structure has been observed to form at pressure in excess of 31.5GPa for Gd1.5Ce0.5Ti2O7, and transform to amorphous under 41.8GPa. |
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