The Structure Transformation Of Cerium-doped Perovskite Zircon Solidified Body-electron Transfer And Stability Study

Zirconolite(CaZrTi2O7)is composed of layered Ti-O polyhedra(TiO6 and TiO5)as(001)planes.Zirconolite possesses 2M,3T,30,4M,and 6T structures issued from different stacking layers.The zirconolite structure is equipped with multiple cation sites,and can accommodate lanthanides(Nd and Ce)and actinides(Pu,Cm,Am,and Np)without significant alteration in its basic structural frame.Continuous addition of rare earths,actinides,and uranium ions would transform zirconolite structure-type from 2M-zirconolite to 4M-zirconolite and cubic pyrochlore.During ceramic immobilization of HLWs,pyrochlore(A2B2O7)structure-type materials could efficiently accommodate large proportions of actinide ions.In pyrochlore structure,A(16d sites)and B(16c sites)ions might be occupied by trivalent and tetravalent actinide ions,respectively.Pyrochlore is a short-range order structure,which can only be stabilized by certain rations of cations radii.Outside this range,fluorite phase is produced with disordered structure,making it more stable.Except for A2B2O7 structure,pyrochlore-like phases are often considered durable crystalline hosts for weapon-usable Pu wastes.Pyrochlore-like phases containing CaMTi2O7 stoichiometry might be considered as distorted fluorite structure,where M represents tetravalent cations,such as Ce,U,and Pu.Pu-doped samples have high chemical toxicities and consuming radioactivities.Hence,they must be handled according to rigorous safety regulations and suitable equipment.For safety,Ce4+ can be used as an imitator of Pu4+due to similarities in valence and ionic radii between both cations.For these reasons,phase evolution of CaZr1-xCexTi2O7(0.0?x?1.0)from zirconolite-type(CaZrTi2O7)to pyrochlore-type(CaCeTi2O7)structures was investigated by X-ray diffraction(XRD)and scanning electron microscopy(SEM)equipped with energy dispersive spectroscopy(EDS).XPS was used to determine the cerium oxidation states(Ce3+ and Ce4+)and quantify their ratios.As well as,the method of the first principles and Monte Carlo were used for the studies of lattice occupation,electron transfer,mechanical properties and radiation resistance.This paper is mainly as follows:(1)Four different phases are identified,namely zirconolite,perovskite,pyrochlore,and cerianite.The 2M-zirconolite could accommodate small amounts of cerium ions(0.05 mol%)without phase separation,and the solubility limit of cerium ions in CaZr1-xCexTi2O7 system is estimated to approximately 0.80.The phase transformation(2M-zirconolite?4M-zirconolite?Ce-pyrochlore)is caused by cations rearrangement as cerium content increases.By adopting the conditions of cerium ions,perovskite(CaTiO3)phase could coexist in wide ranges of composition(x = 0.05 to 1.00)due to reduction of Ce4+ to Ce3+.As cerium ion content rises,the ratio of Ce3+ and Ce4+ and the density significantly decreases,and the value of vickers hardness shows a general trend of fluctuations.(2)The mass solid solubility of CeO2 in zirconolite is 22 wt%,which is much lower than the solid solubility of the formula designed according to stoichiometry.According to the national standard GB7023-86,Ce ion leaching rate was low.After immersed for 42 days,the normalized leaching rate was about 10-6.After the microwave transformation of the TRPO process,the processes of Am/Cm/rare earth element and the Np/Pu in the zirconolite have a solid content of 10 to 20 wt%,and the Np/Pu solidification effect is better.(3)The simulations of first principle shows that Ce4+ ions preferentially occupying the Ca sites and Zr sites.However,the Ce3+ ions only occupying the Ca sites.Because the substitution of Ce4+/Ce3+ ions in Ca sites,the transformation of electronic are from Ce ions to 0 ions.Only few of electronic are transferred due to the substitution of Ce4+ ions in Ca sites.The results of elastic constants suggest zirconolite and Ce-pyrochlore have good mechanical properties.(4)The results of Monte Carlo simulation show that during alpha particle incident with the energy of 10 MeV,around 277 vacancies can be generated at per angstrom thickness and the depth of damage is 40.02 ?m in zirconolite,and around 226 vacancies can be generated at per angstrom thickness and the depth of damage is 40.32?m in Ce-pyrochlore.During Kr+ particle incident with the energy of 10 MeV,around 28004 vacancies can be generated at per angstrom thickness and the depth of damage is 2.77 ?m in zirconolite,and around 24887 vacancies can be generated at per angstrom thickness and the depth of damage is 2.76 ?m in Ce-pyrochlore.These results suggest the radiation resistance of Ce-pyrochlore is better than zirconolite.