First Principles Study On The Bulk Modulus And The Solubility Of Cerium In Lanthanide Titanate Pyrochlores

The excessive exploitation and utilization of fossil fuels leads to depleted resources and increased environmental pollution. So it is one of worldwide concerned problem to search and develop cleaning, effective new energy. Due to the nuclear energy emergence, human can see an opportunity again in the energy field. Nevertheless, amounts of nuclear waste material brings people great challenges. Now, one of the effective ways to solve the problem is the deep landfill of the immobilizing radioactive waste. The choice of immobilization matrix is vital important. Pyrochlores are the one of the most ideal candidate materials for next generation immobilization matrix due to its high radiation resistance, chemical stability and mechanical stability.It is an important research method to investigate the performance of the immobilization matrix by incorporating actinides into pyrochlores. As Pu is not easily studied experimentally, and Pu and Ce share many common chemical and crystallochemical properties, Ce has often been used as a nonradioactive surrogate for Pu. In this paper, density functional theory plus Hubbard U correction(DFT+U) has been applied to investigate the solubility of Ce in La2Ti2O7 and Gd2Ti2O7. The geometrical structure of La2-yCeyTi2O7 and Gd2-yCeyTi2O7 have been analyzed. It is found that there is a slight increase in xO48 f and decrease in lattice constant with increasing cerium content in La2Ti2O7, however, the opposite observations occur in Gd2Ti2O7, which suggests that cerium incorporation may lead to increased radiation resistance in La2Ti2O7 but decreased radiation resistance in Gd2Ti2O7. To investigate Ce solubility in these two pyrochlores, the defect formation energy, incorporation energy and solution energy are calculated. The result shows that cerium is soluble in La2Ti2O7 and Gd2Ti2O7. In this paper, the Bader charge for each ion in the solid solution was calculated to verify the charge state of Ce. The results suggests that cerium in the solid solution exhibits a reduced charge state.The mechanical stability of materials is an important aspect of performance evaluation of immobilization matrix. Thus, in this paper, the bulk modulus of the rare-earth titanate pyrochlore have been investigated by the density functional theory. It is interesting to note that the “bimodal effect” have been observed in Yb2Ti2O7 and Eu2Ti2O7. The bonds of <RE-O48f>(RE = La, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) in RE2Ti2O7 pyrochlores may play a dominant role in determining the value of the bulk modulus.