The structure properties, defect stability and excess properties in Am-doped LnPO4 (Ln = La, Ce, Nd, Sm, Eu, Gd) monazites

Abstract

Understanding how minor actinides affect the synroc stability is very important for waste management. After a careful review of literature, however, existing reports did not reach a satisfactory level on the effect of minor actinides on monazite structural properties. In this study, the electronic properties, structures, defect formation energies and excess properties of Am-doped LnPO4 (Ln = La, Ce, Nd, Sm, Eu, Gd) monazites were investigated by first principles calculations. The calculation results indicate that the inner shell Am-5f orbital can participate in chemical bonding with O-2p orbital in all considered monazites. By analyzing their structures, we find the lengths of Am–O bond in Ln-monazite from La to Gd are gradually shortened, resulting in the shrinkage of [AmO9] polyhedron along the lanthanides, which is different from the evolution of their lattice parameters. This shrinkage plays an important role in the Am defect stability. For instance, the calculated vacancy defect formation energy reveals that the Am has a stronger binding energy in Gd-monazite, making it more difficult to form a vacancy defect. Moreover, after analyzing the thermodynamic excess properties, we find that Am-doped NdPO4 monazite has the highest thermodynamic stability, and all the Ln1-xAmxPO4 solid solutions can remain phase stable above 285 K.