Local Structure and Magnetism of La1-x,MxPO4 (M = Sm, (PU)-P-239, Am-241) Explained by Experimental and Computational Analyses

With their high chemical and self-irradiation stability, crystalline monazites are among the most promising materials for the encapsulation of nuclear wastes. Yet, the local and magnetic structures of the matrices doped with low-content actinide cation, depicted as most resistant, are still unclear. This limits the development of theoretical approaches predicting their behavior under extreme conditions-self-irradiation and long-term leaching. Here, we characterize the model matrices La1-x,MxPO4(0 <= x <= 0.10)-with M = Sm, Pu-239, Am-241-by X-ray diffraction and solid-state P-31 NMR. As an example, we confirm that (La0.96Am0.04PO4)-Am-2-Am-41 has higher self-irradiation resistance compared to (AmPO4)-Am-2-Am-41. Further, computational analyses show that magnetic properties of the Pu complex are strongly affected by the J-mixing and the paramagnetic NMR shifts are dominated by the Fermi contact contribution, arising from delocalization of the spin density of the cation toward the phosphorus through the bonds.