Structural investigations of (La,Pu)PO4 monazite solid solutions: XRD and XAFS study

A fundamental understanding of actinide incorporation processes in envisioned nuclear waste forms, such as monazite ceramics, is required for a reliable prediction of the long-term stability of such ceramic materials for safe nuclear disposal. The present study provides structural insights into the formation of monazite solid solutions by incorporation of PuIII and verifies previous results on surrogate materials, where Eu and Gd served as inactive analogues for trivalent actinides.
A solid state method was used to synthesize La1-xPuxPO4 (x = 0.01, 0.05, 0.10, 0.15, 0.5) solid solutions with monazite structure. XRD measurements of the compounds with x = 0.50 revealed the formation of two phases: (La,Pu)PO4-monazite and a cubic phase (PuO2). Pure-phase La1-xPuxPO4-monazite solid solutions were obtained for materials with x = 0.00-0.15 and confirmed by a linear dependence of the lattice parameters on composition according to Vegard’s law. X-ray absorption spectroscopy (XAS) analysis at the Pu-LIII and La-LIII edges verified the +III valence state of plutonium in the monazite solid solutions. The local environment of Pu is similar as in PuPO4-like along the solid solution series, except for the longest fitted cation-cation distance, which may be an indication of cluster formation consisting of a few Pu-atoms in the La-Pu-monazite lattice.