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

Abstract

A solid state method was used to synthesize La(1-x)Pu(x)PO(4) (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)PO(4)-monazite and a cubic phase (PuO(2)). Pure-phase La(1-x)Pu(x)PO(4)-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-L(III) edges confirmed the +III valence state of plutonium in the monazite solid solutions. The local environment of Pu is PuPO(4)-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.