Spectroscopic studies on monazite-type ceramics for the conditioning of radioactive waste: Infrared, Raman, X-ray Absorption and Site-Selective Time Resolved Laser Fluorescence Spectroscopy

Monazite ceramics are being considered as potential waste forms for immobilization of minor actinides since they exhibit advantageous properties such as high chemical durability and radiation tolerance.
The overall objective of our study is to reveal the mechanisms of solid solution formation as well as the incorporation of the actinides into the crystal structure of the waste matrix. A fundamental understanding of these mechanisms is of great importance with regard to the long-term stability of monazites for safe nuclear waste disposal.
(La,Eu)PO4 and due to quenching effects of high Eu-contents in TRLFS measurements Eu doped (La,Gd)PO4 monazite solid solutions were synthesized by wet chemical methods. Eu serves as surrogate for trivalent actinides. Samples were characterized by XRD, Raman, IR, EXAFS and TRLFS spectroscopies. Structural refinement of XRD data as well as a linear shift of Raman and IR bands towards higher wave numbers shows a linear dependency of lattice parameters on the Eu content according to Vegard’s law. In contrast, EXAFS analysis reveals a decrease only for the La-O distances in the first coordination shell and the first metal-metal distance, while the Eu-O local coordination remains unchanged. TRLFS investigations show that the host cation size in the monazites has very little influence on the Eu3+ incorporation into these materials, but a broadening of the excitation spectra indicate a local disordering of the crystal structure around the dopant