Incorporation of Cm3+ and Eu3+ in LnPO4 ceramics – a site-selective TRLFS study

The chemical durability and structural flexibility of lanthanide phosphates make these ceramics attractive as host phases for the conditioning of long-lived radionuclides produced during the nuclear fuel cycle. In the present work we have studied the structural incorporation of Cm3+ and Eu3+ in various LnPO4 monazite and xenotime phases with site-selective time-resolved laser fluorescence spectroscopy (TRLFS). The europium results indicate a full structural incorporation in the LnPO4 ceramics crystallizing in the nine-fold coordinated monazite structure (LaPO4-GdPO4) independent of the host cation radius. A local disordering can, however, be seen in mixed monazite solid solutions when going from the pure endmembers (LaPO4 and GdPO4) toward the La0.5Gd0.5PO4 composition. The smaller lanthanides crystallizing in the eight-fold coordinated xenotime structure (TbPO4-LuPO4) show only a partial uptake of Eu3+ within the host cation sites. The remainder of the dopant appears to be present as an ill-defined, partially hydrated europium species on or within the xenotime solid. Actinide (Cm3+)-doped LnPO4 samples have been synthesized similarly to the Eu3+ solids. The results of the Cm-TRLFS measurements will be compared to the Eu3+-data and presented at the symposium.