Site-selective TRLFS of Eu(III) doped rare earth phosphates for conditioning of radioactive wastes

Crystalline ceramic materials show promise as potential waste forms for immobilization of high-level radioactive wastes. Rare earth (RE) phosphate ceramics have been found to be extremely stable over geological time scales and they show good tolerance to high radiation doses. These ceramics are able to incorporate radionuclides in well-defined atomic positions within the crystal lattice up to high (~25%) loadings, which will reduce the volume of waste in the radionuclide conditioning process. The dehydrated RE phosphates are known to crystallize in two distinct structures, depending on the ionic radius of the cation: the larger lanthanides from La3+ to Gd3+ crystallize in the nine-fold coordinated monazite structure, while the smaller lanthanides such as Lu3+ form eight-fold coordinated xenotime structures.
In the present work we have used site-selective time-resolved laser fluorescence spectroscopy (TRLFS) to investigate the structural incorporation of Eu3+, an analogue for the actinides Pu3+, Am3+ and Cm3+, in rare earth phosphate ceramics. The very narrow excitation spectra of LaPO4 and GdPO4 monazites doped with 500 ppm Eu3+ indicate that Eu3+ is fully incorporated on the host cation sites in the highly ordered ceramic materials independent of the ionic radii of the host cations. The LuPO4 xenotime phase, however, shows a very low incorporation of the Eu3+ ion within the crystal lattice. The majority of the signal in the Eu3+-LuPO4 excitation spectrum could be assigned to partly hydrated europium in the LuPO4 ceramic. In experiments where we increased the dopant concentration up to 50 % in the xenotime host matrix, a larger amount of Eu3+ incorporation within the crystal structure in relation to the hydrated species could be seen. A similar increase of the dopant concentration in the monazite phases caused a broadening of the excitation spectra as a result of local disordering of the crystal structures. This disordering, however, had no influence on the Ln3+ site symmetry in the monazites.
Our site-selective TRLFS investigations have shown that the host cation size in the monazites has very little influence on the Eu3+ incorporation into these materials. The structure of the ceramic, however, seems to play a decisive role in how well the dopant is substituted within the crystal lattice.