Investigation\nof Radiation Damage in the Monazite-Type\nSolid Solution La<sub>1–<i>x</i></sub>Ce<sub><i>x</i></sub>PO<sub>4</sub>

Crystalline materials such as monazite have been considered\nfor\nthe storage of radionuclides due to their favorable radiation stability.\nUnderstanding their structural chemical response to radiation damage\nas solid solutions is a key component of determining their suitability\nfor radionuclide immobilization. Herein, high-resolution structural\nstudies were performed on ceramics of the monazite solid solution\nLa<sub>1–<i>x</i></sub>Ce<sub><i>x</i></sub>PO<sub>4</sub> (<i>x</i> = 0.25, 0.5, 0.75, 1) in order\nto understand the role of structural chemistry on irradiation stability.\nCeramic samples were irradiated with 14 MeV Au ions with 10<sup>14</sup> ions/cm<sup>2</sup> and 10<sup>15</sup> ions/cm<sup>2</sup> to simulate\nthe recoil of daughter nuclei from the alpha decay of actinide radionuclides.\nThe extent of radiation damage was analyzed in detail using scanning\nelectron microscopy (SEM), Raman spectroscopy, grazing incidence X-ray\ndiffraction (GI-XRD), and high-energy-resolution fluorescence detection\nextended X-ray absorption fine structure (HERFD-EXAFS) spectroscopy.\nSEM and Raman spectroscopy revealed extensive structural damage as\nwell as the importance of grain boundary regions, which appear to\nimpede the propagation of defects. Both radiation-induced amorphization\nand recrystallization were studied by GI-XRD, highlighting the ability\nof monazite to remain crystalline at high fluences throughout the\nsolid solution. Both, diffraction and HERFD-EXAFS experiments show\nthat while atomic disorder is increased in irradiated samples compared\nto pristine ceramics, the short-range order was found to be largely\npreserved, facilitating recrystallization. However, the extent of\nrecrystallization was found to be dependent on the solid solution\ncomposition. Particularly, the samples with uneven ratios of solute\ncations, La<sub>0.75</sub>Ce<sub>0.25</sub>PO<sub>4</sub> and La<sub>0.25</sub>Ce<sub>0.75</sub>PO<sub>4</sub> were observed to exhibit\nthe least apparent radiation damage resistance. The findings of this\nwork are discussed in the context of the monazite solid solution chemistry\nand their appropriateness for radionuclide immobilization.