Investigation of Radiation Damage in the Monazite-Type Solid Solution La_1–<i>x</i>Ce_<i>x</i>PO₄

Crystalline materials such as monazite have been considered for the storage of radionuclides due to their favorable radiation stability. Understanding their structural chemical response to radiation damage as solid solutions is a key component of determining their suitability for radionuclide immobilization. Herein, high-resolution structural studies were performed on ceramics of the monazite solid solution La_1-<i>x</i>Ce_<i>x</i>PO₄ (<i>x</i> = 0.25, 0.5, 0.75, 1) in order to understand the role of structural chemistry on irradiation stability. Ceramic samples were irradiated with 14 MeV Au ions with 10¹⁴ ions/cm² and 10¹⁵ ions/cm² to simulate the recoil of daughter nuclei from the alpha decay of actinide radionuclides. The extent of radiation damage was analyzed in detail using scanning electron microscopy (SEM), Raman spectroscopy, grazing incidence X-ray diffraction (GI-XRD), and high-energy-resolution fluorescence detection extended X-ray absorption fine structure (HERFD-EXAFS) spectroscopy. SEM and Raman spectroscopy revealed extensive structural damage as well as the importance of grain boundary regions, which appear to impede the propagation of defects. Both radiation-induced amorphization and recrystallization were studied by GI-XRD, highlighting the ability of monazite to remain crystalline at high fluences throughout the solid solution. Both, diffraction and HERFD-EXAFS experiments show that while atomic disorder is increased in irradiated samples compared to pristine ceramics, the short-range order was found to be largely preserved, facilitating recrystallization. However, the extent of recrystallization was found to be dependent on the solid solution composition. Particularly, the samples with uneven ratios of solute cations, La_0.75Ce_0.25PO₄ and La_0.25Ce_0.75PO₄ were observed to exhibit the least apparent radiation damage resistance. The findings of this work are discussed in the context of the monazite solid solution chemistry and their appropriateness for radionuclide immobilization.