Investigating Variable Irradiation Damage Response in Monazite, La1– x Ln x PO4 ( Ln = Pr, Nd, and Sm)

Abstract

Monazite (LnPO4) materials are promising waste forms for immobilizing minor actinides, but their radiation damage performance as mixed solid solutions remains poorly described. Herein, we have systemically examined the irradiation response of mixed monazite solid solutions, (La1–xLnx)PO4 (x = 0, 0.5, and 1; Ln = Pr, Nd, and Sm), using 14 MeV 197Au4+ ion irradiations at a fluence of 1 × 1015 ions/cm2 regarding structural, microstructural, and mechanical properties. Amorphization susceptibility was assessed by grazing incidence X-ray diffraction, with the optimal incidence angle determined from stopping and range of ions in matter simulations and X-ray penetration depth calculations. Complementary characterization via scanning electron microscopy, Raman spectroscopy, and Vickers indentation provided insight into microstructural evolution, local chemistry, and mechanical property changes, respectively. The results indicate an increasing radiation damage resistance of end-member LnPO4 materials with decreasing Ln cation size. In mixed solid solutions, (La0.5Ln05)PO4, the radiation damage resistance decreases as the size mismatch between the Ln cations increases, which is attributed to nonideal solid solution cation mixing during irradiation damage. We propose that irradiation-induced lattice displacements increasingly impede cation redistribution and promote structural disorder as the cation size mismatch increases, with measurable consequences for both chemical stability and mechanical integrity. The role of grain boundaries and the susceptibility to changes in local coordination chemistry of cations are further highlighted as determinants for specific irradiation damage response and their ability to act as sinks for ion and defect migration. The investigation underscores the need to evaluate complex solid solutions rather than idealized end-member compositions when assessing ceramic nuclear waste form performance for radionuclide immobilization.