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–
x
Ce
x
PO
4
(
x
= 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 10
14
ions/cm
2
and 10
15
ions/cm
2
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
0.75
Ce
0.25
PO
4
and La
0.25
Ce
0.75
PO
4
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.