Structural analyses of heavy-ion irradiated monazites

Monazites are rare earth phosphates that are potential host matrices for the immobilization of actinides in high-level radioactive waste streams. This is due to their ability to incorporate various cations through different substitution mechanisms as well as their radiation resistance as observed in natural monazite mineral samples. In this study, LnPO4 monazite ceramics and single crystals doped with 500 ppm EuIII as a luminescent probe were irradiated with heavy ions to simulate the recoil of daughter products that occurs during alpha decay of the actinides. More specifically, irradiation experiments were conducted either with 14 MeV Au ions at fluences ranging from 5×1013 – 1×1015 ions/cm2 or with swift 1.7 GeV Au ions at fluences of 5×1011 – 2×1012 ions/cm2.
Irradiated monazite ceramics were analyzed with electron microscopy (SEM), vertical scanning interferometry (VSI), grazing incidence diffraction (GID), Raman spectroscopy, and luminescence spectroscopy to probe long and short range order of the monazite microstructure.
SEM micrographs and VSI data show clear damage of the irradiated regions of the ceramics, in the form of swollen grains and grain boundaries. GID images and powder patterns reveal diffuse scattering and amorphous contributions in irradiated samples. Solid solution compositions show larger damage than corresponding monazite endmembers, while polycrystalline and single crystal samples show similar level of amorphization. In the local coordination environments, Raman spectra of irradiated samples display a shoulder on the ν1 peak, indicating disruption in the vibrational modes of the phosphate tetrahedra. Confocal measurements of the swift heavy-ion irradiated monazites show full amorphization of the surface layers of the monazites samples, and increasing crystallinity with increasing sample depth. Luminescence data illustrate differences in the local LnO9 polyhedral environment in the monazites with irradiation. Integrated excitation spectra show a difference in the intensity and position of the excitation peak with irradiation. Especially single crystal data show a systematic decrease of the local site symmetry of the Eu3+ cation, and a general broadening of emission spectra, indicative for reduced local order following amorphization.