Reissner, and 13 co-authors, reported in 2019 that the onset of thermally induced, heterogeneous structural reorganization of highly radiation-damaged allanite-(Ce) begins at temperatures below 700 K. Three strongly disordered allanite samples (S74 20414: ~ 0.55 wt% ThO2, 22.1 wt% REE oxides, and maximum radiation dose 3.5 × 1018 α-decay/g; LB-1: ~1.18 wt% ThO2, 19.4 wt% REE oxides, and maximum radiation dose 2.0 × 1019 α-decay/g; R1: ~ 1.6 wt% ThO2, 19.7 wt% REE oxides, and maximum radiation dose 2.6 × 1018 α-decay/g) were step-wise annealed to 1000 K in air. Using orientation-dependent nanoindentation, synchrotron single-crystal X-ray diffraction (synchrotron XRD), X-ray powder diffraction (powder XRD), differential scanning calorimetry and thermogravimetric analysis (DSC/TG), mass spectrometry (MS), 57Fe Mössbauer spectroscopy and high-resolution transmission electron microscopy (HRTEM), a comprehensive understanding of the structural processes involved in the annealing was obtained.
As a result of the overall increasing structural order, a general increase of hardness (pristine samples: 8.2–9.3 GPa, after annealing at 1000 K: 10.2–12 GPa) and elastic modulus (pristine samples: 115–127 GPa, after annealing at 1000 K: 126–137 GPa) occurred. The initially heterogeneous recrystallization process is accompanied by oxidation of iron, the related loss of hydrogen and induced stress fields in the bulk material, which cause internal and surface cracking after step-wise annealing from 800 to 1000 K. HRTEM imaging of the pristine material shows preserved nanometer-sized crystalline domains embedded in the amorphous matrix, despite the high degree of structural damage. The results show that hardness and elastic modulus are sensitive indicators for the structural reorganization process.
Thermal treatment of radiation-damaged allanite-(Ce) leads to heterogeneous recrystallization, due to the presence of less-damaged domains in the material. The recrystallization process starts around 550 K, as seen in the synchrotron data, and goes on continuously. Low-temperature annealing at 700 K leads to an increase in the mechanical properties of about 5% in the elastic modulus and about 10% in the hardness in all investigated samples. This further indicates a restoration of long-range order in the structure due to the reorganization process. In the temperature region of ~ 700–1000 K the structural Fe2+ oxidizes in air, which is coupled to the loss of hydrogen. The revealed adverse effect of internal and surface cracking reaches a maximum after annealing at 900 K when almost all Fe2+ is oxidized, and the most hydrogen is lost. The elastic modulus is more sensitive to fracturing than hardness. The initial maximum alpha-radiation dose as well as the assumed degree of structural damage (as estimated from powder XRD do not seem to correlate completely with the H and E order of the three samples.
Although a comparison of samples R1 and S74 20414 show higher H and E values with higher structural order, sample LB-1, showing initially the highest degree of structural order, has intermediate E and H values compared to the other two samples. It is, therefore, assumed that differences in chemistry and/or orientation also have a noticeable impact. Nevertheless, despite their differences in chemistry, radiation damage and orientation, all three samples showed an increase in H and E at the same annealing temperature (700 K) thus indicating a similar behavior in the recrystallization offset. The higher OH− content in sample LB-1 seems to act as a catalyst in the structural reorganization process, as assumed by Zhang et al. (2010) for zircon, leading to a stronger increase in the mechanical properties at higher temperatures (i.e. during the oxidation/dehydration process). In conclusion, the mechanical properties (E and H) are sensitive indicators of the degree of structural reorganization during the annealing process.
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