Radiation-damaged allanite-(Ce) starts to recrystallize at an annealing temperature below 700 K. At the same temperature Fe2+ → Fe3+ oxidation, as well as dehydration occurs. Three radiation-damaged samples (S74 20414: 0.55 wt% ThO2, LB-1: 1.18 wt% ThO2, R1: 1.59 wt% ThO2) as well as one crystalline sample (RS221) were investigated using 57Fe Mössbauer spectroscopy after step-wise annealing. Additionally, the three damaged samples were investigated by in-situ mass spectrometry, analysing the escaping gases during thermal treatment showing the dehydration process. 57Fe Mössbauer spectroscopy revealed not only a general Fe2+ → Fe3+ oxidation, it also showed that this process is not fully completed after annealing at 1000 K in sample LB-1 which is the sample showing the fastest and strongest recrystallization. The crystalline sample also still incorporated Fe2+ as well as Fe3+ after annealing at 1000 K. In addition, a preferred occupation of iron at the position M3 was identified in the crystalline sample which did not occur in the pristine radiation-damaged samples (M1/M3 site distribution: RS221: 17/83, pristine samples R1: 49/51, LB-1: 60/40, S74 20414: 52/48). After annealing at 1000 K sample LB-1 showed a similar distribution as the crystalline sample with a M1/M3 distribution of 15/85. It is therefore proposed that the process of amorphization through α-decay damage changes the distribution of iron atoms on its possible crystallographic sites, and that this process is reversible through thermal annealing.

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% ThO 2 , 22.1 wt% REE oxides, and maximum radiation dose 3.5 × 10 18 α-decay/g; LB-1: ~1.18 wt% ThO 2 , 19.4 wt% REE oxides, and maximum radiation dose 2.0 × 10 19 α-decay/g; R1: ~ 1.6 wt% ThO 2 , 19.7 wt% REE oxides, and maximum radiation dose 2.6 × 10 18 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), 57 Fe 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.