Iron sites in radiation-damaged allanite-(Ce): the effects of thermally induced oxidation and structural reorganization

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 Mossbauer 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 Mossbauer 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.