Impact of FePO4 on the sintering behavior of Th-monazite ceramics: Phase evolution and properties

The rhabdophane–monazite system has been proposed as a promising material for the immobilization of actinides from radioactive waste streams due to its potential for lower processing costs. Assessing the influence of impurities present in such waste streams on monazite properties is therefore critical, particularly given that rhabdophane precipitation is frequently accompanied by the co-formation of FePO 4 ·2H 2 O as a common secondary phase. This study systematically investigates the effects of Fe 3+ incorporation on the sintering behavior and chemical stability of monazite ceramics, with particular focus on the solubility limit of Fe 3+ and the diffusion dynamics of the molten FePO 4 phase. The results reveal that trace amounts of Fe 3+ (up to 2 mol%) can be incorporated into the monazite lattice at 1200 °C via solid-state diffusion during grain growth. When the Fe 3+ doping level exceeds 2 mol%, excess FePO 4 seg = regates at grain boundaries, leading to microcrack formation and significant reductions in density and hardness. Furthermore, leaching experiments demonstrate that Fe 3+ doping below 2 mol% increases the leaching rate of Th 4+ . In contrast, the presence of a secondary FePO 4 phase (>2 mol%) at grain boundaries promotes preferential dissolution, thereby accelerating localized corrosion and the co-leaching of Th 4+ and La 3+ .