The effect of the synthesis route of monazite precursors on the microstructure of sintered pellets

The influence of synthesis route on morphology of La0.5Eu0.5PO4 monazite precursor and the microstructure of pellets after sintering have been investigated. La0.5Eu0.5PO4 precursor materials were synthesised by hydrothermal precipitation in acidic (pH 1) and basic (pH 10.5) media as well as by co-precipitation in acidic media (pH 1) forming needle-like, spherical and rice-shaped particles, respectively. A phase transition from rhabdophane to monazite was observed only for the hydrothermally precipitated precursors at 600-800 °C by TG-DSC. After conventional sintering, all La0.5Eu0.5PO4 powders were found to be single-phase solid solutions with a monazite structure by PXRD and Raman measurements. From microscopic (SEM) and density investigations, it was ascertained that the synthesis route significantly influences the morphology of the precursor material and the microstructure of the sintered pellets. The needle-like materials seem to be the most promising precursor for nuclear waste application since pellets with a density of ∼92.5% TD and intragranular closed porosity are formed during sintering which can serve as cavities to compensate He built up and therefore avoid swelling and crack formation during long-term storage in a repository.