Thorium aspartate tetrahydrate precursor to ThO2: Comparison of hydrothermal and thermal conversions

Abstract

The synthesis of original crystalline thorium aspartate tetrahydrate, Th(C4NO4H6)4.4H2O, was performed using two different wet-chemistry routes, involving either L-asparagine or L-aspartic acid as complexing agent. Characterization of this compound through 13C NMR and PXRD led to confirm the terminal coordination mode of the aspartate group and to suggest a potential cubic lattice (Pn-3 space group). Vibrational spectroscopy data were also collected. The conversion of thorium aspartate tetrahydrate into thorium dioxide was further performed through classical high temperature heat treatment or under hydrothermal conditions. On the one hand, thermal treatment provided a pseudomorphic conversion which retained the starting morphology, and favored the increase of the average crystallite size, as well as the complete elimination of the residual carbon content. On the other, hydrothermal conversion could be used to tune the morphology of the final oxide, ThO2.nH2O microspheres being prepared when starting from L-asparagine.