Formation of a new hydrated sodium–thorium phosphate from thorium dioxide and its subsequent phase evolution

This study investigated the behaviour of nanoscale thorium dioxide in a sodium phosphate buffer under hydrothermal conditions under conditions ranging from weakly acidic (pH ∼ 5) to weakly basic (pH ∼ 8). The hydrothermal syntheses yielded a nanosized hydrated double sodium-thorium phosphate phase. The acidity of the medium affected particle size and elemental composition of the product. The phase, identified in all cases as a hydrated variant of the known sodium-thorium phosphate NaTh₂(PO₄)₃, possesses a framework structure and is able to accommodate water and sodium cations within the channels; notably, the sodium content varied based on the acidity of the synthesis medium. Calcination of the nanosized phase in air produced mixtures of two distinct crystalline sodium-thorium phosphates, NaTh₂(PO₄)₃ and Na₂Th(PO₄)₂, in different ratios. The final product composition was determined by the pH used during synthesis and the related phosphate content of the nanosized phase. Characterization of the materials before and after calcination was carried out with a range of complementary methods: X-ray diffraction, small-angle X-ray scattering, electron microscopy, X-ray absorption spectroscopy, total X-ray scattering with pair distribution function analysis, infrared spectroscopy and ³¹P MAS NMR.