Chemical Conditions of Synthesis of Th4(PO4)4P2O7. Preparation of Thorium Phosphate−Hydrogenphosphate as Precursor

Thorium phosphate−diphosphate, Th 4 (PO 4 ) 4 P 2 O 7, is obtained, at high temperature (800−1250 °C), for a given ratio r = PO 4 /Th equal to 3 / 2 from a thorium compound and a phosphating reactant whatever the chemical conditions. This compound, whose solubility is very low, can be loaded in situ and used as a host matrix for radwaste storage. One of the methods for its synthesis involves the precipitation of a precursor, thorium phosphate−hydrogenphosphate. This precursor is used as an adsorber for low concentrations of radionuclides and, after heating in air in the range 800−1200 °C, it is transformed into doped Th 4 (PO 4 ) 4 P 2 O 7 . The optimum conditions for the precipitation process have been determined. Starting from 0.5−2 M solutions of thorium nitrate or chloride and (NH 4 ) 2 HPO 4 at pH = 9−9.5 at the ratio r = 3 / 2, a compound of general formula Th 2 (PO 4 ) 2 HPO 4 · n H 2 O ( n = 3−7) is synthesized. Its physicochemical properties have also been studied.