HOW TO EXPLAIN THE EXPERIMENTAL DIFFICULTIES IN THE COFFINITE PREPARATION? HINTS FROM THE STUDY OF URANOTHORITE SOLID SOLUTIONS
On this basis, this work aimed at the preparation of Th1-xUxSiO4 uranothorite samples to access the thermodynamic data of coffinite through the solid solution approach. The preparation of the samples was performed from a mixture of thorium and uranium in hydrochloric medium and sodium metasilicate. The starting reagents were mixed in the desired stoichiometry then heated in hydrothermal conditions at 250°C for 24 hours.
The resulting powdered samples were then extensively characterized by XRD, SEM, TEM as well as µ-Raman and IR spectroscopies. First, the Rietveld refinement of XRD patterns revealed the presence of two distinct phases in the samples, i.e. uranothorite and uranothorianite. The first one present the classic bypiramidal morphology associated to the zircon-type structure while the second appeared to be nanosized. Also, the amount of dioxide by-product was found to increase significantly with the amount of uranium initially introduced. Finally, the spectroscopic data pointed out the absence of water molecules or hydroxide groups in the compound, thus confirming the chemical formula of the uranothorite series. It also allowed extrapolating the positions of coffinite vibration modes which remained unknown up to now.