Preparation of Optimized Uranium and Thorium Bearing Brabantite or Monazite/Brabantite Solid Solutions

The simultaneous incorporation of thorium and uranium(IV) in the rare-earth orthophosphates from a mixture of powdered AnO2, (NH4)(H2PO4), and Ca(HPO4)·2H2O was successfully performed, leading to the formation of single-phase Th-brabantite (Ca0.5Th0.5PO4), U-brabantite (Ca0.5U0.5PO4), and (Th,U)-brabantites (Ca0.5Th0.5−yUyPO4). The repetition of several cycles composed of a grinding step (30 Hz, 15 min) and of a heat treatment (T=1200°C, 10 h, Ar atmosphere) allowed preparation of single-phase and homogeneous compounds. The complete characterization of the samples by the means of X-Ray powder diffraction, electron probe microanalyses, and μ-Raman spectrometry did not reveal either the presence of minor phases or the partial oxidation of uranium(IV) into uranyl. This study also evidenced the formation of a complete Ca0.5Th0.5−yUyPO4 solid solution with the associated linear decrease of a, b, and c unit cell parameters and β increase versus the uranium mole loading. Even if the samples were found to be single phase, the significant improvement of the cation distribution in the materials was reached by the mean of the repetition of grinding/heating cycles. The same solid-state process was applied with success to the preparation of homogeneous and single-phase (Th,U)-monazite/brabantite solid solutions with general formula Ln1−2xCaxThx−yUyPO4 (with 0.1≤x≤0.4 and y=x/5)