Incorporation of U(iv) in monazite–cheralite ceramics under oxidizing and inert atmospheres

Abstract

This work is the first attempt to prepare Nd_1−xCa_xU_xPO₄ monazite–cheralite with 0 < x ≤ 0.1 by a wet chemistry method. This method relies on the precipitation under hydrothermal conditions (T = 110 °C for four days) of the Nd_1−xCa_xU_xPO₄·nH₂O rhabdophane precursor, followed by its thermal conversion for 6 h at 1100 °C in air or Ar atmosphere. The optimized synthesis protocol led to the incorporation of U and Ca in the rhabdophane structure. After heating at 1100 °C for 6 h in air, single-phase monazite–cheralite samples were obtained. However, α-UP₂O₇ was identified as a secondary minor phase in the samples heated under Ar atmosphere. The U speciation in the samples converted in an oxidising atmosphere was carefully characterized using synchrotron radiation by combining HERFD-XANES and XRD. These results showed the presence of a minor secondary phase containing hexavalent uranium and phosphate with a stoichiometry of U : P = 0.78. This highly labile uranyl phosphate phase incorporated 21 mol% of the uranium initially precipitated with the rhabdophane precursor. This phase was completely removed by a washing protocol. Thus, single-phase monazite–cheralite was obtained through the wet chemistry route described in this work with a maximum U loading of x = 0.08.