Synthesis, phase structure and microstructure of monazite-type Ce_(1-x)Pr_xPO_4 solid solutions for immobilization of minor actinide neptunium

摘要

Praseodymium was used as the surrogate for trivalent minor actinide neptunium, and a complete series of pure monazite-type Ce_(1-x)Pr_xPO_4 (x = 0–1) solid solutions were successfully prepared by the solid state reaction. The effects of calcining temperature, holding time and Pr content on the structure of Ce_(1-x)Pr_(x-)PO_4 solid solutions were investigated. The results show that although Pr_6O_(11) (Pr_2~(3+)Pr_4~(4+)O_(11)) exists two stabilized oxidation states, there has been no tetravalent praseodymium phosphate during the synthesis process. The optimized temperature for the synthesis of Ce_(0.8)Pr_(0.2)PO_4 solid solution is more than 1100 ?C, and a hypothetical reaction mechanism is also proposed. Besides, the crystalline grains coarsen as the increasing of holding time. The linear variation of unit cell parameters and a gradual hypsochromic shift in the Raman spectra are observed with the increase of Pr content, indicating that cerium is progressively replaced by praseodymium and Ce_(1-x)Pr_xPO_4 solid solutions were prepared.