Synthesis and microstructure of fluorapatite-type Ca10-2xSmxNax(PO4)6F2 solid solutions for immobilization of trivalent minor actinide

Ca10-2xSmxNax(PO4)6F2 (x = 0.8-1.2) solid solutions were successfully synthesized by the solid state reaction method using samarium (Sm) as the surrogate for trivalent minor actinide neptunium (Np). The influences of calcining temperature, holding time and Sm doping content on the phase composition and microstructure of Ca10-2xSmxNax(PO4)6F2 were investigated. The results indicated that the optimized calcining temperature and holding time for preparing Ca10-2xSmxNax(PO4)6F2 were 1000 °C and 2 h. Ca10-2xSmxNax(PO4)6F2 were confirmed to be the discontinuous solid solutions and solid solubility limit of Sm in the Ca10-2xSmxNax(PO4)6F2 was 1.2 formula units. The rod-like grains of Ca10-2xSmxNax(PO4)6F2 had typical hexagonal characteristics with a diameter of 3-5 μm and a length of 10-20 μm. No significant changes on the microstructures of Ca10-2xSmxNax(PO4)6F2 were observed with the increase of Sm doping content. The Ca, Sm, Na, P, O and F elements were nearly distributed uniformly in the Ca8Sm1Na1(PO4)6F2 solid solution.