Abstract: Novel phosphates, Na_5-2xYb₂xCe_1-x(PO₄)₃·yH₂O(x=0-1, y=0-1), were synthesized under mild hydrothermal conditions at 240℃ with Ce(SO₄)₂·H₂O, Ce(SO₄)₂·2(NH₄)₂SO₄· 4H₂O, CeO₂, Yb₂O₃, 85 % H₃PO₄ and NaOHas the starting materials.The products were sensitively influenced by the pHvalue of the systems and reaction temperature.Under an acidic condition, CePO₄ was the product at 240℃, in which Ce(Ⅳ ) species was reduced to Ce(Ⅲ) species.Under a weak basic condition, Na₂Ce(PO₄)₂·0.79H₂Owas produced.Under a medium basic condition, the expected compound Na₅Ce(PO₄)₃ was the product.Under a basic condition, CeO₂ was the final product, amorphous phase was also produced in some cases.The structure of the compounds were investigated by X-ray diffraction, SEM, IR, Raman, ³¹P MAS NMR and XPS spectroscopies.To determine the valence of Ce in the compound, EPRand DRSof the compound were also recorded.The thermal properties of the compounds were determined with DTA-TGanalysis.Na₅Ce(PO₄)₃ was decomposed into Na₃Ce(PO₄)₂, Na₄P₂O₇ and O, at 694℃, in which Ce(Ⅳ) was reduced to Ce(Ⅲ) species.Na₃Yb₂(PO₄)₃·H₂O lost water and was transferred into a new phase at a high temperature 682℃.The thermal stability of Na_5-2xYb₂xCe_1-x(PO₄)₃·yH₂O could be improved by increasing x value.When x=0.54, the compound was stable even at the temperature ca.1000℃.The ionic conductivities of the compounds were increased when the amount of Yb in the compound was increased and when x=0.33, it reached the highest value (σ_300℃= 3.96 ×10^-6 S·cm^-1).

Key words: Cerium, Ionic conduction, Mild hydrothermal synthesis, Rare earth phosphate, Ytterbium