Abstract: Using nanoparticles of CeO₂ and ZrO₂ prepared by chemical precipition method as the starting materials, the single phase cubic Ce_0.5 Zr_0.5 O₂ solid solution has been synthesized under high pressure and high temperature for the first time. The structural characteristics and valence state of the single phase cubic Ce_0.5 Zr_0.5 O₂ solid solution were investigated by XRD, TG-DTA, Raman, XPSand EPRmeasurements. The XRDresults showed that the nanocrystalline mixtures of CeO₂-50% ZrO₂ can form a cubic Ce_1-x Zr_x O₂ solid solution above 1.67 GPa and above 1073 K, but the product is in a mixture state. The single phase cubic Ce_0.5 Zr_0.5 O₂ solid solution can be obtained only under the certain conditions of 3.1 GPa at 1073 K. The high pressure promotes the process of the solid state reaction and the temperature of forming CeO₂ ZrO₂ solid solution decreases under high pressure. The DTAcurve of the single phase cubic Ce_0.5 Zr_0.5 O₂ solid solution showed a broad exothermic peak between 403 Kand 742 K. But the XRD pattern and Raman spectra displayed that the structure of the solid solution had not been changed before and after annealing at 773 Kfor 1 h. It revealed that the cubic Ce_0.5 Zr_0.5 O₂ solid solution was stable in structure below 773 K. The EPRspectra recorded at room temperature gave an evidence of the presence of Ce ³⁺ and O^2- in CeO₂ nanoparticles. However, the signals of Ce ³⁺ and O^2- have not been detected in Ce_0.5 Zr_0.5 O₂ solid solution. An unknown signal( g=1.990) has been observed in cubic Ce_0.5-Zr_0.5 O₂ solid solution which has not been changed after annealing at 773 K for 1 h, exhibiting that there exists no Ce ³⁺ in cubic Ce_0.5 Zr_0.5 O₂ solid solution and the Ce ⁴⁺ has not been reduced into Ce ³⁺ after annealing. The hyperfine structure in Zr_3d XPS spectrum has been observed for the Ce_0.5 Zr_0.5 O₂ solid solution. It shows that there may exist some Zr ions with a low valence state. The ionic conductivity measured by impedance spectroscopy showed that the cubic Ce_0.5 Zr_0.5 O₂ solid solution exhibits only the bulk conductivity. The bulk conductivity( σ=1.2× 10^-5 S/cm at 823 K, σ=2.1×10^-3 S/cm at 1123 K) is the same as that of CeO₂, but smaller than that of Y₂O₃-stabilized ZrO₂. Amarked curvature at T=823 Khas been observed in the Arrhenius plot of the bulk conductivity. The activation energy below 823 Kis lower than that above 823 K, and the reason has been discussed.

Key words: Cerium oxide, Zirconium oxide, Nanocrystalline, High-pressure and high-temperature, Solid solution