Valence characteristics and structural stabilities of the electrolyte solid solutions Ce1-xRExO2-delta (RE = Eu, Tb) by high temperature and high pressure

Solid solutions Ce1-xRExO2-delta (RE = Eu, Tb) were prepared by a high-temp erature and -pressure method. The products were characterized by X-ray diff raction (XRD), TG, electron paramagnetic resonance (EPR), X-ray photoelectr on spectroscopy (XPS), and Mossbauer spectroscopy. XRD data analysis showed that all solid solutions crystallized in a single-phase cubic fluorite str ucture. The nonlinear relationships between the lattice parameter and dopan t content for both series of solutions were ascribed to the results of cati on substitutions and variations of the relative content of oxygen vacancy V o and defect associations {RE'(Ce)Vo} and {Ce'(Ce)Vo}. EPR and XPS measurem ents confirmed the presence of Ce3+ ions in the solid solutions. For the so lid solutions Ce1-xEuxO2-delta, all Eu ions were determined to be trivalent by XPS and Eu-151 Mossbauer measurements. For the solutions Ce1-xTbxO2-del ta, all Tb ions were also stabilized in the trivalent state. This result is different from that of the counterpart by hydrothermal conditions, in whic h a mixed valence of Tb3+/Tb4+ prevails at a higher dopant content. The pre pared solutions Ce1-xTbxO2-delta were metastable. With increasing temperatu re, they would be destabilized and decompose into two fluorite phases, acco mpanied by partial oxidation from Tb3+ to Tb4+. Alternating current impedan ce spectroscopy showed primarily bulk conduction for all samples. For the s olutions Ce1-xEuxO2-delta, the temperature dependence of the ionic conducti vity was linear within the temperature range measured with activation energ ies of 1.05, 0.82, and 0.87 for x = 0.2, 0.38, and 0.5, respectively. For t he decomposition product of the solid solution Ce0.71Tb0.29O2-delta, the co nductivity gave two linear regions with smaller activation energies; i.e., the activation energy was 0.60 eV below 600 degrees C and 0.39 eV above 600 degrees C. The higher ionic conductivity (1.1 x 10(-2) S/cm at 720 degrees C) for the decomposition phases of the solution Ce0.71Tb0.29O2-delta was a scribed to an electronic component involved in; relation to the presence of the mixed valence of Tb3+/Tb4+ and Ce3+/Ce4+.