Atomic Displacement Parameters of Ceria Doped with Rare-Earth Oxide Ce0.8R0.2O1.9 (R = La, Nd, Sm, Gd, Y, and Yb) and Correlation with Oxide-Ion Conductivity

Abstract

To examine the correlation between the atomic displacement parameters of oxygen atom and oxide-ion conductivity of rare-earth-oxide-doped ceria materials, Ce0.8R0.2O1.9 (R = La, Nd, Sm, Gd, Y, and Yb), their crystal structures have been refined by the Rietveld analyses of high-resolution (d > 0.38 Å) synchrotron powder diffraction data measured in air in situ at 29.5, 408.0, and 675.0 °C. As a reference material, pure nondoped ceria CeO2 was also investigated by the same method. Both atomic displacement parameters of cations and anions increase with temperature, which indicates the thermal vibrations and dynamic positional disorders. At a given temperature, the thermal parameters of cation in Ce0.8R0.2O1.9, UC(R), are larger than those in CeO2, UC(CeO2), which indicates that the positional disorder of cations in Ce0.8R0.2O1.9 is also larger compared with that in CeO2. Similarly, at a given temperature, the atomic displacement parameters of anions in Ce0.8R0.2O1.9, UO(R), are larger than those in CeO2, UO(CeO2), which indicates that the positional disorder of oxide ions in Ce0.8R0.2O1.9 is larger than that in CeO2. The larger positional disorders of oxide ions in Ce0.8R0.2O1.9 would be the structural origin of higher ionic conductivities of Ce0.8R0.2O1.9, compared with those of undoped CeO2. In Ce0.8R0.2O1.9, both atomic displacement parameters of cations UC(R) and of anions UO(R) decrease with increasing ionic radius of R3+ ions for the coordination number of eight r(R) from R = Yb to R = Gd, while they increase with r(R) from R = Gd to R = La. Thus, both UC(R) and UO(R) exhibit minima at R = Gd in the range from R = Yb to R = La. The mismatch between dopant rare-earth oxide and host ceria would yield the positional disorder, local distortion, and lattice strain, which leads to the larger atomic displacement parameters. The present paper discusses the correlation between the oxygen thermal parameter UO(R) and oxide-ion conductivity of Ce0.8R0.2O1.9.