The grain boundary properties of CGO10 (Ce0.9Gd0.1O1.95) and CGO20 (Ce0.8Gd0.2O1.95) have been
investigated by impedance spectroscopy and high resolution scanning transmission electron
microscopy-electron energy loss spectroscopy (STEM-EELS). High density polycrystalline ceramic
specimens with a range of grain sizes (0.17-2.7μm) have been prepared from high purity (SiO2 ~120-210
ppm) commercially available powders. Impedance measurements obtained for these ionic conducting
materials were interpreted using the brick layer model. The specific grain boundary conductivity [delta*GB]was found to be 2-3 orders of magnitude lower than the grain bulk conductivity [delta G]. The grain
boundary blocking effect in these materials has been attributed to the formation of grain boundary
space charge layers. The brick layer model was used to estimate the space charge layer thickness (δ)
from impedance spectroscopy measurements and was found to be in the range 1-2 nm.
High resolution STEM-EELS measurements were carried out over a number of grain boundaries in
CGO10 and CGO20. These measurements showed an increase in the gadolinium to cerium and oxygen
to cerium atomic ratios (Gd:Ce and O:Ce respectively) at the grain boundary region. These findings are in
accordance with the space charge layer model. From these measurements the space charge layer
thickness (δ) was shown to be in the range 1-2 nm. Analysis of the cerium M4,5-edge fine structure
showed the cerium oxidation state to remain unchanged in the space charge layer in all grain
boundaries studied
