Influence of microstructural characteristics on ionic conductivity of ceria based ceramic solid electrolytes

Abstract Solid electrolyte powders based on gadolium-doped and samaria-codoped ceria of compositions Ce 0.8 Gd 0.2−x Sm x O 1.9 (x = 0.00; 0.01; 0.03; 0.05) were sintered using the polymeric precursor method (Pechini). The X-ray diffractometry results confirmed the formation of a single crystalline phase, that is, a solid solution. Test specimens were compacted using uniaxial cold pressing followed by two-stage sintering. Relative densities were higher than 96% of theoretical density in all the samples. Ionic conductivity was determined using impedance spectroscopy, obtaining values of 10 −2  S cm −1 at 700 °C, with the best results for codoped electrolytes, primarily the Ce 0.8 Gd 0.15 Sm 0.05 O 1.9. system. This study aimed at correlating ionic conductivity variations of codoped electrolytes with microstructural alterations (on a nanometric scale) resulting from the addition of codopants. Energy dispersive spectroscopy (EDS) was used to analyze the grain boundary compositions of the electrolytes produced and assess possible dopant segregation in these regions. Interplanar spacings corresponding to the crystallographic planes of the cubic (fluorite-type) phase were identified using high-resolution transmission electron microscopy (HRTEM) images. Extra diffraction spots and diffuse stains were observed in the doped and codoped electrolytes, using selected area electron diffraction (SAED), confirming the presence of amorphous nanodomains at the atomic level. However, the codoped system exhibited diffraction patterns with no diffuse stains, indicating that codoping favored obtaining nanodomain-free regions. These regions displayed better structural homogeneity, which may explain the enhanced ionic conductivity in codoped systems.