On the symmetry of defects in perovskite-type protonic conductors:A Sc-45 NMR study

Sc doped perovskite-type protonic conductors were investigated by Sc-45 nuclear magnetic resonance (NMR), focusing on the local coordination environment around the dopant cation. Peaks observed in Sc-45 NMR spectra were assigned to 5-coordinated Sc with one oxygen vacancy (ScO5), 6-coordinated Sc (ScO6) and 6-coordinated Sc with a proton residing in its vicinity (ScO6-H). Quadrupole coupling constant, CQ, which is sensitive to local coordination environment of the probe nucleus was determined for ScO5. CQ of ScO5 decreased with decreasing the tolerance factor of the host lattice, indicating the distortion from cubic perovskite structure results in lower CQ of ScO5. This result suggests ScO5 polyhedra have tendency to become more isotropic with the distortion of the lattice. The oxygen vacancy concentration around Sc determined from integral intensities of NMR spectra was higher in isotropic ScO5 polyhedra.

► Sc-45 NMR can quantify defects in various perovskite protonic conductors.
► The local symmetry of 5 coordinated Sc defect changed with composition.
► Sc doped CaZrO3 showed higher oxygen vacancy concentration than BaZrO3.
► The stability of defects was suggested to be correlated to the symmetry of defects.