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 (ScO ₅), 6-coordinated Sc (ScO ₆) and 6-coordinated Sc with a proton residing in its vicinity (ScO ₆-H). Quadrupole coupling constant, C _Q, which is sensitive to local coordination environment of the probe nucleus was determined for ScO ₅. C _Q of ScO ₅ decreased with decreasing the tolerance factor of the host lattice, indicating the distortion from cubic perovskite structure results in lower C _Q of ScO ₅. This result suggests ScO ₅ 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 ScO ₅ polyhedra.