  The oxygen-exchange behavior has been studied in half-doped manganese and
cobalt perovskite oxides. We have found that the oxygen diffusivity in
Gd_{0.5}Ba_{0.5}MnO_{3-\delta} can be enhanced by orders of magnitude by
inducing crystallographic ordering among lanthanide and alkali-earth ions in
the A-site sublattice. Transformation of a simple cubic perovskite, with
randomly occupied A-sites, into a layered crystal GdBaMn_2O_{5+x} (or
isostructural GdBaCo_2O_{5+x} for cobalt oxide) with alternating lanthanide and
alkali-earth planes reduces the oxygen bonding strength and provides
disorder-free channels for ion motion, pointing to an efficient way to design
new ionic conductors.
