Oxygen Ordering and Mobility in YBaCo<sub>4</sub>O<sub>7+δ</sub>

Extraordinary oxygen ordering and mobility have been found in an oxygen-nonstoichiometric mixed-valence cobalt oxide, YBaCo<sub>4</sub>O<sub>8.5</sub>. The excess oxygen atoms appear to be incorporated in an orderly way into the YBaCo<sub>4</sub>O<sub>7</sub> parent lattice with different configurations as the oxygen content varies. Intense electron-beam irradiation was used to observe recurrent oxygen migration in the lattice, causing reversible structural modulation transitions. The oxygen migration can be attributed mainly to the electron-beam heating effect. This study highlights the high degree of freedom of surplus oxygen within the investigated structure and advances our understanding of the oxygen diffusion process in related transition-metal oxide systems.