Ruddlesden–Popper Oxychlorides Ba₃Y₂O₅Cl₂, Sr₃Sc₂O₅Cl₂, and Sr₂ScO₃Cl: First Examples of Oxide-Ion-Conducting Oxychlorides

Oxide-ion conductors are attractive materials due to their wide applications such as solid oxide fuel cells (SOFCs). Crucial to the developments of the applications is the discovery of a class of oxide-ion conductors. Some Ruddlesden–Popper (RP) oxychlorides are expected to show high oxide-ion conductivity, because they have larger free and lattice volumes than RP oxides, leading to low activation energies. Here, we discover RP oxychlorides Ba3Y2O5Cl2, Sr3Sc2O5Cl2, and Sr2ScO3Cl as a class of oxide-ion conductors. Rietveld analyses of X-ray powder diffraction data indicate that Ba3Y2O5Cl2 and Sr3Sc2O5Cl2 are tetragonal I4/mmm n = 2 RP phases and that Sr2ScO3Cl is a tetragonal P4/nmm n = 1 RP phase. The predominant conducting species in Ba3Y2O5Cl2, Sr3Sc2O5Cl2, and Sr2ScO3Cl is strongly suggested to be the oxide ion by (i) direct-current (DC) polarization measurements, (ii) electrolyte domains observed in the oxygen partial pressure dependence of DC electrical conductivities at constant temperatures, and (iii) wide optical band gaps. DFT calculations of Ba3Y2O5Cl2 show a lower energy barrier for oxide-ion migration than that for chloride-ion migration, which supports the oxide-ion conduction. The DFT results indicate that oxide ions in Ba3Y2O5Cl2 two-dimensionally migrate mainly through equatorial oxygen O1 sites along the O1–O1 edge of the YO5 square pyramid. The oxide-ion conductivity of Ba3Y2O5Cl2 is considerably higher than that of Sr3Sc2O5Cl2, mainly due to a lower activation energy for conductivity, which can be ascribed to the larger free and lattice volumes of Ba3Y2O5Cl2. The DFT energy barrier for oxide-ion migration and experimental activation energy for bulk conductivity in Ba3Y2O5Cl2 are as low as those of a leading oxide-ion conductor La0.9Sr0.1Ga0.95Mg0.05O3−δ. Thus, the present work suggests that RP oxychlorides are a class of potential oxide-ion conductors.