Oxygen vacancy ordering phenomena in the mixed-conducting hexagonal perovskite Ba7Y2Mn3Ti2O20

A 21R-type mixed conducting oxygen-deficient hexagonal perovskite Ba7Y2Mn3Ti2O20 was synthesized and its structure characterized by variable temperature neutron powder diffraction. The structure is characterized by the presence of both BaO2 and BaO3 layers. In the oxygen-stoichiometric material, the vacancies are ordered in c'-BaO2 layers. Extra oxygen is accommodated by intergrowth of the vacancy-ordered c'-BaO2 layers with cubic c-BaO3-x layers (x congruent to 1), which have disordered 2/3 oxygen occupancy. This material displays mixed electronic and oxide ionic conduction at high temperature, and the role of the two distinct and partially occupied anion sites within the BaO2 layers in permitting the oxygen mobility is discussed.