Unconventional Oxygen Storage/Release Properties of Melilite-Type Ba 2 MnGe 2 O 7+δ Associated with Complex Structural Transformation

High Resolution Image Download MS PowerPoint Slide Here, we report the synthesis and characterization of melilite-type A 2 Mn C 2 O 7 ( A = Sr, Ba; C = Si, Ge) and the discovery of its unconventional oxygen storage and release properties. Unlike conventional Mn-containing oxygen storage materials driven by the Mn 2+ /Mn 3+ redox couple, Ba 2 MnGe 2 O 7+δ (BMG) does not require highly reducing atmospheres for oxygen release and exhibits reversible oxygen storage/release under oxygen-rich conditions at moderate temperatures (200–500 °C). Comprehensive compositional and structural analyses utilizing X-ray absorption spectroscopy, synchrotron in situ powder X-ray diffraction, single-crystal X-ray diffraction, and powder neutron diffraction revealed that the oxygen storage/release processes involve changes in the local coordination environment. Specifically, MnO 4 tetrahedra in the reduced phase change into MnO 5 trigonal bipyramids in the oxidized phase, accompanied by a distinct transformation from the fundamental melilite-type structure of Ba 2 MnGe 2 O 7 (tetragonal, space group P 4̅2 1 m ) to a 5 a × 5 a × 1 c superstructure of Ba 2 MnGe 2 O 7.455(4) (tetragonal, P 4̅). BMG exhibits a maximum oxygen storage capacity of δ ≈ 0.45 and, notably, develops a distinctive blue color upon oxygen storage. This characteristic response suggests promising potential for various oxygen-related applications, such as oxygen sensors and oxygen-sensitive inorganic pigments.