Oxygen-deficiency-induced 6H-polymorph of hexagonal perovskite Ba ₄YMn ₃O _11.5-δ: Synthesis, structure and properties

The 12-layer hexagonal perovskite Ba ₄YMn ₃O _11.5 (R3m, referred to as 12R) was transformed to a 6-layer mixed valent Mn ^3+/4+ hexagonal perovskite Ba ₄YMn ₃O _10.7 (P6 ₃/mmc, referred to as 6H) via a partial-reduction in a N ₂ flow. This phase transformation between the 12R and 6H phases is redox-reversible. In contrast with the 12R-type Ba ₄YMn ₃O _11.5 structure containing c-BaO _2.75 and h-BaO ₃ layers with a (cchh) ₃ stacking, the 6H-type Ba ₄YMn ₃O _10.7 structure consists of the cubic (c) BaO _2.87 and hexagonal (h) BaO _2.33 layers with a (cch) ₂ stacking, showing a preference of oxygen vacancy distribution in the hexagonal layers over the cubic layers. The h-BaO _2.33 layer in the 6H-type Ba ₄YMn ₃O _10.7 transforms two-thirds of face-sharing octahedral Mn ₂O ₉ dimers into edge-sharing pyramidal Mn ₂O ₈ units, sharing corners with Y octahedra/pyramids. Impedance measurements suggested that the 6H-type material is insulating with a bulk electrical resistivity of ∼10 ⁷ Ω cm at 303 K, significantly higher than that for the 12R-type Ba ₄YMn ₃O _11.5 by ∼4 orders of magnitude.