Oxygen-deficiency-induced 6H-polymorph of hexagonal perovskite Ba4YMn3O11.5−δ: synthesis, structure and properties†
The 12-layer hexagonal perovskite Ba4YMn3O11.5 (Rm, referred to as 12R) was transformed to a 6-layer mixed valent Mn3+/4+ hexagonal perovskite Ba4YMn3O10.7 (P63/mmc, referred to as 6H) via a partial-reduction in a N2 flow. This phase transformation between the 12R and 6H phases is redox-reversible. In contrast with the 12R-type Ba4YMn3O11.5 structure containing c-BaO2.75 and h-BaO3 layers with a (cchh)3 stacking, the 6H-type Ba4YMn3O10.7 structure consists of the cubic (c) BaO2.87 and hexagonal (h) BaO2.33 layers with a (cch)2 stacking, showing a preference of oxygen vacancy distribution in the hexagonal layers over the cubic layers. The h-BaO2.33 layer in the 6H-type Ba4YMn3O10.7 transforms two-thirds of face-sharing octahedral Mn2O9 dimers into edge-sharing pyramidal Mn2O8 units, sharing corners with Y octahedra/pyramids. Impedance measurements suggested that the 6H-type material is insulating with a bulk electrical resistivity of ∼107 Ω cm at 303 K, significantly higher than that for the 12R-type Ba4YMn3O11.5 by ∼4 orders of magnitude.