(a) The ball-and-stick and polyhedral composite representation showing parts of the Mn—As—O framework. The polyhedral units represent AsO4 tetrahedra. One of the [Mn4O18]28− units is located in the area outlined by a dotted ellipsoid. The only unshared oxygen atom, O6 of the As1O4 tetrahedra, forms a bond with a Ba cation. (b) A tetrameric unit formed by two edge-sharing Mn1O6 octahedra and two corner-sharing Mn2O6 octahedra. (c) Three [Mn4O18]28− units showing the intertetramer interaction through long Mn1—O5 and Mn2—O5 bonds (dotted lines).