New Ba M 2 (SeO 3 ) 3 · n H 2 O ( M = Co, Ni, Mn, Mg; n ≈ 3) Zemannite‐Type Frameworks: Single‐Crystal Structures of BaCo 2 (SeO 3 ) 3 ·3H 2 O, BaMn 2 (SeO 3 ) 3 ·3H 2 O and BaMg 2 (SeO 3 ) 3 ·3H 2 O

The syntheses and characterisation of a new family of isostructural mixed‐metal selenites based on the zemannite‐type framework are described. These Ba M 2 (SeO 3 ) 3 · 3H 2 O phases are built up from three‐dimensional networks of M O 6 ( M = Co, Ni, Mn, Mg) octahedra and SeO 3 pyramids. The octahedral groups form isolated M 2 O 9 face‐sharing pairs, which are linked by the selenite anions into a three‐dimensional anionic framework. The framework encapsulates one‐dimensional 12‐ring channels occupied by the barium cations and water molecules. There are two types of channels: in one, the cations and water molecules are well ordered, and the channels are fully occupied; in the other, the same species are disordered, and the channels are partially occupied. There is little if any evidence of zeolitic reversible de/rehydration and cation ion‐exchange behaviour for these materials.