(NH4)4Cd(HSeIVO3)2(SeVIO4)2: a new structure type with kröhnkite‐like chains

Tetra­ammonium cadmium di­hydrogenselenite(IV) diselen­ate(VI), (NH₄)₄Cd(HSe^IVO₃)₂(Se^VIO₄)₂, is the third example of a compound containing both hydrogen selenite and selenate anions, and has a new structure type. It contains kröhnkite‐like heteropolyhedral chains in which CdO₆ octahedra are linked via bridging HSeO₃ groups, having their remaining two trans apices decorated by SeO₄ groups. The charge‐balancing NH₄ groups are involved in weak hydrogen bonding, whereas the H atom of the HSeO₃ group provides a strong hydrogen bond O⋯O = 2.614 (5) Å]. The average Cd—O bond length is 2.298 Å. All atoms are on general positions except Cd (on ). Relations to the kröhnkite‐type compounds Na₂Mg(SO₃)·2H₂O, Ba₂CoCl₂(SeO₃)₂ and Ba₂Ca(HPO₄)₂(H₂PO₄)₂, and to the mineral curetonite are dis­cussed. Unit‐cell data are given for an isotypic Mn^II analogue.