The cadmium oxidotellurates(IV) Cd₅(TeO₃)₄(NO₃)₂ and Cd₄Te₅O₁₄

Monoclinic single crystals of Cd₅(TeO₃)₄(NO₃)₂ (space group P2₁/c), penta-cadmium tetra-kis-[oxidotellurate(IV)] dinitrate, and of Cd₄Te₅O₁₄ (space group C2/c), tetra-cadmium penta-oxidotellurate(IV), were obtained under the same hydro-thermal conditions. Whereas the crystal structure of Cd₅(TeO₃)₄(NO₃)₂ is distinctively layered, that of Cd₄Te₅O₁₄ exhibits a tri-periodic framework. In Cd₅(TeO₃)₄(NO₃)₂, the three Cd^II atoms have coordination numbers (CN) of 7, 6 and 6. The two types of [CdO₆] and the [CdO₇] polyhedra [bond lengths range from 2.179 (3) to 2.658 (2) Å] share corners and edges, resulting in layers extending parallel to (100). Both Te^IV atoms are coordinated by three oxygen atoms in a trigonal-pyramidal shape. The oxygen atoms of the isolated [TeO₃] groups [bond lengths range from 1.847 (3) to 1.886 (3) Å] all are part of the cadmium-oxygen layer. The electron lone pairs ψ of the Te^IV atoms are directed away from the layer on both sides. The available inter-layer space is co-occupied by the nitrate group, which is directly connected with two of its O atoms to the layer whereas the third O atom is solely bonded to the N atom and points towards the adjacent layer. In Cd₄Te₅O₁₄, all three unique Cd^II atoms are coordinated by six oxygen atoms, considering Cd-O distances from 2.235 (2) to 2.539 (2) Å. By edge- and corner-sharing, the distorted [CdO₆] polyhedra form an open framework that is partially filled with three different stereochemically active Te^IV atoms. All of them exhibit a CN of 4, with Te-O bonds in a range from 1.859 (2) to 2.476 (2) Å. The corresponding [TeO₄] units are linked to each other by corner- and edge-sharing, forming infinite helical ¹ _∞[Te₁₀O₂₈] chains extending parallel to [203]. The connectivity in the chains can be described as (⋯-⋄-⋄=⋄-⋄-⋄-⋄-⋄=⋄-⋄-⋄-⋯) where '⋄' denotes a [TeO₄] unit, '-' a linkage via corners and '=' a linkage via edges. Such a structural motif is unprecedented in the crystal chemistry of oxidotellurate(IV) compounds.