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A new form of Cd₃TeO₆ revealing dimorphism.

ABSTRACT: Phase-formation studies in the system CdO-TeO₃ using a CsCl/NaCl melt at comparatively low temperatures revealed that tricadmium orthotellurate(VI), Cd₃TeO₆, is dimorphic. The new modification of Cd₃TeO₆ is denoted as the β-form and adopts the rhombohedral Mg₃TeO₆ structure type with one Cd and two O sites in general positions, and two Te sites with site symmetry each. In comparison with the previously reported monoclinic cryolite-type α-form that was prepared at higher temperatures, β-Cd₃TeO₆ has a much lower density and most likely represents a metastable modification. Whereas the [TeO₆] octa-hedra in both polymorphs are very similar and show only minor deviations from ideal values, the polyhedra around the Cd^II sites are different, with a distorted [CdO₆] octa-hedron in both modifications but an additional [CdO₈] polyhedron with a [4 + 4] coordination in the α-form.

SUBMITTER: Weil M

PROVIDER: S-EPMC6218891 | biostudies-literature | 2018 Nov

REPOSITORIES: biostudies-literature

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A new form of Cd3TeO6 revealing dimorphism.

Weil Matthias M Veyer Théo T

Acta crystallographica. Section E, Crystallographic communications 20181012 Pt 11

Phase-formation studies in the system CdO-TeO3 using a CsCl/NaCl melt at comparatively low temperatures revealed that tricadmium orthotellurate(VI), Cd3TeO6, is dimorphic. The new modification of Cd3TeO6 is denoted as the β-form and adopts the rhombohedral Mg3TeO6 structure type with one Cd and two O sites in general positions, and two Te sites with site symmetry each. In comparison with the previously reported monoclin ...[more]

PMID: 30443380

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Project description: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 CdII 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 TeIV 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 TeIV 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 CdII 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 TeIV 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 (⋯-⋄-⋄=⋄-⋄-⋄-⋄-⋄=⋄-⋄-⋄-⋯) n 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.

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A new form of Cd₃TeO₆ revealing dimorphism.

Publications

A new form of Cd<sub>3</sub>TeO<sub>6</sub> revealing dimorphism.

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