Linked Octahedra Framework Structures

4) 3-dimensionally Linked Octahedra (Framework Structures) a) Structural Element MeFa - = [MeF6/2]  

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The crystal structure of U6(S04)6(0H)404 (Fig. 17) contains both additional O and OH" anions that are not bonded to The U atoms and additional anions form the [U604(0H)4] polynuclear cations shown in Fig. 15b. The U atom coordination is a regular octahedron, whereas the O and OH anions are under each triangular face of the octahedron. The [U604(0H)4] complexes are linked into frameworks through sulfate tetrahedra. The ions are in square antiprismatic coordination. It is noteworthy that the compound is isostructural with its Ce analogue [89]. 

Figure 15-4. Structures of some zeolites a) basket-like unit (truncated octahedron) of linked Si04 tetrahedra b) framework of baskets as in a), joined at square faces c) space-filling framework of truncated octahedra and cubooctahedra.

The structure of Na2[(U02)(Si04)] [68] contains in an unusual distorted octahedral coordination, with four short bonds ( 2 A) and two longer ( 2.3 A) bond lengths. Each octahedron shares four of its vertices with silicate tetrahedra, and the silicate tetrahedra are linked into four-membered rings. The Na cations are located in voids within the framework (Fig. 23a). 

WO, and ReO, may be regarded as the limiting cases of A-site vacancy perovskites. Both the oxides possess corner-linked framework of the octahedra, but unlike ReO, WO3 is never cubic. It shows several polymorphic transitions starting from the low temperature triclinic structure to more symmetric forms with increasing temperature. The transitions arise from temperature-dependent displacements of the tungsten atom from the centre of the WOg octahedron, ... 

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