Framework structures containing octahedra

Mixed framework structures containing octahedra and tetrahedra. Silicates,... 

The structure of CaB contains bonding bands typical of the boron sublattice and capable of accommodating 20 electrons per CaB formula, and separated from antibonding bands by a relatively narrow gap (from 1.5 to 4.4 eV) . The B atoms of the B(, octahedron yield only 18 electrons thus a transfer of two electrons from the metal to the boron sublattice is necessary to stabilize the crystalline framework. The semiconducting properties of M B phases (M = Ca, Sr ", Ba, Eu, Yb ) and the metallic ones of M B or M B5 phases (Y, La, Ce, Pr, Nd ", Gd , Tb , Dy and Th ) are directly explained by this model . The validity of these models may be questionable because of the existence and stability of Na,Ba, Bft solid solutions and of KB, since they prove that the CaB -type structure is still stable when the electron contribution of the inserted atom is less than two . A detailed description of physical properties of hexaborides involves not only the bonding and antibonding B bands, but also bonds originating in the atomic orbitals of the inserted metal . ... 

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). 

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]. 

We have already discussed the structure of a crystal containing atoms with octahedral coordination, the Prussian blue crystal. Each of the iron atoms in the framework of this crystal forms six bonds directed toward the corners of a regular octahedron (see plates 27 and 28). 

For tetranuclear clusters to incorporate main-group elements interstitially, they must adopt a more open skeletal arrangement. The M4 butterfly cluster framework can be derived from the tetrahedron by cleaving a metal-metal edge alternatively, it can be viewed as an orac/mo-octahedron (Scheme 1). Butterfly clusters containing semi-interstitial B, C, N, and O atoms have been observed and they all adopt similar structures in which the heteroatom interacts with all four metal atoms and is located in a rather exposed position midway between the wing-tip atoms of the M4 skeleton. 

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