Octahedra, stacking

This compound is isostructural with brucite (Mg(OH)2) and Cdl2. The unit cell is hexagonal with a = 0.3258 nm and c = 0.4605 nm. The structure consists of sheets of corner-sharing, trigonally distorted Fe(OH)6 octahedra stacked along the [001] direction. The Fe" ions occupy only half the available octahedral interstices and this results in a structure in which each filled layer of sites alternates with an empty layer of sites. The OH radical behaves as a single entity. Amakinite is a rare mineral of the composition (Fe,Mg,Mn)(OH)2, also with brucite structure. Fe(OH)2 is readily oxidized by air and even by water, upon which the colour changes from white to brownish. The structure can be maintained up to a replacement of one tenth Fe" by Fe " (Bernal et al., 1959). 

Finally, the crystal structure of [ ( 2)8 ]2[( 12)( 14)(1 )2] [24] features layers of corner-shared nominal Aul6 octahedra stacked along the a axis, with... 

The isopoly anions may be considered to be portions of a closest packed array of oxide ions with the metal ions occupying the octahedral holes. The edge-sharing array found in [V n02S]6 consists of ten octahedra stacked as shown in Fig. 16.10a. This seems to be the largest stacked-octahedral isopoly anion cluster compatible with metal-metal repulsions, and the remaining edge-shared structures represent portions of this unit.46... 

Figure 56 Octahedra stacked along c-axis in the rutile structure (looking down the c-axis).

Very often, metal oxide surfaces will reconstruct when exposed to water as was discussed for MgO. We will illustrate the importance of such a reconstruction by comparing the reactivity of the (001) surface of anatase (Ti02) with that of the rutile (110) surface of Ti02- Both are the siufaces of lowest energy for the respective phases. The difference between anatase and rutile relates to the stacking of the TiOe octahedra in both phases. In anatase, the octahedra stack tetrahedrally, whereas in rutile they stack parallel. Quantum-chemical results are available for both phases as well as for their corresponding surfaces. We refer here to the work of Arrouvel et al. on the (100) siuface of Ti02 anatase and of Lindan et al. on the (110) siuface of rutile. The respective surfaces are shown in Fig. 5.4a and b. 

Fig. 24. Anderson and Hyde model for forming a crystallographic shear plane in WO3. The perfect structure consists of WOg octahedra stacked as shown. In the Anderson and Hyde model anion vacancies are imagined to collect as shown on the left-hand side of the figure and then collapse to give the structure shown on the right. Along the fault the stacking arrangement of the WOj tetrahedra is not the same as in good material and also the structure is sheared along the fault plane by the vector 5cj[TT0]. From Hyde and Bursill. ...

NbOF3 and TaOF3 are characterized by very low stability, are sensitive to moisture and decompose in air even at ambient temperature. Nevertheless, Kohler et al. [217] succeeded in investigating these compounds. Both compounds crystallize in a SnF4 type structure forming planes that share oxyfluoride octahedrons with each other, at four comers. These planes are stacked in a three-dimensional lattice via van der Waals interactions [217]. 

From Fig. 17.1 we realize another fact. The octahedron centers are arranged in planes parallel to the a-b plane, half-way between the layers of spheres. The position of the octahedron centers corresponds to the position C which does not occur in the stacking sequence ABAB... of the spheres. We designate octahedral interstices in this position in the following sections by y. By analogy, we will designate octahedral interstices in the positions A and B by a and j3, respectively. 

The crystal structures of the 2,3-516 and 2,6-dicarboxylates51 have been determined. In the former, the silver atom lay on a centre of symmetry and was strongly bound to the 2-carboxyl group (Ag—O 213.1 pm) and to the nitrogen atom (Ag—N 212 pm). The overall structure was a tetragonally distorted octahedron, since stacking occurred and weak interactions existed between the silver atom and the ketonic group of the 3-carboxylic acid (Ag—O 298 pm).516... 

Clay minerals are formed by two building blocks [93] tetrahedrons of oxygen with Si4+ ions in their centers or octahedrons of oxygen with Al3+ or Mg2+ in their centers. The tetrahedrons share oxygens and form hexagonal rings. Some oxygen atoms form hydroxyls, in particular when the clay is filled with Ca2+. This pattern can be repeated ad infinitum to form flat tetrahedral sheets. Similarly, the octahedrons are linked to form octahedral layers. The tetrahedral and octahedral sheets can be stacked on top of each other in various forms to build the different kinds of clays. 

The cubic analogue to the NiAs type is the rocksalt structure. It may be considered as a stacking variant of the former. Here both anions and cations are coordinated by an octahedron formed by atoms of the other kind. In NiAs only the metals have an octahedral environment while the non-metal atoms have a trigonal-prismatic coordination. 

The cores of 406 and 407 are each 3/4/3 stacks (alternatively described as a bi-octahedron with a broken shared edge or as two distorted fused trigonal prisms) with idealized symmetry C2h which encloses a C2 unit. The Co and Ni atoms cannot be distinguished and are probably distributed randomly. The carbides are in distorted seven-vertex cages (capped trigonal prisms) with M-C distances between 1.90-2.15 A (mean 2.07 A). 

In junoite, Cu2Pb3Big(S, Se)ig (Fig. 31), described by Mumme , Bi2Pb(S, Se)5 layers of B1 (lll)f c.c. type are stepped by repeated offset through one octahedron width after each period of two H subcells. The T layers are Bl (100)t,c.c, type fragmented into ribbons 3 primitive T subcells wide (of composition Bi4Pb2Sg) that are offset, en echelon in the stacking direction by their own thickness, and united (at the gap between two adjacent ribbons, and between the steps of the two adjacent H layers) by the insertion of pairs of tetrahedrally-coordinated Cu atoms. T ribbons and H layers share a common S atom row at each step. 

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