Tetrahedra face-sharing

In the sphalerite structure the anions form a cubic close packed array. The structure has a single adjustable parameter, the cubic cell edge. The 0 ions are too small for them to be in contact in this structure (see Fig. 6.4) so ZnO adopts the lower symmetry hexagonal wurtzite structure which has three adjustable parameters, the a and c unit cell lengths and the z coordinate of the 0 ion, allowing the environment around the Zn " ion to deviate from perfect tetrahedral symmetry. In the sphalerite structure the ZnX4 tetrahedron shares each of its faces with a vacant octahedral cavity (one is shown in Fig. 2.6(a)), while in the wurtzite structure one of these faces is shared with an empty tetrahedral cavity which places an anion directly over the shared face as seen in Fig. 2.6(b). The primary coordination number of Zn " in sphalerite is 4 and there are no tertiary bonds, but in wurtzite, which has the same primary coordination number, there is an additional tertiary bond with a flux of 0.02 vu through the face shared with the vacant tetrahedron. 

Figure 8-11 The three face-sharing tetrahedra of Pb atoms in the Pb60(0H) + cluster. The middle tetrahedron has an O atom at its center. The end tetrahedra have OH groups.

Figure 25.13 Metal frameworks of some Ru and Os carbonyl clusters with interstitial atoms, (a) [Ru8(H)2(CO)2i] (octahedron and face-sharing trigonal bipyramid) the second H is probably at the centre of the octahedron. (b) [Ru8(C)2(CO)n(PPh2)2] (octahedron and face-sharing square pyramid) PPh2 ligands bridge the pairs of shaded Ru atoms, (c) [Os7(H)2C(CO)i9] (tetrahedron and 3 irregularly spaced metal atoms) H atoms probably bridge two edges of the tetrahedron.

Figure 5. Schematic representation in two dimensions of a dehydrated but fully hydroxylated colloidal silica particle. The fourth oxygen coordinated with Si is above or below the plane of the paper. The figure is only a diagram, not a model. In an amorphous silica model the Si-O-Si bond angle may vary, but the Si-O distances are constant each oxygen ion is linked to not more than two cations the coordination number of oxygen ions about the control cation is 4 or less oxygen tetrahedra share comers, not edges or faces and at least two comers of each tetrahedron are shared.

Cation-cation repulsion in (a) corner-shared tetrahedrons, (b) edge-shared tetrahedrons, (c) face-shared tetrahedrons, (d) corner-shared octahedrons, (e) edge-shared octahedrons, and (f) face-shared octahedrons. The degree of cation-cation repulsion increases with the degree of sharing and the oxidation number of the cation and decreases with an increase in the coordination number. [Reproduced from Klein, C. Manual of Mineral Science, 22nd ed., Wiley New York, 1977. This material is reproduced with permission of John Wiley Sons, Inc.]... 

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