Layered Structured Hexagonal Oxide

Figure 8.14 Layer structure of oxides with composition Lii-yNii+yCU (a) Nii.o202, with the oxygen atoms in hexagonal... ABAB. .. sequence and (b) nominal compositions between Lio.o7Ni02-Lii.oNi02 have the oxygen atoms in cubic. .. ABCABC... sequence.

The structure of La203 [Section 5.5.10, 3 5/3POPPO(h), Figure 5.64) is known as the A-type M203 or A-type rare-earth structure. It is rather complex. The unit cell is hexagonal, but oxide ions are in an A, B, C sequence. La3+ ions fill two O layers with the next O layer vacant. This is a layer structure because the sequence is POP POP, but the first and sixth P layers are identical thus, five layers POP PO repeat. La has CN 7 the seventh oxide ion is from the next POP sandwich. 

When pitch binder is pyrolyzed during the carbon bake operation, it is converted from an isotropic liquid, with no structural order, to a liquid-crystal (called mesophase) having a layered structure which is finally converted to layers of carbon atoms in a hexagonal lattice of graphite crystallites. These crystallites of binder coke become more disordered and crosslinked into a more-isotropic coke as the pitch QI content increases. Such moderately-isotropic coke, in contrast to highly-anisotropic microstructure (10), is preferred binder coke because it forms both physical and chemical bonds between filler coke particles which are stronger and more oxidation-resistant (8,9). 

An idealized three-dimensional representation of calcium montmorillonite structure at the atomic level is shown in Figure 1. The 2 1 layered structure is composed of upper and lower layers of silicon oxide tetrahedra linked in hexagonal arrays to form two-dimensional silicate sheets extending in the a,b-directions. Sandwiched between these sheets are partially filled two-dimensional sheets composed of... 

For films of titania grown on metal substrates, it is readily apparent that the overlayer is an oxide rather than an alloy as the superstructures only form in the presence of oxygen and oxygen is seen in XPS. X-ray photoelectron diffraction (XPD) has also been employed to probe the layer structure and most phases (including all of the hexagonal phases) the overlayers are proposed to form with a Pt-Ti-0 stacking sequence in the monolayer [29],... 

For specificity, let us consider the application of these ideas to a-CraOa. Its crystal structure can be represented as a hexagonal close-packed lattice of oxide ions (the closed-packed layers of oxide ions alternate ababab. ..) in which two-thirds of the octahedral holes are filled with Cr3+ ions in a systematic fashion. Suppose the crystal to be cleaved in a close-packed plane in the presence of water. To preserve electrical neutrality, the oxide ions in this plane must be equally divided between the surfaces of the faces being formed. As a result, each Cr3+ in the layer below these oxide ions would be five-coordinate and in a square pyramidal configuration. Each ion would react with a molecule of water following which a proton would move from each adsorbed water molecule to an adjacent oxide ion. Thus, the outer face would consist of a close-packed layer of hydroxide ions. This is shown in Fig. 2. The basic point is that electrical neutrality and six-coordination can both be preserved by replacing what would be a plane of oxide ions in bulk by an equivalent plane of hydroxide ions at the surface. Similar ideas obtain on alumina. 

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