T layers

O 1 t layer atom 2nd layer atom 0 3rd layer atom... 

The least compHcated clay minerals are the 1 1 clay minerals composed of one tetrahedral (T) layer and one octahedral (O) layer (see Fig. 1). These 1 1 clay minerals are also referred to as TO minerals. The TO package has a basal spacing (nominal thickness) of 0.7 nm (7 E) and they are commonly referred to as 7 E minerals. Kaolinite, the dioctahedral 1 1 mineral, has filling two of three octahedral sites, and serpentine [12168-92-2J, (Mg)3Si205(0H)4, the trioctahedral 1 1 mineral has filling all three octahedral sites. The kaolin minerals have limited substitution in the octahedral... 

Capacity. This property refers to the loading of solute per weight of extraction solvent that can be achieved in an extrac t layer at the plait point in a Type I system or at the solubihty hmit in a Type II system. 

Figure 7.87 shows a AG -concentration diagram for Fe(j,-Zn( ). It was constructed from the experimental data shown in Table 7.37. The method of construction is described elsewhere. Figure 7.87 can now be used, by applying the constraints imposed by the tangency rule, to explain why in Fig. 7.88a and b, where the chemical potentials (shown in the diagram) of zinc vapour varied between 0 and - 1 - 81 kJ molthe total interaction surface layer consisted of T, T, 6, and flayers in Fig. 7.88c at a chemical potential only slightly lower ( — 2-11 kJmol ) only T and T, layers were present whilst at -2-55 kJ mol only a F outermost layer was formed. 

On the experimental front, Burrows and Forrest 155] have measured the electric field and thickness dependence of the current and radiance from bilayer devices with various HTLs and Alqs as the ETL. The data were analyzed in temis of trap-limited transport in the Alq t layer, with the assumption that the voltage drop across the HTL is negligible. However, this assumption was challenged by Vestweber and Riess [ I56 and Giebcler et al. 1157], who demonstrated that HTL plays an important role in determining the efficiency of bilayer OLEDs. 

The intrinsic material fabricated at the frequencies reported above was incorporated in p -i-n solar cells [493]. The p- and n-layer were prepared by the conventional 13.56-MHz discharge. The device quality films indeed yield good solar cells, of 10% efficiency, as is shown in Figure 60. This cell is manufactured with a 500-nm-thick t -layer made at 65 MHz with a power density of 42 mW/cm , resulting in excellent properties. The deposition rate still is 2-3 times higher than... 

CS348 Daniels, 1. R. and G. T. Layer. How should gynaecomastia be managed ... 

The least complicated clay minerals are llu- 1 1 clay minerals composed of one tetrahedral [Pg.387]

The loss of mannan caused by the mannanase treatment of sprucewood holocellulose seems to be similar in the Si and T layers. This has also been reported by Hoffmann and Parameswaran (19). 

The removal of xylan by the xylanase appeared more pronounced in T than in Si. This is in disagreement with microdensitometrical evaluations of substance losses in cell walls of spruce- and pinewood holocellulose reported by Boutelje and co-workers (5,6). They found that the hydrolysis of xylan by xylanase treatment occurred mainly in the S2 layer the Si and in some cases also the S3 (T) layer appeared not to be affected. In the same way, they showed that the Si layer is more resistant to cellulolytic attack than the other cell wall layers. 

In the intermediate layer a surplus of negative charge arises. This is compensated by cations of the O layer [2,3,4], initially Mg2+ and Fe2+. The thus formed vacancy in the O layer is filled with a Si4+ from the T layer [4] or an Al3+ from the O layer. By now the disintegration has reached an advanced stage. An example of erosion in a chemical equation is the formation of the clay mineral kaolinite from orthoclase, an alkaline feldspar. 

A number of Si4+ ions in the T layer can have been replaced by Al3+. Again, there is a shortage of positive charges which is compensated in the same way as described in 3. 

All P, O, and T layers have the same hexagonal close-packed arrangement within each layer. The two T layers are equivalent for ccp and hep, and for ccp, only P and O layers are interchangeable, and together they are equivalent to the two T layers (considered together). Because of these similarities, ccp, hep, the simple cubic structure, and even bcc structures can be handled in the PTOT system. It also applies to much more complex structures. The PTOT system provides a framework for considering the mechanism of formation and transformation of crystal structures. The transformations of structures of metals, ccp, hep, and bcc, are of particular interest. These are considered in detail in Chapter 4. 

It is important to note that each sphere is part of two tetrahedra, so there are twice as many T sites as P sites or between any two close-packed P layers there are two T layers, T+ and T. The positions (A, B, or C) of the T sites correspond to the apex P positions. For hep, these are A and B. In the sequence from the Pa layer, for the first T+ layer each tetrahedron has its apex in the second P layer (Pg), and in the second T layer, each tetrahedron has its apex in the first layer (Pa) giving PaTbTaPbTaTb Pa— Figure 3.2c shows three P layers and the tetrahedra between these with apices in the Pg layer. The bases are staggered (Pa and Pc). 

The PO structures involve P and O layers with both T layers empty. The most common structure of PO type is the NaCl structure described by 3 2PO. The larger Cl- ions are in the ccp P layers and Na+ ions fill all O layers (Figure 3.6). In a ccp PO structure the P and O layers are identical. We could reverse the labels in the figure, letting the light balls represent Cl- and the dark ones Na+. NiAs is an example of an hep PO structure or 2 2PO, but this structure is not common for hep because of interactions of atoms in O layers, all in C positions (see Section 5.2.1). 

The PT structure has filled P layers and one T layer filled between P layers. Zinc sulfide has two modifications zinc blende or sphalerite... 

In Figure 3.7, we can see that P and T layers occupy only A and B positions. Let us focus on a Pg layer. Just below and above the Pg layer there are Ta layers. These T sites are very close, with no shielding. For hep structures no examples are encountered for PTT or PTOT with both T layers filled without unusual features (pp. 139-144). Partial filling of both T layers avoids repulsion involving adjacent T sites. 

Tj,0/jTf.. The O layers are in a ccp sequence (CABC...) and the T layers have a ccp sequence (for T+BG4. .. and for T ABCBecause each P layer has two close T layers (e.g., T PgTc) and each O layer has two close T layers (e.g., T OgTc) with exactly the same relationships, we can reverse designations of P and O layers. This is true for ccp, but not for hep. The full 3 4PTOT ccp structure is shown in Figure 3.8. Note the equivalence of P and O layers, exactly as for NaCl. We could also reverse the roles of pairs of T layers (T+ and T ) with the roles of P and O together. The first T layer above a P layer is T+, and this becomes O and T becomes P. P and O layers become T layers ... 

In the PTOT scheme some P, O, and T layers can be empty or partially filled. This is determined by the stoichiometry of the compound, relative sizes of ions or atoms, and preferences for coordination number (CN). Each P site has 12 nearest P sites (CN = 12). An O site has six near P sites, and a T site has four nearest P sites. 

The partial filling of a P, O, or T layer can be shown as Lp2, L2/3, etc. This can result from empty sites or in cases where more than one kind of atom form the same layer. The common cases are described below. 

The zinc blende structure of ZnS (3 2PT) has S2- filling ccp P layers and Zn2+ filling one T layer. For Li20 (3 3PTT, the antifluorite structure), the O2- ions fill ccp P layers with Li+ ions filling both T layers. The NaCl structure has the 3 2PO structure, with CP ions filling ccp P layers and Na+ ions filling O layers. Cadmium chloride (CdCh) has a... 

Not all of these patterns are encountered for hep structures because there are strong interactions between O sites because they are all in C positions without shielding. There is also strong interaction between close adjacent T layers preventing full occupancy of both T layers for hep. These problems are avoided for ccp structures because P, O, and T sites are staggered at A, B, and C positions. 

Figure 4.13. The 3 2PT structure of diamond. The packing (P) layers are in a ccp pattern. All sites are occupied by C, but here the balls in the T layers are lighter. The cubic unit cell is outlined by double lines.

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