Hexagonal cell layers

Figure 10. Projection of of the crystal of lithiophorite, (Li,Al)Mn02(0H)2. along the LUO] direction of the hexagonal cell [58], The connections within the Mn06 and (Li,Al)(OH)6 octahedra layers are emphasized. For a better understanding the O - H bridging bounds between the two layer types are not shown.

The outermost layer of the skin appearing in the exploded epidermal sketch of Fig. 1(b) represents the stratum corneum (the horny layer). The principal barrier element of the skin, it is an essentially meta-bolically inactive tissue comprised of acutely flattened, stacked, hexagonal cell building blocks formed from once-living cells. These cellular building blocks are... 

Cl 6c +(u,u,u +m, + m, + m + w, + w, +m) w= This structure has a rhombohedral symmetry but is usually described in terms of a triple-volume hexagonal cell, which makes comparison with the idealized Cdl2 structure simpler. The idealized unit cell is adequate for the purposes of this book. In this representation the anion layers are in cubic closest packing. .. ABC ABC ABC... The metal and nonmetal stacking sequence is... 

Figure 3.20. A lateral view of different stacking sequences of triangular nets. They correspond to some typical close-packed structures. The first layer sequence shown corresponds to a superimposition according to the scheme ABABAB... (equivalent to BCBCBC... or CACACA... descriptions) characteristic of the hexagonal close-packed, Mg-type, structure. With reference to the usual description of its unit cell, the full stacking symbol indicating the element, the relative position of the superimposed layers and their distance is Mg Mg. The other sequences correspond to the schemes ABC.ABC. (Cu, cubic), ABAC.ABAC. (La, hexagonal), ACACBCBAB. (Sm, hexagonal). For Cu the constant ch of the (equivalent, non-conventional) hexagonal cell is shown which may be obtained by a convenient re-description of the standard cubic cell (see 3.6.1.3). ch = cV 3, body diagonal of the cubic cell.

Figure 7.17. The As-hR6 structure (a) a puckered layer of As atoms is viewed along the c axis of the hexagonal cell and (b) lateral view of two superimposed layers. Notice the coordination 3 of the atoms, characteristic for the element of the 15th group (five valence electrons).

Fig. 37. Photographs of wash-coat layer on monolithic substrate with square cells (left) and hexagonal cells (right) (see Plate 1 in Color Plate Section at the end of this book).

BENARD CONVECTION CELLS. When a layer of liquid is heated from below, the onset of convection is marked by the appearance of a regular array of hexagonal cells, the liquid rising in the center and falling near the wall of each cell. The criterion for the appearance of the cells is that the Rayleigh number should exceed 1700 (for rigid boundaries). 

A single layer of flat hexagonal cells of the corneal endothelium covers the posterior corneal surface and hydrates the cornea. The corneal endothelium can allow diffusion of molecules of dimensions up to 20 nm.65 The stroma has a highly organized hydrophilic tissue structure that comprises 90 percent of cornea.63 It has an open structure that can allow molecules up to 500,000 Da in size to pass through. However, the stroma may be a diffusion barrier to lipophilic drugs.65,84 Drug-melanin interactions such as the one with timolol can form a barrier and reduce bioavailability.85... 

The structure of BaNiCh is closely related to perovskite (CaTiC>3, 3 2Pi/4 3/4O1/4), but the packing sequence is hep rather than ccp as for perovskite. The hexagonal cell contains two molecules, CB , C63me, a0 = 5.580 and c0 = 4.832 A. As shown in Figure 5.40, each Ni atom is at the center of an octahedron formed by six O atoms. With P layers at B and C positions all Ni atoms in O layers are at A positions, sharing three O atoms with another Ni atom. The Ba atom is at the center of a trigonal prism formed by six Ni atoms. The packing sequence of this structure is shown below ... 

Figure 5.42. A projection on the base of the BaMn03 hexagonal cell. The small heavy circles are Mn and the larger ones are Ba. Above there is a packing drawing along the c axis. The Mn atoms in O layers are black. Oxygen atoms are dot shaded and smaller than the Ba atoms.

Figure 5.47. A projection on the base of the VF3 hexagonal cell and a packing drawing with layers labeled (V atoms are small and black).

Introducing vacancies into the simple 2-2PT structure causes more complexity. For Tn2Se3, there are one-third vacancies in the T layer occupied, giving 2 6PT2/3 with 12 layers in the repeating unit. The Se atoms are in an hep pattern with In atoms in two-thirds of one T layer as shown in Figure 6.8. There are six molecules in the hexagonal cell, Cg, P65, a0 = 7.11, and c0 = 19.3 A. 

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