Puckered layers

Figure 13.1 Puckered layer structure of As showing pyramidal coordination of each As to 3 neighbours at a distance rj (252 pm). The disposition of As atoms in the next layer (r2 312 pm) is shown by dashed lines.

In graphite each carbon atom is bound to three others in the same plane and here the assumption of inversion of a puckered layer is improbable, because of the number of atoms involved. A probable structure is one in which each carbon atom forms two single bonds and one double bond with other atoms. These three bonds should lie in a plane, with angles 109°28 and 125°16,l which are not far from 120°. Two single bonds and a double bond should be nearly as stable as four single bonds (in diamond), and the stability would be increased by the resonance terms arising from the shift of the double bond from one atom to another. But this problem and the closely related problem of the structure of aromatic nuclei demand a detailed discussion, perhaps along the lines indicated, before they can be considered to be solved. 

Figure 1. Metal-atom layers (A, B, C) and B-atom layers H, H are planar layers K, K are puckered layers .

In contrast to sulfurous acid, H2S03, which is not known as pure substance, the respective selenium compound can be obtained in crystalline form. H2Se03 crystallizes with the non-centrosymmetric space group P2 2,2] and contains pyramidal H2Se03 molecules.54 Within the molecules the bond lengths Se-O and SeOH differ by about 10 pm and the molecules are connected to puckered layers by strong hydrogen bonds. 

This structure can be described as a (puckered) layer type (See Fig. 7.17). In each layer the As-As distance (dx) is considerably shorter than the As-As interlayer... 

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).

Notice, in comparison with A1B2, the different value of c/a and the free parameter in the positions of Cd atoms (0.42 instead of 0.509) which results in puckered layers. 

This bilayer, which is also called a puckered layer, consists of a first layer of adsorbed water molecules combined directly with surface metal atoms and a second layer of water molecules hydrogen-bonded with the first layer of adsorbed... 

The 3D potential map was examined section by section perpendicular to the c-axis. There are totally 6 layers stacked along the c axis in each unit cell. Only two of these 6 layers are unique, one flat layer occurring twice (at z = 0.25 and 0.75) and one puckered layer occurring four times (at z 0.10, 0.40, 0.60 and 0.90). Sections corresponding to the flat (F) and puckered (P) layers are shown in Figs. 6a and b, respectively. The flat layers coincide with mirror planes. The stacking sequence is PFP (PFP" ) , where P relates to P via a mirror reflection on the flat layer, and the (PFP ) block is related to the PFP block by a 63 operation along the c axis. 

The potential maps were remarkably clear (Figs. 5a and b) with most of the peaks perfectly round and well resolved. 63 and 68 peaks were peak found from the density maps in the flat (Fig. 5a) and puckered layers (Fig. 5b), respectively. Out of the 131 peaks, 7 were removed due to too short interatomic distances. It was possible to distinquish Al and transition metal atoms according to the peak height and chemical knowledge. There are in total 129 unique atoms in the unit cell, 62 atoms in the flat layer and 67 atoms in the puckered layer. The structure model and the potential map are superimposed in Fig. 5. 

There is sometimes ambiguity as to which nearest neighbours to assign to the local coordination polyhedron. This is illustrated by the two different puckered layer structure types which are displayed by the most stable polymorphs of phosphorus and arsenic respectively. They may both be thought of as resulting from the breaking of three bonds about each atom on a simple cubic lattice as shown in Fig. 1.5. The layers then distort Black... 

The 8-N rule states that the number of bonds (or local coordination, x) equals 8 minus the number of the periodic group. This rule is illustrated in Fig. 1.2 where we see that for N — 7 the halogens take dimeric structure types with x = 1, for N = 6 the chalcogenides selenium and tellurium take helical chain structures with x = 2, for N = 5 the pnictides arsenic, antimony, and bismuth take a puckered layer structure with x = 3, and for N = 4 the semiconductors... 

Another useful member of the vanadium oxide group is the LiVsOg vanadium bronze. Its basic structure is formed by octahedral and trigonal bipyramids arranged to form puckered layers between which the Li+ ions... 

Figure 4.18. Four cells of As showing the puckered layers. Positions of close-packed layers are shown.

Figure 9.24. The structure of Calri2. Ca atoms are centered between hexagons in puckered layers Larger circles are Ca atoms.

Fig. 1. The puckered layers of [MF4J units in the structures of (a) silver and (b) copper difluorides. Single circles show atoms in one puckered layer and dashed circles those in the adjacent layer below. Those atoms from the adjacent layer above, which complete the distorted octahedral coordination, are shown hatched. (In silver difluoride they are directly above the dashed circles.)...

The change in symmetry from copper to silver is related to rather subtle structural differences. If only the four short M—F bonds are considered, then both structures consist of puckered layers formed by joining the square, [MF4] units through corners. These puckered layers lie directly over one another, related by a translation in copper difluoride, but are displaced alternately, related by a glide plane, in silver difluoride (27), (see Fig. 1). In both structures, the fluorine atoms are arranged in considerably distorted hexagonal close-packed arrays, the distortion being more marked for silver. 

If the formula of orthorhombic black phosphorus is written simply as P, then Z = 8. The special positions of space group Cmca are 4(a) 2/m, 4(b) 2/m, 8(c) 1,8(d) 2, 8(e) 2, 8(f) m, and general position 16(g) 1. The independent P atom actually is located in 8(f), generating a continuous double-layer structure, which is a heavily puckered hexagonal net in which each P atom is covalently bound to three others within the same layer. In Fig. 9.6.12, two puckered layers in... 

The structure of SnCl2 is shown in Fig. 14.5.1(a). In the crystalline state it has a layer structure with chains of corner-shared, trigonal-pyramidal SnCb units. The commercially available solid hydrate SnCl2-2H20 also has a puckered-layer structure. 

N,N,-bis(4-pyridylmethyl)-l,4-diaminobutane, and its subsequent reactions to yield linear polymers, a puckered layer, and a molecular triangle. 

While the diamagnetic AuSb2 is a metallic and even superconducting (27) d7 pyrite, the corresponding Cu and Ag phosphides are both diamagnetic semiconductors (33, 134). The cation, therefore, must be either mono- or trivalent. The monoclinic structure of CuP2 has recently been determined by Olofsson (135) [C h — P2i/c, all atoms in 4(e)], The phosphorus atoms form infinite puckered layers almost identical to those met... 

This form of carbon is almost invariably found with the cubic structure shown in Fig. 7-1. There is also a hexagonal form (lonsdaleite),5 found in certain meteorites and also available synthetically, in which the puckered layers are stacked in an ABAB- pattern instead of the ABCABC- pattern. The hexagonal form is probably unstable toward the cubic, since unlike the cubic, it contains some eclipsed bonds. 

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