Layer hexagonal

Figure 6.27 Comparison of the hexagonal layer structures of BN and graphite. In BN the atoms of one layer are located directly above the atoms of adjacent layers with B - N contacts in graphite the C atoms in one layer are located above interstices in the adjacent layer and are directly above atoms in alternate layers only.

The dimensions of the unit in the basal plane closely approximate those for the similarly pseudohexagonal crystal hydrargillite, Al(OH)3, as well as of the hexagonal layers in two forms of silica, /3-tridymite and 0-cristo-balite. The monoclinic (pseudohexagonal) unit of structure of hydrargillite8 has a = 8.70 A, b = 5.09 A, c = 9.76 A, and 0 = 85°29, and contains 8A1(0H)3. The crystal is composed of layers of octahedra,... 

The compound LisZn2Ge3 can be expressed as SLi , 2Zn, SGe in terms of oxidation numbers, or as 8 Li , 2 Zn, 3 Ge with regard to the Zintl-Klemm concept and valence considerations. It closely relates to Li5AlSi2 and the corresponding [Li3Al3Si 5] hexagonal layer in which Zn and Ge would play the role of A1 and Si. With the Zn vacancy compensated by a Hthium dumbbell, the [Li4ZrL4Gei5] anionic network is isoelectronic. 

Three views of the hexagonal layers contained within a face-centered cubic array, (a) A view perpendicular to the hexagonal layers, with all but one atom removed from the top layer. (Z>) A side view showing an outline of the cube and atoms from three successive layers of the cubic array, (c) The same side view, with two hexagonal layers screened for emphasis. 

Tc or c cubic closest-packing of spheres Th or h hexagonal closest-packing of spheres Ts stacking sequence AA... of hexagonal layers Qs stacking sequence AA... of square layers... 

For additional symbols of further packings cf. [38, 156], T (triangular) refers to hexagonal layers, Q to layers with a periodic pattern of squares. The packing Qs yields a primitive cubic lattice (Fig. 2.4), Qf a body-centered cubic lattice (cf. Fig. 14.3, p. 153). Sometimes the symbols are set as superscripts without the angular brackets, for example Ti[Ca03]c. 

Oxygen in the solid state consists of 02 molecules. From 24 K to 43.6 K they are packed as in a-F2. Under pressure (5.5 GPa) this packing is also observed at room temperature. Below 24 K the molecules are slightly tilted against the hexagonal layer. From 43.6 K up to the melting point (54.8 K) the molecules rotate in the crystal as in /3-F2. Under pressure oxygen becomes metallic at approximately 100 GPa, but it remains molecular. 

Arrangement of spheres in a hexagonal layer and the relative position of the layer positions A, B and C... 

Spheres are drawn smaller than their actual size. Spheres with the same coloring form hexagonal layers as in Fig. 14.1... 

The number of different possible stacking variants increases with increasing numbers of hexagonal layers in one periodically repeating slab of layers ... 

Aside from the ordered stacking sequences we have considered so far, a more or less statistical sequence of hexagonal layers can also occur. Since there is some kind of an ordering principle on the one hand, but on the other hand the periodical order is missing in the stacking direction, this is called an order-disorder (OD) structure with stacking faults. In this particular case, it is a one-dimensionally disordered structure, since the order is missing only in one dimension. When cobalt is cooled from 500 °C it exhibits this kind of disorder. 

Usually, the composition of the compound is fulfilled in all of the hexagonal layers. This facilitates a rational classification of the extensive data it is only necessary to draw a sketch of the atomic arrangement in one layer and to specify the stacking sequence... 

Tetrahedra in hexagonal closest-packing (a) view of the hexagonal layers (b) view parallel to the hexagonal layers (stacking direction upwards)... 

Relative arrangement of the octahedra in hexagonal and in cubic closest-packing in the direction of stacking of the hexagonal layers... 

A simple structure which can be described in terms of superposition of (even if far away, not close-packed) hexagonal layers is that of graphite Chyt C%u The hexagonal net is also called graphitic net. 

Similar results were found for the decomposition on surface carbides of tungsten. The W(IOO) surface reconstructs to form a W(IOO) (5 x 1)C structure, as shown in Fig. 2. This structure has been interpreted to be a hexagonal layer of W2C on top of the (100) surface 110) with the carbon atoms in interstitial positions under the first layer of tungsten atoms 111). The presence of the carbon atoms strongly modified the reactivity of the surface. Rather than dissociating formic acid completely like W(IOO), the carbide surface selectively formed the formate. The decomposition proceeded at 540 K with... 

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