Hexagonal structure H

All common natural gas hydrates belong to the three crystal structures, cubic structure I (si), cubic structure II (sll), or hexagonal structure H (sH) shown in Figure 1.5. This chapter details the structures of these three types of hydrate and compares hydrates with the most common water solid, hexagonal ice Ih. The major contrast is that ice forms as a pure component, while hydrates will not form without guests of the proper size. 

Natural gas clathrate hydrates normally form either in the primitive cubic structure I, in the face-centered cubic structure II, or in the hexagonal structure H. 

Cubic structure I predominates in the earth s natural environments with small (0.4-0.55 nm) guests and cubic structure II generally occurs with larger (0.6-O.7 nm) guests in mostly man-made environments. Hexagonal structure H may occur in either environment, but only with mixtures of both small and the largest (0.8-0.9 nm) molecules. The smallest hydrated molecules (Ar, Kr, Oj and Nj) with diameters... 

As a rule, gas hydrates obtained under the P-T conditions close to those in the nearsurface have one of the three following structures cubic structure 1 (si), cubic structure II (sll) or hexagonal structure H (sH). Each of these structures has its own set of polyhedral cavities of different sizes. The better that the size and molecular shape of the guest... 

Fig. 4 Clathrate hydrate structures. Hydrates of cubic Structures I, II. and hexagonal Structure H are illustrated to indicate the stacking of the polyhedra.

Figure 6. (a) Model of a section of the hypothetical polymeric network of the body centered tetragonal structure (b.c.t.-4) of carbon and BN suggested by R. Hoffmann et al. [72]. (b) Model of a section of the hypothetical hexagonal structure (H-6) of carbon postulated by M. Tamor and K. Hass [73,74], In both structures, each carbon atom is considered to be trigonally coordinated and sp -bonded, exclusively. First published in [152] and reproduced with permission. 

Like carbon, boron nitride (BN) exists in two main crystalline structures, a hexagonal structure (h-BN) and a cubic zinc blend structure (c-BN). The c-BN is... 

Boron nitride, for instance, having electronic properties that resemble carbon can exist in a hexagonal structure h-BN similar to the graphite layered geometry. Much like graphene sheets, BN sheets can be grown on more or less lattice-matched transition metal surfaces (Corso et al. 2004 Huda and Kleinman 2006). A model BN sheet is shown in O Fig. 27-14. 

Figure 1. Hexagonal C-H—N based channels in the crystal structure of complex 5 formed by 4,4-dicyanobiphenyl and urea. Synthon III is highlighted. Note that there are two geometrical variations of this synthon.

Figure 4. Hexagonal C-H—O channels in the synthon VI based structure of the 1 1 complex 4,4r-dinitrobiphenyl urea. The similarity to Figure 1 is obvious.

To observe the nascent Si(lll) structure, all the dangling bonds have to be capped with something. In silicon technology, this process is called passivation. A standard method is to treat it in NH F solution, and every unsatisfied dangling bond is capped with a H atom. Higashi et al. (1991) has successfully imaged the hexagonal structure of the passivated Si(lll) surface with STM, as shown in Fig. 1.13. 

The particles formed are in most cases spherical, although rods, ellipsoids, platelets, and hexagonal structures have also been produced. Solids composed of spherical particles are as a rule amorphous, while those of other morphologies are crystalline. In general, aging times of 2 h at approximately 100°C were sufficient to produce the desired results however, in some instances much longer times were necessary to complete the precipitation process. 

As indicated by XRD patterns, there exist just 2-3 broad peaks in the calcined acid-made materials (Fig. 3A). Moreover, the N2 adsorption/desorption isotherm shown in Fig. 3B, the calcined acid-made mesoporous silica indeed possesses a broad capillary condensation at the partial pressure p/p0 of ca. 0.2-0.4, indicating a broad pore size distribution with a FWHM ca. 1.0 nm calculated from the BJH method. This is attributed to the occurrence of partial collapse of the mesostructure during the high temperature calcination. The hexagonal structure completely collapsed when subjected to further hydrothermal treatment in water at 100 °C for 3 h. Mesoporous silica materials synthesized from the acid route are commonly believed to be less stable than those from the alkaline route [6,7]. 

Similar to Jeffrey s hypothetical structure IV, structure H is a hexagonal crystal of space group P6/mmm. However, in contrast to structure IV, structure H comprises 435663 and 51268 cavities in addition to 512 cavities. On the basis of size considerations (including the relative size of the guests, the size of the cages in I-VII, and the unit cell parameters for sH), the structure was incompatible with Jeffrey s structures. Therefore, sH is not one of Jeffrey s (1984) known or hypothetical structures. Single crystal diffraction data for structure H have been obtained by Udachin et al. (1997b, 2002) and Kirchner et al. (2004). 

Dixon, Stevens, and Herschbach88 have carried out accurate calculations on a number of possible transition states for the H2 + D2 2HD reaction. The most likely candidate for a concerted process is a trimolecular, hexagonal structure, which has an energy of 288 kJ mol-1 above three separated molecules. This is to be compared with 517 kJ mol-1 for the square bimolecular species67 68 and 432 kJ mol-1 for dissociation of H2 into atoms. Other H4n+2 species would be allowed intermediates according to the W-H rules, but only H6 has an energy lower than is required for the atomic process. 

The index usually indicates the sequence cubic (3), hexagonal (2), or double hexagonal (4) for the ABAC sequence. The Pearson symbols (Table 2.5) can clarify cases such as hexagonal structures with an ABC sequence and a = 3 for the index. The symbols t for tetragonal, o for orthorhombic, m for monoclinic, and h for hexagonal, rhombohedral, or trigonal indicate the type of distortion of an idealized structure. Without distortion, a = 3 is for a cubic structure and a = 2 is for a hexagonal (or rhombohedral) structure. 

Figure 5.15. The 2 2PO structure of NaH(P03NH2). (a) The dimensions of the PO3NH22 ion. (b) Two projections of the hexagonal structure. The Na+ ions are heavy lined circles and the O atoms are largest circles. In the packing drawing the P03NH2 ions are orientated as in (a) and layers are labeled with Na+ in Oc layers. H atoms are not shown.

The analogous calculation of the solubility of the interstitial impurity as a function of the H atom concentration, temperature, long-range order have been carried out for AB and AB3 alloys of other hexagonal structures. The results of such calculations are listed below [19-22],... 

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