Cubic structure I

According to these authors all gas hydrates crystallize in either of two cubic structures (I and II) in which the hydrated molecules are situated in cavities formed by a framework of water molecules linked together by hydrogen bonds. The numbers and sizes of the cavities differ for the two structures, but in both the water molecules are tetrahedrally coordinated as in ordinary ice. Apparently gas hydrates are clathrate compounds. 

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... 

Fig. 1 The cubic structure I (CS-I) hydrate with methane molecules accommodated in the dodecahedra] (D) and in the tetradecahedral (T) cavities. (View this art in color at www. dekker.com.)...

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

Beta-iron (P-Fe). When heated above its Curie temperature of 769 C, alpha-iron loses its ferromagnetic properties but retains its body-centered cubic structure (i.e., second-order transition). This particular form of iron is called beta-iron, which is considered a different allotropic form owing to its paramagnetic properties. However, because no changes in the crystal lattice structure occurs, it is customary to consider it nonmagnetic alpha-iron ... 

CO2 hydrate is perhaps one of the best understood clathrate hydrate regarding structural, thermodynamic, as well as other physical properties. Figure 7(a) and (b) show the CO2 molecules with full symmetry in small and large cages in cubic structure I analyzed by the Rietveld refinement with the direct-space technique. In the large cage, the best... 

Figure 2 Cubic structure I clathrate hydrate, showing the two constituent cages, the space group, and typical lattice parameter.

Huang Ming-Zhu and Ching WY 1993 Caicuiation of opticai excitations in cubic semiconductors, i. Eiectronic structure and iinear response Phys. Rev. B 47 9449-63... 

In this illustration, a Kohonen network has a cubic structure where the neurons are columns arranged in a two-dimensional system, e.g., in a square of nx I neurons. The number of weights of each neuron corresponds to the dimension of the input data. If the input for the network is a set of m-dimensional vectors, the architecture of the network is x 1 x m-dimensional. Figure 9-18 plots the architecture of a Kohonen network. 

Structure Although massive chromium has a body-centred cubic structure, electrodeposited chromium can exist as two primary modifications, i.e. a-(b.c.c.) and (c.p.h.). The precise conditions under which these forms of chromium can be deposited are not known with certainty. Muro" showed that at 40°C and 2-0-22 A/dm the deposit was essentially a-chromium but small amounts of 0- and 7- were present, while Koch and Hein observed... 

It would be too much to expect that the same diffusing mechanism would exist for all types of spinels (yes. there are many types of spinels, since spinel" is a particular composition, i.e.- AB2O4. having a cubic structure. What we need to do is to determine which of these cases is the most relevant. To do this, we need to generate some experimental data. 

Only large clusters usually adopt the face-centered cubic structure of metallic platinum. A novel cuboctahedral cluster [Pt15Hx(CO)8(PBut3)6] has been reported by Spencer et al.512 and the first octahedral cluster [Pt6(CO)6(/i-dppm)3]2+ was only reported recently.573... 

Silver nitrate complexed with ethylenediamine and glucose and other reducing agents have been used to grow silver films by SILAR. After annealing at 300 °C the films showed cubic structure and the I-V curves exhibited linear behavior, whereas the as-grown films showed nonlinear behavior.125... 

As expected for a material with cubic sites, the RT spectrum of stoichiometric wiis-tite consists of a single line. The spectrum of the non-stoichiometric material shows an asymmetrically broadened doublet with contributions from Fe " resonance and from two quadrupole split Fe" doublets. The component peaks which contribute to this spectrum have not been fully resolved owing to the range of Fe" environments in the structure (i. e. the variation in Fe content and vacancy level). At 77 K the Mossbauer spectrum consists of a broad doublet with contributions from Fe" and Fe ". Fe(OH)2 shows a sextet corresponding to a Bhf of 16.6 T at 20 K (Miyamoto, 1976 Genin et al. 1986) and to -20 Tat 4K (Refait et al. 1999). The Tn is at 34 K (Miyamoto et al., 1967). The spectrum of high pressure FeOOH at room temperature consists of a sextet (Peret et al. 1973). 

Mixed crystals of type I are very stable and can be used for electrochemical investigations. Recently, it has been shown that such solid solutions maintain the regular cubic structure and only a single... 

I would also like to mention one type of non-spherical compartment that is much less popular than micelles or vesicles, hut in my view very interesting. These are the cubic phases, so called because of their cubic symmetry. Many different types of cubic structures have been described (Mariani et al., 1988 Lindblom and Rilfors, 1989 Fontell, 1990 Seddon, 1990 Seddon etal., 1990 Luzzati et al., 1993). 

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