Structure of clathrate

Figure 4. Structure of clathrate hydrate 1 (left) and clathrate hydrate II (right).

All the polyhedra intermediate between the dodecahedron and truncated icosahedron can be realized except /s = 12,/ = 1, and some in more than one form (different arrangements of the 5-gon and 6-gon faces). A number of these polyhedra are of interest in connection with the structures of clathrate hydrates (p. 543), because certain combinations of these solids with dodecahedra form space-filling assemblies in which four edges meet at every vertex. Two of these polyhedra, a tetrakaidecahedron and a hexakaidecahedron, are illustrated in Fig. 3.4. 

In order to determine the structure of clathrate hydrates formed in this system, we carried out an X-ray powder diffraction investigation. The powder diffraction patterns are shown in Figure 3. 

Polar deep ice. Polar deep ice is made of snow under a compressing process. The dielectric properties of polar ice core samples have been reported as having small values of relaxation time rand activation energy.The observation of small rvalues for the core ice samples suggests that Bjerrum defects are more numerous in polar ice than in ordinary ice. The impurity concentration of polar ice is not sufficiently high to decrease the rvalue the HCl concentration is about 2x10 mol/1 for Byrd core ice. Since we know that polar deep ice has structures of clathrate gas hydrate, imperfection in the structures and the existence of gas molecules in the ice lattice seem to affect the dielectric properties.It is well known that the dielectric properties of ice samples derived from polar deep ice that has melted and refrozen are similar to those of ordinary ice. ... 

The structure of liquid water was dealt with in detail in Sect. 1.1.2. Once a solute, whether an ion or a neutral solute and whether hydrophilic or hydrophobic, is placed in the water, it is reasonable to expect it to affect the structure around it. The effects may be limited to a hydration shell surrounding the solute that has a structure differing from that in pure water. For instance, around monatomic ions the water molecules in the hydration shell are oriented towards the ion in a more or less spherical symmetry. Around hydrophobic solutes cages of water molecules are formed, that may be icelike but also resemble the structure of clathrates or crystal hydrates. In many cases the effects of the presence of an ion are manifested also beyond the hydration shell or shells. 

Chapters 4, 11 and 12 are dealing with three powerful characterization methods. The first one is the in situ X-ray diffraction study of the influence of pressure on the stracture of clathrates, using a high pressure cell and synchrotron radiation as incident beam. The second one concerns the contribution of neutron diffraction studies to the structure of clathrates and the third one is about the mechanical properties of clathrates, which can be considered as hard materials ... 

Crystallographic Disorder and Complexity of the Crystal Structures of Clathrates... 

Fig. 2.4 Defects in the crystal structure of clathrate-I KgSn44n2 (top) environment of the Snl position in the (100) plane middle) displacement ellipsoids in the (100) plane without taking into account the splitting of the Sn3 position left) and considering the splitting into Sn31 and Sn32 right) together with the difference density distribution at r = 300 K and T = 80 K bottom) different local atomic arrangements without left) and with right) defect at the Snl position...

Attempts to gain further information on bonding in Si- and Sn-based cationic clathrates from the Si and Sn NMR spectra were less informative because of the broadening of the spectral lines due to statistical disorder of atoms in the crystal structure [27, 36, 40]. Although this method is unable to probe the local environment of a target atom, it can be used for investigating the extended electronic structure of clathrates. In particular, the analysis of the chemical shifts of type-I and type-III clathrates in the Si-P-Te system confirmed that the latter clathrate is charge balanced whereas the former is electron-deficient [36, 40]. 

Fig. 5.11 Crystal structure of clathrate-IX. Two types of guest atoms are shown first ones are located inside the 20-vertex polyhedral cage, The remaining cations (black large circles) are disordered over the open zeolite-like channel system [76]...

Fig. 5.12 Columns of 24-vertex cages (tetrakaidecahedra) found in the structures of clathrates-I A8Sn44D2 (A = Rb, Cs). In the a-form (superstructure, space group la- id), the partially occupied tin site denoted as open square is distributed along 4i axis, while in the fS-form (space group Pm3n) the partially occupied site (open square) is distributed along a 42 axis...

Fig. 5.13 Location of sodium atoms in the crystal structure of clathrate-in Cs3oNai 33 j.ioSni72-j . When Na(l) is present the adjacent Sn(17) atoms are missing, when Na(l) is missing only one of the Sn(17) atoms is present. Na(2) is replacing Sn(14)-Sn(16) dumbbells. Atoms numbering is taken from [75]...

Figure 25 Part of the crystal structure of clathrate (94b)2-(p-xylene). Color code H, light blue Cl, orange host C, green and guest C, purple.

J. J. Molaison and N. Pradhan,/. Chem. Phys., 136, 2012, 054502 and refs therein. (Structures of clathrates)... 

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