Clathrate inverse

Because in the cyclodextrin hydrates the cavity is occupied by water molecules, they can be considered as the inverse of the clathrate hydrates discussed in Part IV, Chapter 21. In these, the water molecules form the host structure and the organic molecule is the guest. 

The guest molecules in clathrate hydrates are confined in nearly spherical cavities. The symmetry of the cavities is high, most having fourfold, threefold, inversion, or other symmetry elements and their combinations.Therefore, the guest molecules of low symmetry (e.g., polar molecules) have several equivalent orientations in the cavity and, hence, are disordered. The disorder persists to 4.2 K, as dielectric and NMR measurements show for, e.g., dimethyl ether in a Structure II hydrate. A phase transition to ail ordered phase may be expected to occur. 

The historically first experiment of this kind for clathrates was an attempt to synthesize a noble gas clathrate XegGe46 by chemical transport of Ge with GeLt under high Xe gas pressure. While the existence of the noble gas clathrates is still under discussion, the first inverse clathrate IgGe43.33I2.67 [95] was obtained in this way. The technique was also successfully applied for the preparation of phases such as GesgAsgIg [18]. Later it was developed into a fruitful synthetic tool with the possibility to obtain well-developed single crystals [19, 20]. 

Chemistry and Physics of Inverse (Cationic) Clathrates and Tin Anionic Clathrates... 

J.V. Zaikina, T. Mori, K. Kovnir, D. Teschner, A. Senyshyn, U. Schwarz, Yu. Grin, A.V. Shevelkov, Bulk and surface structure and high-temperature thermoelectric properties of inverse Clathrate-III in the Si-P-Te System. Chem. Eur. J. 16, 12582—12589 (2010)... 

A common characteristic of all inteimetallic clathrates, independent of composition, is a relatively large, complex unit cell. For type 1 clathrates, the number of atoms per primitive unit cell, N, is 54. The effect of N on Kl for crystalline solids at high temperature was analyzed, using relatively simple models, by Slack [25]. A well-known inverse correlation between N and /Cl at the Debye temperature D for a crystal is described by the relation [25]... 

The electronic properties of inorganic clathrates are often discussed in terms of a rigid band model [63]. According to this approximation electropositive guest atoms donate their valence electrons to the sp bonded framework atoms. Compositions where electronegative guest elements contribute holes to the framework orbitals are known as inverse [64] (cf. Chap. 5). In order to consider clathrates for TE applications it is necessary to optimize their electronic properties thereby maximizing the power factor in Eq. 6.1. From the relationship between the electrical resistivity, p, and cr [32]... 

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