Cationic clathrate

As these are low melting compounds, the field of cationic clathrates could be developed only by the careful control and systematic use... 

Chapters 2, 3, 5, 9 and 10 concern the synthesis, the structure and the physical properties of numerous anionic and cationic clathrates with light or heavier elements as host lattices and various other ones as guest species. Some of them exhibit very interesting thermal properties and are promising thermoelectric materials, a point which is treated in details in Chap. 6 ... 

J.V. Zaikina, K.A. Kovnir, F. Haarmann et al.. First silicon-based cationic clathrate III with exceptional high thermal stability. Chem. Eur. J. 14, 5414-5422 (2008)... 

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

Crystal Structure of Cationic Clathrates 5.2.1 Proper Type-I Structure... 

Five different types of superstructure that lead to an increase in the unit cell volume have been documented for the type-I clathrates in the literature [9,33,46-49]. Two of these types were observed for the cationic clathrates. 

Qualitatively, the semiconducting behavior of the germanium- and tin-based cationic clathrates can be rationalized within the frames of the Zintl counting scheme, assuming the reversed host-guest polarity. In traditional Zintl phases of a general... 

Band structure calculations were performed for various type-I cationic clathrates and semiclathrates [9, 22, 35]. Several important features were found in these calculations. All compounds were calculated to be semiconductors with a band gap of 0.01-0.3 eV depending on the phase composition and the method used for calculations. Guest atoms do not contribute to the states in vicinity of the Fermi level. Typically, they show very little dispersion, suggesting that their interaction with the framework atoms is quite weak, and carry a negative charge which, in the case of iodine, was found to be close to the formal oxidation state -1. Although in... 

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]. 

Notes on Synthesis and Thermal Stability of Cationic Clathrates... 

Tin-based cationic clathrates are stable in moist air for months. They readily oxidize upon heating in air. The destruction temperature varies depending on the chemical composition, achieving the maximum of 650 K for the clathrates of the Sn-In-As-I system [29]. Also, they decompose upon heating in vacuum between 730 and 970 K, liberating volatile tin halides. 

The crystal structures of tin-based cationic clathrates readily contract upon cooling [31, 56, 58]. The dependence is linear with temperature down to about 100 K and then shows lesser increment with further cooling (Fig. 5.8). Thermal... 

Experimentally observed thermal conductivity k of the cationic clathrates is characterized by low values. Among this group of compounds, tin-base cationic clathrates display the lowest thermal conductivity ranging from 0.36 to about 2.0 W m at... 

Fig. 5.9 Top A C/7 versus 7 plot for Sn24Pi9.3l8 with the contributions of one Debye and two Einstein modes, bottom Thermal conductivity versus temperature for some cationic clathrates...

Total thermal conductivity is a sum of the lattice and electronic parts, K = Ki + Ke- The lattice part of the thermal conductivity describes the scattering of phonons on the vibrations of atoms, whereas the electronic part describes thermal conductivity appearing due to conduction electrons and is related to the electrical conductivity Wiedemann-Franz equation, = a T Lo, where T is the absolute temperature and Lq is the ideal Lorenz number, 2.45 X 10 Wf2K [64]. The electronic part of the thermal conductivity is typically low for low-gap semiconductors. For the tin-based cationic clathrates it was calculated to contribute less than 1% to the total thermal conductivity. The lattice part of the thermal conductivity can be estimated based on the Debye equation /Cl = 1 /3(CvAvj), where C is the volumetric heat capacity, X is the mean free path of phonons and is the velocity of sound [64]. The latter is related to the Debye characteristic temperature 6 as Vs = [67t (7V/F)] . Extracting the... 

Debye temperature from either heat capacity or structural data and assuming that the mean free path of phonons is the average distance between the guest atoms, the lattice part of the thermal conductivity was calculated for several tin-based cationic clathrates to be in the range of 0.7-0.9 W m K , which is in good agreement with the experimental data [31, 32, 56, 58]. 

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