Intermetallic clathrates

Fig. 2.2 Relationship between the clathrate-I-type crystal structures of the clathrate hydrates (Zg(H20)46, kft) and the intermetallic clathrates (NagSi46, right)...

This chapter deals with the chemical composition, crystal structures and their crystallographic features, chemical bonding and preparation routes of intermetallic clathrates. 

Fig. 2.3 Crystal structures of intermetallic clathrates represented as packing of cage polyhedrons 20-atom dodecahedron blue), 24-atom tetrakaidecahedron (green), 26-atom pentakai-decahedron (red) and 28-atom hexakaidecahedron (yellow). The example compositions for different clathrate families (hydrate, cationic, and anionic) are given for comparison...

The composition of clathrates with crystal structures built-up of space-filling polyhedrons (e.g. clathrates I, II, HI and IV) can be calculated using the formalism mentioned above for the sodalite type of structure. For these clathrates, the composition results from the fact that each framework atom is shared by four polyhedral cages. For the first observed intermetallic clathrate I, NagSUe [17, 31-33] with two NaSiao and six NaSi24 polyhedrons in the unit cell, the content of the unit cell is... 

Various synthesis routes to intermetallic clathrates have been successfully applied covering the whole spectrum of preparation methods in sohd state chemistry. The applicabihty of each method is dependent on the elements forming the target material and some thermodynamic properties, such as formation energy and vapor pressure of components at the reaction temperature. Although it is straightforward for many systems to prepare a clathrate phase at one certain composition. 

Similar features in crystal chemistry amongst many of the known intermetallic clathrate compositions aid the prediction of new compositions. Charge balance is a key consideration in the rational design of many intermetallic clathrate compositions. A significant number of reported intermetallic clathrate-1 compositions appear to adhere relatively well to two rules of thumb (i) there is significant (often nearly complete) transfer of valence electron density from the guest to the... 

Many intermetallic clathrate compositions of interest are members of ternary and higher order systems. In such cases the phase equilibria can be rich and complex, while the potential for thermodynamically or kinetically stable... 

In many cases, direct reaction of the elements is not always feasible, especially when the physical characteristics for the constituents (melting point, vapor pressure, diffusion coefficients, etc.) or kinetics of the reaction are unfavorable, or if the target product is thermodynamically metastable (in such cases, other synthetic routes discussed elsewhere in this chapter may be more effective). Nevertheless, direct reaction of the elements or preformed preciu-sors has to date been the most commonly used approach to prepare intermetallic clathrates, producing the largest variety of compositions synthesized to date (cf. Table 3.1 for representative reactions). 

Intermetallic clathrates were first discovered in the coiu se of investigations on the thermal decomposition of the binary compounds hltTti [48,49], where M = alkali metal and Tt = group 14 element. Systematic investigations carried out in the seminal work by Cros et al. [1-3,50-52] demonstrated that a variety of alkali metal silicon and germanium-based compositions are prepared via this route, which continues to be employed for preparation of clathrate materials in contemporary investigations [53-64]. 

Table 3.1 Selected examples of intermetallic clathrate compositions prepared by direct reaction of the elements to obtain polycrystalline specimens (representative reactions)... 

Soft chemical methods for the preparation of solid-state materials possess many advantages over conventional synthesis approaches. The introduction of additional synthetic variables allows for additional ways to influence the resulting products that form, and the comparatively low temperatures used in these reactions allow access to kinetically stable (thermodynamically metastable) compounds that cannot be prepared by conventional methods. The use of low temperature chemical oxidation of precursors in heterogenous reactions was recently introduced as an effective method for synthesis of intermetallic clathrates (see also Chap. 2 of the present volume) [83, 84]. We summarize here the compositions obtained by this approach. 

The majority of synthetic efforts have overwhelmingly focused on the preparation of intermetallic clathrates in bulk form by the methods described above. To date, comparatively fewer efforts to prepare thin film and nanostructured materials have been reported. 

Table 3.5 Silicon-based intermetallic clathrate compositions prepared by high-pressure reaction...

Due to remarkable flexibility in composition intermetallic clathrates display a wide variety of electrical behavior, from very good metals to semiconductors (as will be discussed in Sect. 6.4). Thus, in some compositions the charge carriers dominate k... 

Intermetallic clathrates, due to their structural characteristics fulfilling the phonon glass-electron crystal concept (PGEC) [1], are considered as promising thermoelectric materials and thus the thermoelectric properties of these compounds have... 

In contrast to the scarce knowledge on hardness and elastic properties of intermetallic clathrates, there is an enormous amount of data on Einstein and Debye temperatures available in the literature. The exceptional vibrational features of these cage compounds were extensively smdied in recent years and will be discussed in the frame of this work. Needless to say that DPT calculations of electron and phonon density of states, of thermoelectric properties and of elastic properties have greatly supported clathrate research. In this context attention should be drawn to a recent work of Karttunen et al. [29], who employed the Perdew-Burke-Emzerhof hybrid density functional with localized atomic basis sets composed of Gaussian-type functions, to calculate the elastic properties of 14 different types of clathrate frameworks (for elemental structures of C, Si, Ge, Sn) predicting bulk and Young s moduli comparing them with their diamond-like, dense so called ot-phases. 

In order to fill the knowledge gap on the mechanical performance of clathrates, the aim of this work is, to provide experimental data on hardness and elastic properties of intermetallic clathrates, covering a wide range of different compositions, and to compile, evaluate and discuss all data hitherto available in the literature on (i) hardness, (ii) elastic properties, (iii) Debye and Einstein temperatures, and (iv) on thermal expansion. 

---