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Abstract
Exploratory studies in the systems Rb–Ga–Sn and Cs–Ga–Sn yielded the cubic type-I clathrates with refined compositions Rb8GaxSn46−x and Cs8GaxSn46−x (6.9 < x < 7.5). Nearly single-phase materials with good crystallinity were obtained from stoichiometric reactions of the elements. The structures were characterized by means of single-crystal X-ray diffraction methods. Both Rb8GaxSn46−x and Cs8GaxSn46−x represents cases, where a Group 13 element randomly substitutes a Group 14 element in the structure. The extent of Ga/Sn mixing is apparently governed by the drive of the system to achieve an optimal valence electron count, and hence, Rb8GaxSn46−x and Cs8GaxSn46−x (x ≈ 8) can be regarded as Zintl phases. This notion is supported by structure refinements on a multitude of single-crystal X-ray diffraction data, which also confirm that both types of cages in the cubic type-I structure are fully occupied by Rb and Cs atoms. The open-framework, comprised of 46 nodes per formula unit, adapts to the incorporation of nearly eight Ga atoms within the matrix of Sn, whereby small, short-range distortions result. The exact nature of these effects is still unclear, as so far, the structural variations could only be modeled as both positional and occupational disorder at one of three framework sites. Since vacancies in the structures of the binary type-I clathrates A8Sn46−x☐x (A = Rb, Cs; ☐ = missing Sn atom) are also known to cause local distortions, the latter were also synthesized with the same protocols used for the synthesis of A8GaxSn46−x and structurally re-analyzed. The results from the latter studies confirm that homogeneity issues abound, and that the final structures/compositions are an intricate function of the experimental conditions.
Highlights
In the past two to three decades, thermoelectric materials have played an important role in the development of new technologies for primary power generation and solid-state energy conversion.Such materials are capable of converting waste heat into electricity, which may be useful for a vast amount of applications
An alternative flux-growth technique calls for using the metal halides as a modified flux medium, might be suggested as means to obtain bulk samples with good crystallinity [42]. These could be beneficial for studying the physical properties of Rb8 Gax Sn46−x and Cs8 Gax Sn46−x (x ≈ 8); the lesson learned from the present studies of the crystal chemistry of the title clathrate phases is that the outcomes of the synthesis are very much dependent on the reaction conditions
One can argue the information from the published crystal structures ought to be used with caution, and that single-crystal X-ray diffraction work should be employed to verify the received final structures/compositions
Summary
In the past two to three decades, thermoelectric materials have played an important role in the development of new technologies for primary power generation and solid-state energy conversion. Such materials are capable of converting waste heat into electricity, which may be useful for a vast amount of applications. Among prospective materials for thermoelectric applications, intermetallic clathrates are of particular interest due to their unique structures and properties [1] Such compounds, with host–guest type interactions, have been known for a while, whereas isotypic compounds based on the elements of Group. Germanium, and tin form the host framework, which can be partially substituted by elements of Group 13, late transition metals or Group 15 elements
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