K and Ba distribution in the structures of the clathrate compounds K(x)Ba(16-x)(Ga,Sn)136 (x = 0.8, 4.4, and 12.9) and K(x)Ba(8-x)(Ga,Sn)46 (x = 0.3)

Synthesis and structural characterization of the type-I clathrates K8AlxSn46-x and Rb8AlxSn46-x (x ≃ 6.4-9.7)

Exploratory studies in the systems A-Al-Sn (A = K and Rb) yielded the clathrates K₈Al_xSn_46-x (potassium aluminium stannide) and Rb₈Al_xSn_46-x (rubidium aluminium stannide), both with the cubic type-I structure (space group Pm-3n, No. 223; a ≃ 12.0 Å). The Al:Sn ratio is close to the idealized A₈Al₈Sn₃₈ composition and it is shown that it can be varied slightly, in the range of ca ±1.5, depending on the experimental conditions. Both the (Sn,Al)₂₀ and the (Sn,Al)₂₄ cages in the structure are fully occupied by the guest alkali metal atoms, i.e. K or Rb. The A₈Al₈Sn₃₈ formula has a valence electron count that obeys the valence rules and represents an intrinsic semiconductor, while the experimentally determined compositions A₈Al_8±xSn_38∓x suggest the synthesized materials to be nearly charge-balanced Zintl phases, i.e. they are likely to behave as heavily doped p- or n-type semiconductors.

Structure and Transport Properties of Dense Polycrystalline Clathrate-II (K,Ba)16(Ga,Sn)136 Synthesized by a New Approach Employing SPS

Tin clathrate-II framework-substituted compositions are of current interest as potential thermoelectric materials for medium-temperature applications. A review of the literature reveals different compositions reported with varying physical properties, which depend strongly on the exact composition as well as the processing conditions. We therefore initiated an approach whereby single crystals of two different (K,Ba)₁₆(Ga,Sn)₁₃₆ compositions were first obtained, followed by grinding of the crystals into fine powder for low temperature spark plasma sintering consolidation into dense polycrystalline solids and subsequent high temperature transport measurements. Powder X-ray refinement results indicate that the hexakaidecahedra are empty, K and Ba occupying only the decahedra. Their electrical properties depend on composition and have very low thermal conductivities. The structural and transport properties of these materials are compared to that of other Sn clathrate-II compositions.

Synthesis and Structural Characterization of the New Clathrates K₈Cd₄Ge42, Rb₈Cd₄Ge42, and Cs₈Cd₄Ge42

This paper presents results from our exploratory work in the systems K-Cd-Ge, Rb-Cd-Ge, and Cs-Cd-Ge, which yielded the novel type-I clathrates with refined compositions K₈Cd_3.77(7)Ge_42.23, Rb₈Cd_3.65(7)Ge_42.35, and Cs_7.80(1)Cd_3.65(6)Ge_42.35. The three compounds represent rare examples of clathrates of germanium with the alkali metals, where a d¹⁰ element substitutes a group 14 element. The three structures, established by single-crystal X-ray diffraction, indicate that the framework-building Ge atoms are randomly substituted by Cd atoms on only one of the three possible crystallographic sites. This and several other details of the crystal chemistry are elaborated.

On the possibility for Rb- and Eu-cation ordering in type-I clathrates: synthesis and homogeneity range of the novel compounds Rb(8-x)Eu(x)(In,Ge)46 (0.6 ≤ x ≤ 1.8)

Studies in the Rb-Eu-In-Ge system confirm the existence of the phase Rb(8-x)Eu(x)(In,Ge)46 (0.6 ≤ x ≤ 1.8), crystallizing with the cubic clathrate type-I structure. The In and Ge content can be varied, concomitant with changes in the Rb-Eu ratio. Two of the three framework sites are occupied by statistical mixtures of Ge and In atoms, while the site with the lowest multiplicity is taken by the In atoms only. Based on the three refined formulae [heptarubidium europium nonaindium heptatriacontagermanide, Rb7.39(3)Eu0.61(3)In8.88(5)Ge37.12(5), and two forms of hexarubidium dieuropium decaindium hexatriacontagermanide, Rb6.30(3)Eu1.70(3)In9.76(4)Ge36.24(4) and Rb6.24(2)Eu1.76(2)In10.16(5)Ge35.84(5)] and the explored different synthetic routes, it can be suggested that the known ternary phase Rb8In8Ge38 and the hypothetical quaternary phase Rb6Eu2In10Ge36 represent the boundaries of the homogeneity range. In the former limiting composition, both the (Ge,In)20 and the (Ge,In)24 cages are fully occupied by Rb atoms only, whereas Rb6Eu2In10Ge36 has Rb atoms encapsulated in the larger tetrakaidecahedra, with Eu atoms filling the smaller pentagonal dodecahedra. For the solid solutions Rb(8-x)Eu(x)(In,Ge)46, Rb and Eu are statistically disordered in the dodecahedral cage, and the tetrakaidecahedral cage is only occupied by Rb atoms.