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 K8AlxSn46-x (potassium aluminium stannide) and Rb8AlxSn46-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 A8Al8±xSn38∓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.