Synthesis and structural characterization of the type-I clathrates K₈Al_xSn_46–xand Rb₈Al_xSn_46–x(x≃ 6.4–9.7)

Abstract

Exploratory studies in the systems A –Al–Sn ( A = K and Rb) yielded the clathrates K 8 Al x Sn 46– x (potassium aluminium stannide) and Rb 8 Al x Sn 46– x (rubidium aluminium stannide), both with the cubic type-I structure (space group Pm \overline{3} n , No. 223; a ≃ 12.0 Å). The Al:Sn ratio is close to the idealized A 8 Al 8 Sn 38 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 A 8 Al 8 Sn 38 formula has a valence electron count that obeys the valence rules and represents an intrinsic semiconductor, while the experimentally determined compositions A 8 Al 8± x Sn 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.