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

Abstract

Exploratory studies in the systemsA–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 groupPm\overline{3}n, No. 223;a≃ 12.0 Å). The Al:Sn ratio is close to the idealizedA8Al8Sn38composition and it is shown that it can be varied slightly, in the range ofca±1.5, depending on the experimental conditions. Both the (Sn,Al)20and the (Sn,Al)24cages in the structure are fully occupied by the guest alkali metal atoms,i.e.K or Rb. TheA8Al8Sn38formula has a valence electron count that obeys the valence rules and represents an intrinsic semiconductor, while the experimentally determined compositionsA8Al8±xSn38∓xsuggest the synthesized materials to be nearly charge-balanced Zintl phases,i.e.they are likely to behave as heavily dopedp- orn-type semiconductors.