Semiconducting clathrates meet gas hydrates: Xe₂₄[Sn ₁₃₆]

Semiconducting Group 14 clathrates are inorganic host-guest materials with a close structural relationship to gas hydrates. Here we utilize this inherent structural relationship to derive a new class of porous semiconductor materials: noble gas filled Group 14 clathrates (Ng_x[M₁₃₆], Ng=Ar, Kr, Xe and M=Si, Ge, Sn). We have carried out high-level quantum chemical studies using periodic Local-MP2 (LMP2) and dispersion-corrected density functional methods (DFT-B3LYP-D3) to properly describe the dispersive host-guest interactions. The adsorption of noble gas atoms within clathrate-II framework turned out to be energetically clearly favorable for several host-guest systems. For the energetically most favorable noble gas filled clathrate, Xe ₂₄[Sn₁₃₆], the adsorption energy is -52 kJ mol ^-1 per guest atom at the LMP2/TZVPP level of theory, corresponding to -9.2 kJ mol^-1 per framework Sn atom. Considering that a hypothetical guest-free Sn clathrate-II host framework is only 2.6 kJ mol^-1 per Sn atom less stable than diamond-like α-Sn, the stabilization resulting from the noble gas adsorption is very significant.