Guest-Framework Interaction in Type I Inorganic Clathrates with Promising Thermoelectric Properties: On the Ionic versus Neutral Nature of the Alkaline-Earth Metal Guest A in A8Ga16Ge30 (A=Sr,Ba)

Periodic density functional calculations, using pseudopotentials and a local basis set have been performed on the type-I clathrates, A8Ga16Ge30 (A = Sr,Ba). Both are known to show promising thermoelectric properties. Ab-initio wave functions have been analyzed within the Quantum Theory of Atoms in Molecules. This has enabled us to analyse both the charge transfer, and bonding properties of the clathrate, from a rigorous quantum mechanical viewpoint. The Ba and Sr are found to be largely ionic (charge is about +1.7 e) both in the smaller 20-atom and in the larger 24-atom cages, reconciling theory to a Zintl phase view of these type I clathrates. The assertion that the Sr atoms in the different cages have similar oxidation states is shown to be consistent with multi-wave length diffraction experiments for Sr8Ga16Ge30; while the assertion of ionicity of the Sr is supported by the observation that the adsorption edge lies close to that previously found in the Sr K-edge XANES spectra of Sr(OH)2·8H2O. As such, this work contradicts previous experimental and theoretical studies that claim that the guest atoms are neutral. We show that the discrepancy to be related to the definitions used for electron transfer. Definitions based on electron displacement (rearrangement) in space, as done in previous works, do not account for the variation in shape and volume of the atomic catchment's regions upon change in the number and average locations of the particles in the system. Eventually, such definitions lead to underestimation of charge transfer. The large binding energy found in earlier work for Ba and Sr in these materials is found to be consistent with a simple picture of charge transfer from the guest to the frame. Preliminary investigations on a clathrate of perfect stoichiometry, appear to rule out any important relationship between the observed increase in the thermoelectric figure of merit with increasing external pressure, and the host-guest charge transfer.