Direct verification of Ga-Ga bond avoidance in the type-I clathrate Ba8 Ga16 Sn30 from its x-ray absorption fine structure

Experiments in the past have only been able to suggest that Ga-Ga bonds are not favored in the cage structure of Ba8 Ga16 Sn30 and other type-I clathrates. Here we show definitive evidence that this is indeed the case. Using the extended x-ray-absorption fine structure technique, we are able compare the backscattering functions for the first neighbors about Ga to the calculated functions for Ga-Ga and Ga-Sn bonds. The result is that only ∼15% of the Ga nearest neighbors are Ga. Combining this result with diffraction data on occupational parameters, we propose one possible arrangement of Ga and Sn in the unit cell of Ba8 Ga16 Sn30. Additionally, we find significant disorder in the Ga/Sn lattice; the Ga-Sn bond is 0.07 Å and the Ga-Ga 0.2 Åshorter than the average bond length, which must contribute to the smaller thermal conductivity.