Comparison of local distortions in Ba₈Ga₁₆X₃₀ (X = Si, Ge, Sn): An EXAFS study

We report an extended X-ray fine structure (EXAFS) analysis of the type-I clathrates Ba₈Ga₁₆X₃₀ (X = Si, Sn) and compare the results with the results of previous studies on X = Ge. The distribution of Ga on the three crystallographic cage sites is not random, with the Ga preferentially having X as the nearest neighbor. Our results show that for X = Si, Sn the average pair distances within the cages (Ga-Sn, Ga-Ga, Ga-Si, Sn-Sn) are significantly different from the distances found in X-ray diffraction, with some much shorter bonds and some much longer bonds. These results suggest a substantial buckling of the cages, particularly for Ba₈Ga₁₆Sn₃₀. The environment about Ba, extracted from Ba K edge EXAFS, becomes increasingly disordered from Ge to Si to Sn, and for Ba₈Ga₁₆Sn₃₀ the nearest Ba neighbor distance is very short, consistent with severe buckling. This buckling contributes to the increased local disorder for Ba₈Ga₁₆Si₃₀ and Ba₈Ga₁₆Sn₃₀, and provides an explanation for a higher resistivity and a lower ZT than for Ba₈Ga₁₆Ge₃₀.