Comparison of local distortions in Ba sub(8)Ga sub(16)X sub(30 ) (X = Si, Ge, Sn): an EXAFS study

We report an extended X-ray fine structure (EXAFS) analysis of the type-I clathrates Ba sub(8)Ga sub(16)X sub(30 ) (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 Ba sub(8)Ga sub(16)Sn sub(3 0). The environment about Ba, extracted from Ba K edge EXAFS, becomes increasingly disordered from Ge to Si to Sn, and for Ba sub(8)Ga sub(16)Sn sub(3 0) the nearest Ba neighbor distance is very short, consistent with severe buckling. This buckling contributes to the increased local disorder for Ba sub(8)Ga sub(16)Si sub(3 0) and Ba sub(8)Ga sub(16)Sn sub(3 0), and provides an explanation for a higher resistivity and a lower ZTthan for Ba sub(8)Ga sub(16)Ge sub(3 0).