Energetics of endohedral atoms in type-I clathrates observed by soft x-ray spectroscopy

We present an extensive soft x-ray photoelectron spectroscopy study on the core levels of the endohedral atoms of Ba8 Ga16 Ge30 (BGG), Sr8 Ga16 Ge30 (SGG), Eu8 Ga16 Ge30 (EGG), and Sr8 Ga16 Si30 (SGS) single crystals, performed at a high-energy facility. Unexpected features evidencing the different shift for the 2a and 6d sites between Ba4d and Sr3d are observed for BGG and SGG. The detailed analyses including theoretical support by first-principles band-structure calculations lead to the surprising conclusion that the component distributions of the larger tetrakaidecahedral cage are different depending on the endohedral atoms, which is in contrast to the past consensus that BGG and SGG have the same framework structure. The differences are further supported by analysis of the Ga3d core levels. This may suggest thorough reconsiderations on earlier interpretations of experimental data.