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

Abstract

We present an extensive soft x-ray photoelectron spectroscopy study on the core levels of the endohedral atoms of ${\text{Ba}}_{8}{\text{Ga}}_{16}{\text{Ge}}_{30}$ (BGG), ${\text{Sr}}_{8}{\text{Ga}}_{16}{\text{Ge}}_{30}$ (SGG), ${\text{Eu}}_{8}{\text{Ga}}_{16}{\text{Ge}}_{30}$ (EGG), and ${\text{Sr}}_{8}{\text{Ga}}_{16}{\text{Si}}_{30}$ (SGS) single crystals, performed at a high-energy facility. Unexpected features evidencing the different shift for the $2a$ and $6d$ sites between $\text{Ba}\text{ }4d$ and $\text{Sr}\text{ }3d$ 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 $\text{Ga}\text{ }3d$ core levels. This may suggest thorough reconsiderations on earlier interpretations of experimental data.