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 Ba 8 Ga 16 Ge 30 ͑BGG͒, Sr 8 Ga 16 Ge 30 ͑SGG͒, Eu 8 Ga 16 Ge 30 ͑EGG͒, and Sr 8 Ga 16 Si 30 ͑SGS͒ single crystals, performed at a high-energy facility. Unexpected features evidencing the different shift for the 2a and 6d sites between Ba 4d and Sr 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 Ga 3d core levels. This may suggest thorough reconsiderations on earlier interpretations of experimental data.

Soft x-ray photoelectron spectroscopy study of type-I clathrates

“…The separations of two nearest peaks between the 2a and the 6d sites are 0.89 eV on average for Ba 4d. In order to check whether the observed separation is theoretically reasonable, we have performed first-principle band calculations on BGG using the WIEN2k package program as discussed previously [15,16]. The calculations predict that the orbital level of the 2a-site Ba is lower by 0.95 eV than that of the 6d-site one, being in good agreement with the experimental results.…”

“…The guest atoms were put in high-symmetry positions for both kinds of sites. The details have been reported elsewhere [16].…”

“…have recently been reported for type I clathrates, M 8 Ga 16 Ge 30 (M = Ba : BGG, M = Sr : SGG and M = Eu : EGG) [7,8], and applications are already under consideration due to the observed high energy conversion efficiency [9]. The energetics of the endohedral atoms, confined under the potentials created by the surrounding spaces, is very important, especially for the PGEC concept.…”

“…The energetics of the endohedral atoms, confined under the potentials created by the surrounding spaces, is very important, especially for the PGEC concept. Rattling phonons have been proposed as the key issue, ever since elastic neutron diffraction studies in M 8 Ga 16 Ge 30 demonstrated that the smaller-size Sr atom shows a wider distribution of its density map than that of the larger-size Ba, while the Eu atom shows an even wider quartic density distribution [8].…”

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AbstractExtensive soft x-ray photoelectron spectroscopy studies are performed on Ba 8 Ga 16 Ge 30 (BGG) and Sr 8 Ga 16 Ge 30 (SGG) single crystals ranging from Fermi to core levels, at a high-energy facility. Valence band x-ray photoelectron spectroscopy (XPS) experiments with theoretical calculations revealed that the valence band is mainly constructed by the Ge/Ga 4s and 4p wave functions with little contribution of the Ba/Sr atomic orbitals.

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