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

Abstract

Extensive 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. Surprisingly, unexpected features evidencing the different shift for the 2a-and 6d-sites between Ba 4d and Sr 3d are observed. The detailed analyses including theoretical support by first-principles band-structure calculations lead to the conclusion that the component distributions of the larger tetrakaidecahedral cage are different depending on the endohedral atoms, which contrasts with the past consensus that BGG and SGG have the same framework structure. This may give thorough reconsiderations on earlier interpretations of experimental data.

Electronic properties across metal-insulator transition in β -pyrochlore-type CsW2O6 epitaxial films

“…Actually, it is revealed from the Rietveld analysis of powder X-ray and neutron diffractions that the Debye-Waller factor of the Cs atom significantly decreases in comparison to that of W and O atoms in the LT phase [9]. Furthermore, similar anomalous core-level spectra of rattling atoms are reported for cage materials [31][32][33][34], including a rattling superconducting material of Ba24Ge100, where the 12d-site Ba exists inside of the large cage of Ge. The 12d-site Ba 4d core levels split into two components (12d-1 and 12d-2) at 300 K reflecting the rattling motion.…”

Realizing p-type NbCoSn half-Heusler compounds with enhanced thermoelectric performance via Sc substitution

“…Therefore, it has been quite challenging to develop highly efficient thermoelectric materials. The strategic goals of TE research are to discover new materials with high TE performance and/or improve the performance of the existing well-known materials, such as SiGe alloy [10], PbTe [11], chalcogenides [12], skutterudites [13,14], clathrates [15,16], Zintl phases [17] as well as full/half-Heusler compounds [18,19] by band engineering and carrier filtering effect [20][21][22], phonon engineering [23,24], reducing the dimension of materials [25], or spin fluctuation [26].…”