Electronic structure of Si and Ge gold-doped clathrates

Abstract

The electronic properties of single phase type-I clathrate compounds, Ba 8 Au 6 (Si or Ge) 40 and Ba 8 Ge 46Ϫx have been investigated. The crystal structure, electrical resistivities, magnetic susceptibilities, the density of states at the Fermi level, and the band gap ͑where applicable͒ were determined. Ba 8 Au 6 Si 40 and Ba 8 Au 6 Ge 40 show a metallic behavior whereas Ba 8 Ge 46Ϫx is semiconducting. On a basis of this work, a first evidence is presented of the existence of type-I mixed-clathrates containing both silicon and germanium.

Refinement of the crystal structure of K8Ge44⬜2, an intermetallic clathrate I

“…The framework defects in K 8 Ge 44 £ 2 have been confirmed in recent works from powder diffraction data [12,13]. Moreover, the validity of the split atom model has been proven by the detection of superstructures with ordered defect arrangement (space group Ia3d )for the clathrate-I phases Rb 8Sn44£2 [14], Cs8Sn44£2 [15] and Ba 8Ge43£3 [9,16,17]. Interestingly, vacancies occur for the larger tetrel elemenst but not for the corresponding silicide.…”

Thermoelectric Properties of the Clathrate I Ba8Ge43□3

“…Experimentally, it was found that the compound shows semiconducting behavior. 5,6 The apparent discrepancy from the simple valence rules has been tentatively ascribed to an ordering of the framework vacancies resulting in modifications of the electronic band structure near the Fermi level. 5 On the other hand, investigations on the ternary system Ba 8Àx Eu x Ge 43 (x < 0.6) revealed metallic behavior for the whole homogeneity range.…”

Superconductivity and Magnetism in Silicon and Germanium Clathrates

“…In Ba 8 46 exhibits an unbroken framework, as established through crystallography and NMR [27,62]. However its analog Ba 8 Ge 46 does not form a fully populated framework, instead favoring spontaneous vacancy formation [63,64], with the configuration Ba 8 Ge 43 3 , denoting the vacancies, with a superstructure as shown in Figure 11. According to a Zintl picture four vacancies, with 4 electrons each, might be expected to accept the 16 electrons from Ba 2+ , completing a network of filled bonds.…”