Positional disorder of Ba in the thermoelectric germanium clathrate Ba6Ge25

Abstract

The local structure of Ba 6 Ge 25 has been studied by x-ray diffraction and the atomic pair distribution function technique at 40 K and room temperature. Unambiguous evidence has been found that two out of three types of Ba atoms in Ba 6 Ge 25 move off their positions and become locked in split sites at low temperatures.

Ba6Ge25: low-temperature Ge–Ge bond breaking during temperature-induced structure transformation

“…Analysis of our preceding low-temperature X-ray powder diffraction data on Ba 6 Ge 25 [9] indicated that the cubic symmetry of Ba 6 Ge 25 is maintained in the whole temperature range (10-295 K), and that the Ba2 and Ba3 positions displayed pronounced atomic displacements during the transition (this was confirmed by independent investigations on powder samples [13]). However, variation in the germanium framework is difficult to be recognized from powder data.…”

“…Thus, the split site of the Ba1 atom (at 8c site) is called Ba1 0 (24e), Ba2 (12d) is described by a central site Ba2 0 (12d) and the satellites Ba2 00 (24e) and Ba2 000 (24e), while the split position of Ba3 (4a) is named Ba3 0 (8c) (see Figs. 5,13,14). The major and minor sites of Ge4 are called Ge4 0 (12d) and Ge4 00 (24e), respectively (see Fig.…”

The Intrinsic Shape of the cubic cP124 Clathrate Structure

“…[21] have analysed this phase transition in detail. [21] with the 20 tetrahedral centers of gravity inside the M 12 E positions in T center in T center in T type m(T) Ba 6 Ge 25 [19] β the Ge-Ge network by breaking bonds and localization of the excess electrons in non-bonding orbitals of germanium atoms. This intrastructural chemical process leads to a drastical change in physical behaviour.…”

High-pressure Raman study of the Ba-doped silicon clathrateBa24Si100up to27GPa

“…Ba 24 Ge 100 undergoes a two-step first-order phase transition between 230 and 180 K from metallic behavior to a high-resistivity state, 19,20 which is due to the Geu Ge bond breaking combined with the displacement of neighboring Ba atoms to the split positions. [21][22][23] However, BCS-like superconductivity occurs at low temperature T c = 0.24 K. By applying pressure, the phase transition is shifted to lower temperature, but T c increases remarkably to 3.8 K at 2.8 GPa. [19][20][21][22] These unusual properties are understood by the density of states at the Fermi energy.…”

“…[21][22][23] However, BCS-like superconductivity occurs at low temperature T c = 0.24 K. By applying pressure, the phase transition is shifted to lower temperature, but T c increases remarkably to 3.8 K at 2.8 GPa. [19][20][21][22] These unusual properties are understood by the density of states at the Fermi energy. 22,24 On the other hand, Ba 24 Si 100 showed a metallic conductivity without the phase transition down to 2 K at atmospheric pressure.…”