Ba₈Ge₄₃ revisited: a 2a′×2a′×2a′ Superstructure of the Clathrate‐I Type with Full Vacancy Ordering

Abstract

The reinvestigation of the crystal structure of Ba8Ge43□3 (space group $Ia{\bar 3}d$, no. 230; a = 21.3123(5) Å; Z = 8; Pearson symbol cI408) shows a full ordering of the vacancies (□) in the germanium framework. This ordered crystal structure can be considered as a derivative of an ideal “Ba8Ge46” clathrate‐I type structure ($Pm{\bar 3}n$, a′ = a/2) in which three Ge vacancies (per formula unit) are allowed to order in a cubic superstructure with a doubled unit cell parameter (□ at the 24c site, space group $Ia{\bar 3}d$). In the resulting Ge framework, each vacancy □ is surrounded by four three‐bonded (3b)Ge‐ species. The ordering in crystals of as‐cast samples (cooled in argon atmosphere, non‐annealed) is partially disrupted. For the “as‐cast” crystals, a short‐range model is proposed based on the partial distribution of Ge on the 24c and 24d sites. From the electron balance, Ba8Ge43 can be considered as a metallic Zintl phase with four excess electrons per formula unit. The Ba8Ge43 phase is stable in the temperature range 770 ‐ 810 °C and exists in equilibrium with Ba6Ge25 and Ge. By decomposition of undercooled (metastable) Ba8Ge43, a new metastable binary BaGe˜5 phase is formed.

NMR experiments and electronic structure calculations in type-I BaAlGe clathrates

“…Samples are denoted by Al x , for example Al 16 representing Ba 8 Al 16 Ge 30 . Powder x-ray diffraction ͑XRD͒ measurements showed the Al 13 and Al 16 samples to be pure type-I clathrate with no additional phases within experimental resolution, while Al 12 and Al 10 also exhibited a small amount of diamond-phase Ge. Rietveld analysis yielded best fits of Ba 8.0 Al 15.1 Ge 27.8 for the nominal Al 16 sample, and Ba 7.0 Al 12.6 Ge 33.5 for Al 12 .…”

“…Powder x-ray diffraction ͑XRD͒ measurements showed the Al 13 and Al 16 samples to be pure type-I clathrate with no additional phases within experimental resolution, while Al 12 and Al 10 also exhibited a small amount of diamond-phase Ge. Rietveld analysis yielded best fits of Ba 8.0 Al 15.1 Ge 27.8 for the nominal Al 16 sample, and Ba 7.0 Al 12.6 Ge 33.5 for Al 12 . For Al 16 these yielded 2.0, 5.2, and 7.9 Al atoms on the 6c, 16i, and 24k framework sites respectively in the type-I structure ͑space group Pm3n, #223͒, and for Al 12 , 1.5, 2.0, and 9.1 Al atoms.…”

“…For Al 16 these yielded 2.0, 5.2, and 7.9 Al atoms on the 6c, 16i, and 24k framework sites respectively in the type-I structure ͑space group Pm3n, #223͒, and for Al 12 , 1.5, 2.0, and 9.1 Al atoms. Lattice constants gradually decreased from 10.852 Å for Al 16 to 10.838 Å for Al 12 and 10.814 Å for Al 10 . Resistivities exhibited positive temperature coefficients, typical of metals or heavily doped semiconductors, with, for example, Al 16 having a low-temperature value of 170 ⍀-cm, within the range observed previously for Ba 8 Al 16 Ge 30 samples.…”

Thermoelectric Properties of the Clathrate I Ba8Ge43□3

“…Ba 8 Ge 43 h 3 is thermodynamically stable in the temperature range from 770°C to 810°C and cannot be obtained as a pure phase by quenching of sealed reaction containers. 4 To avoid the peritectoid decomposition below 770°C, the elements were placed in a glassy carbon crucible, heated to the melt with an induction furnace, and quenched between two steel plates in less than 1 s to room temperature. The reaction product typically consists of a circular plate, $10 mm in diameter and $1 mm in thickness.…”

“…Ba 8 Ge 43 h 3 was originally predicted to show metallic behavior according to the 8-N rule, 3,4 because only 12 of the 16 valence electrons related to the Ba atoms are necessary to satisfy the electronic requirements of 12 Ge anions surrounding three vacancies. Experimentally, it was found that the compound shows semiconducting behavior.…”