Ba 8 Ge 43 revisited: a 2 a ′×2 a ′×2 a ′ Superstructure of the Clathrate‐I Type with Full Vacancy Ordering

The reinvestigation of the crystal structure of Ba 8 Ge 43 □ 3 (space group Ia 3d , no. 230; a = 21.3123(5) Å; Z = 8; Pearson symbol cI 408) shows a full ordering of the vacancies (□) in the germanium framework. This ordered crystal structure can be considered as a derivative of an ideal “Ba 8 Ge 46 ” clathrate‐I type structure ( Pm 3n , 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 24 c site, space group Ia 3d ). 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 24 c and 24 d sites. From the electron balance, Ba 8 Ge 43 can be considered as a metallic Zintl phase with four excess electrons per formula unit. The Ba 8 Ge 43 phase is stable in the temperature range 770 ‐ 810 °C and exists in equilibrium with Ba 6 Ge 25 and Ge. By decomposition of undercooled (metastable) Ba 8 Ge 43 , a new metastable binary BaGe ˜5 phase is formed.