Ba8Ge43 revisited:: a 2a′ x 2a′ x 2a′ superstructure of the clathrate-I type with full vacancy ordering

The reinvestigation of the crystal structure of Ba(8)Ge(43)square(3) (space group Ia (3) over bard, no. 230; a = 21.3123(5) Angstrom; Z = 8; Pearson symbol cI408) shows a full ordering of the vacancies (square) in the germanium framework. This ordered crystal structure can be considered as a derivative of an ideal "Ba8Ge46" clathrate-I type structure (Pm (3) over barn, 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 (square at the 24c site, space group Ia (3) over bard). In the resulting Ge framework, each vacancy square 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 degreesC and exists in equilibrium with Ba6Ge25 and Ge. By decomposition of undercooled (metastable) Ba8Ge43, a new metastable binary BaGesimilar to5 phase is formed.