Crystal structure of the novel chiral clathrate, Ba6In4Ge21
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Source of material: The compound was synthesized from the elements under Ar in a sealed Nb capsule (ratio 4:6:21; heating up to 1320 K; cooling down (1 d)). The silvery grey, brittle compound is a semiconductor, and stable against air, moisture and dilute acids and bases. The chemical analysis with selected crystals gave Ba6.2ln4.3Ge20.7(AES-ICP) (see ref. 1). Ваб1п4Се21 forms a novel chiral structure (cPlTA, see refs. 1, 2) which was also found with other compounds in the last years (see refs. 3, 4). The refinement of the occupancies gave the composition Ваб1плСе25-л with η = 4.06 ± 0.13 . The structure is characterized by a network of condensed pentagondodecahedra (pdod). Each pdod shares faces (shaded in figure b) with three others and is connected to a fourth via an additional skelton bond (figure c). The essential pdod (figure a) consists of 4 threefold (black) and 16 fourfold bonded atoms, leading to 108 bonding electrons per formular unit: [In4Ge2i] Each pdod contains the complete information about the chirality (chains E3-E1-E1, see figure b). Fragments of the network are (4,4,4)-barrelans (figure d) as well as (4,4,4)-propeUans (one wing see figure c) formed by condensed pdods. Hie general pattern of the cP124 structure is described by i(MM'2M"3[(3b)Y8(4b)Xi7]), M = Баз, M' = Bal, Μ" = Βώ, with 8 Ge atoms at the threefold bonded (3b)Y nodes (as Ge") and with 17 E atoms at the fourfold bonded (4b)X nodes (as In~, Ge®). The structure is a hierachical derivative of β-Μη and AI2CM03 (Bal, ВаЗ, Ва2) and contains elements of clathrates and zeolites. Bal center the pdod, Ba2 and ВаЗ are in the larger and smaller open cavities of the channels. Ibe anisotropic displacements (or split positions) of Ba2 and ВаЗ extend along the channels, which follow the path of the i i i ~ У graph. The bond lengths in the Eioonet vary between 2.526 A and 2.654 Â.