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Structure and Bonding of an Intergrowth Phase Ca 7 Ag 2+ x Ge 7– x ( x ≈ 2/3) Featuring a Zintl‐Type Polyanionic Chain
Author(s) -
Ponou Siméon,
Doverbratt Isa,
Lidin Sven,
Miller Gordon J.
Publication year - 2016
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.201501176
Subject(s) - chemistry , zintl phase , antibonding molecular orbital , crystallography , germanide , electron localization function , electronic structure , ternary operation , crystal structure , context (archaeology) , germanium , electron , atomic orbital , computational chemistry , physics , paleontology , organic chemistry , quantum mechanics , silicon , computer science , programming language , biology
Single crystals of a new ternary phase, Ca 7 Ag 2+ x Ge 7– x [ x = 0.48(3)], were obtained from as side‐product of high‐temperature solid‐state reactions, and its crystal structure determined by X‐ray diffraction methods. Following the Zintl concept, the anionic substructure consists of a novel pentameric Zintl anion [Ge 5 ] 12– with C 2 v local symmetry, as well as [Ag x Ge 2– x ] (6–3 x )– units accounting for [Ge 2 ] 6– dimers and isolated Ge 4– (and Ag + ) species sharing the same atomic sites. DFT‐level band structure calculations were carried out on a hypothetical, fully ordered model ( x = 0). We found that the electronic structure associated with the planar W‐shaped [Ge 5 ] 12– polyanions is more suited to optimize the structure's stability than the helical configuration of the isoelectronic [Se 5 ] 2– , in the context of incomplete charge transfer from the electropositive metal Ca. Thus, the antibonding states at the Fermi level that are centered on the two [Ge n ] (2 n +2)– oligomers can be depopulated by means of Ge‐to‐Ca electron back donation, strengthening at the same time the Ge–Ge bonds. These antibonding states also endow the system with substantial electronic flexibility, which may result in some phase width. Finally, plausible local ordering models of Ag/Ge mixing in [Ag x Ge 2– x ] (6–3 x )– units for x = 2/3, as expected from the Zintl concept, are briefly discussed within the “coloring problem” approach.