Defusing Complexity in Intermetallics: How Covalently Shared Electron Pairs Stabilize the FCC Variant Mo2CuxGa6鈥?i>x (x 鈮?0.9)

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• 作者：Brandon J. Kilduff ; Vincent J. Yannello ; Daniel C. Fredrickson
• 刊名：Inorganic Chemistry
• 出版年：2015
• 出版时间：August 17, 2015
• 年：2015
• 卷：54
• 期：16
• 页码：8103-8110
• 全文大小：581K
• ISSN：1520-510X

文摘

Simple sphere packings of metallic atoms are generally assumed to exhibit highly delocalized bonding, often visualized in terms of a lattice of metal cations immersed in an electron gas. In this Article, we present a compound that demonstrates how covalently shared electron pairs can, in fact, play a key role in the stability of such structures: Mo₂Cu_xGa_6鈥?i>x (x 鈮?0.9). Mo₂Cu_xGa_6鈥?i>x adopts a variant of the common TiAl₃ structure type, which itself is a binary coloring of the fcc lattice. Electronic structure calculations trace the formation of this compound to a magic electron count of 14 electrons/T atom (T = transition metal) for the TiAl₃ type, for which the Fermi energy coincides with an electronic pseudogap. This count is one electron/T atom lower than the electron concentration for a hypothetical MoGa₃ phase, making this structure less competitive relative to more complex alternatives. The favorable 14 electron count can be reached, however, through the partial substitution of Ga with Cu. Using DFT-calibrated H眉ckel calculations and the reversed approximation Molecular Orbital (raMO) method, we show that the favorability of the 14 electron count has a simple structural origin in terms of the 18 鈥?n rule of T鈥揈 intermetallics (E = main group element): the T atoms of the TiAl₃ type are arranged into square nets whose edges are bridged by E atoms. The presence of shared electron pairs along these T鈥揟 contacts allows for 18 electron configurations to be achieved on the T atoms despite possessing only 18 鈥?4 = 14 electrons/T atom. This bonding scheme provides a rationale for the observed stability range of TiAl₃ type TE₃ phases of ca. 13鈥?4 electrons/T atom, and demonstrates how the concept of the covalent bond can extend even to the most metallic of structure types.