An Unusual Electron Count and Electron-Deficient Multi-Center Bonding in One Class of Intermetallics: The BaAl4, CaAl2Zn2, CeMg2Si2 and FCC Al Structures
An Unusual Electron Count and Electron-Deficient Multi-Center Bonding in One Class of Intermetallics: The BaAl4, CaAl2Zn2, CeMg2Si2 and FCC Al Structures
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一类金属间化合物中不寻常的电子计数和缺电子多中心键合：BaAl4、CaAl2Zn2、CeMg2Si2 和 FCC Al 结构
DOI：
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发表时间：
1986
期刊：
影响因子：
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通讯作者：
R. Hoffmann
中科院分区：
文献类型：
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作者：
C. Zheng;R. Hoffmann
所讨论的结构类型可以被认为是由从FCC晶格中切割出来的二维层组成的。每一层都有一个正方形的亚晶格，上面和下面是四倍空的顶端原子。构建了这种人工智能层的电子结构-每个人工智能有八个低洼带。其中两条条带位于顶端，指向离层的方向。在堆叠时，顶端Al之间形成键。导致1个Αla-ΑΙa σ*带被推高，7个带后的间隙被填满。这是BaAl4和Al42-。另一幅图是每个空穴的离域五中心六电子成键，与B5H9中的缺电子成键密切相关。CeAl2Ga2和CeMg2Si2中的键合可以通过扰动原始的BaAl4得到。给出了四个原子的电子计数超过~ 16时从BaAl转换到CeMg2Si2结构类型的基本原理。最后，我们将这些论点与铝金属的高结合能联系起来。
The structural types discussed may be thought of as being composed of two-dimensional layers cleaved out of the FCC lattice. Each layer has a square sublattice capped by apical atoms above and below four-fold hollows. The electronic structure of such an Al layer is constructed - it has eight low-lying bands per tour AI. and two of these bands are localized in apical Al s. pointing away from the layer. On stacking, bonds are formed between apical Al′s. leading to the pushing up of one Αla-ΑΙa σ* band, and a gap after 7 bands are filled. This is BaAl4, with Al42- . An alternative picture is of delocalized five-center six-electron bonding in each hollow, closely related to the electron deficient bonding in B5H9. The bonding in CeAl2Ga2 and CeMg2Si2 can be derived similarly, by perturbing the original BaAl4 case. A rationale for switching from BaAl, to the CeMg2Si2 structural type for electron counts exceeding ∼ 16 for four atoms is given. Finally we relate these arguments to the high cohesive energy of Al metal.