An Unusual Electron Count and Electron-Deficient Multi-Center Bonding in One Class of Intermetallics: The BaAl4, CaAl2Zn2, CeMg2Si2 and FCC Al Structures
Open Access
- 1 March 1986
- journal article
- Published by Walter de Gruyter GmbH in Zeitschrift für Naturforschung B
- Vol. 41 (3) , 292-320
- https://doi.org/10.1515/znb-1986-0304
Abstract
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 Al4 2- . 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.Keywords
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