An Unusual Electron Count and Electron-Deficient Multi-Center Bonding in One Class of Intermetallics: The BaAl₄, CaAl₂Zn₂, CeMg₂Si₂ and FCC Al Structures

Abstract

<jats:p>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 Αl<jats:sub>a</jats:sub>-ΑΙ<jats:sub>a</jats:sub> σ* band, and a gap after 7 bands are filled. This is BaAl<jats:sub>4,</jats:sub> with Al<jats:sub>4</jats:sub><jats:sup>2-</jats:sup> . An alternative picture is of delocalized five-center six-electron bonding in each hollow, closely related to the electron deficient bonding in B<jats:sub>5</jats:sub>H<jats:sub>9</jats:sub>. The bonding in CeAl<jats:sub>2</jats:sub>Ga<jats:sub>2</jats:sub> and CeMg<jats:sub>2</jats:sub>Si<jats:sub>2</jats:sub> can be derived similarly, by perturbing the original BaAl<jats:sub>4</jats:sub> case. A rationale for switching from BaAl, to the CeMg<jats:sub>2</jats:sub>Si<jats:sub>2</jats:sub> 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.</jats:p>

Chemistry of layeredd-metal pnictide oxides and their potential as candidates for new superconductors

“…Furthermore, possible approaches to the design of new superconductors based on these layered d-metal pnictide oxides are proposed. layers are also found in diverse non-oxide compounds, and one of their simplest forms, ThCr 2 Si 2 -type compounds, has extensively been studied both experimentally [34][35][36][37][38] and theoretically [39][40][41]. On the other hand, the anti-fluorite-type [Ln 2 O 2 ] layers are often found in oxychalcogenides and oxyhalides (also called 'oxide halides'), and one of their simplest forms, PbClF-type compounds, also has a large number of members [42,43].…”

Electronic Structure and Superconductivity of Compressed Metal Tetrahydrides

“…A detailed extended Hückel analysis of the electronic structure of one member of this family, BaAl 4 , was carried out to understand the origins of the magic electron count of ~14. [58] The BaAl 4 structure contains two types of aluminum atoms, those in apical (Al a ) positions are found on Wyckoff site e, and those in the basal (Al b ) positions are found on Wyckoff site d. In BaAl 4 , as in most main group examples of this structure, a bond forms between two atoms on the apical positions because only the σ bonding level is filled. The distance between an apical and four basal atoms in BaAl 4 is also quite short rendering the basal atoms fourcoordinate, and the apical atoms five-coordinate.…”

NMR Spectroscopy of Intermetallic Compounds: An Experimental and Theoretical Approach to Local Atomic Arrangements in Binary Gallides

“…Ga(5b) and Ga(4b) are involved in two-center and multi-center bonding, respectively. [49,50] Figure 9 clearly shows that Ga atoms are able to arrange with different bonding situations. The variation of the bonding situation results from the different occupation of p x -, p y -, and p z -like states.…”