Fragmentation of the Fluorite Type in Fe₈Al_17.4Si_7.6: Structural Complexity in Intermetallics Dictated by the 18 Electron Rule

Abstract

This Article presents the synthesis, structure determination, and bonding analysis of Fe(8)Al(17.4)Si(7.6). Fe(8)Al(17.4)Si(7.6) crystallizes in a new monoclinic structure type based on columns of the fluorite (CaF(2)) structure type. As such, the compound can be seen as part of a structural series in which the fluorite structure-adopted by several transition metal disilicides (TMSi(2))-is fragmented by the incorporation of Al. Electronic structure analysis using density functional theory (DFT) and DFT-calibrated Hückel calculations indicates that the fluorite-type TMSi(2) phases (TM = Co, Ni) exhibit density of states (DOS) pseudogaps near their Fermi energies. An analogous pseudogap occurs for Fe(8)Al(17.4)Si(7.6), revealing that its complex structure serves to preserve this stabilizing feature of the electronic structure. Pursuing the origins of these pseudogaps leads to a simple picture: the DOS minimum in the TMSi(2) structures arises via a bonding scheme analogous to those of 18 electron transition metal complexes. Replacement of Si with Al leads to the necessity of increasing the (Si/Al):TM ratio to maintain this valence electron concentration. The excess Si/Al atoms are accommodated through the fragmentation of the fluorite type. The resulting picture highlights how the elucidating power of bonding concepts from transition metal complexes can extend into the intermetallic realm.

Magnetically stabilized Fe8(μ4-S)6S8 clusters in Ba6Fe25S27

“…For each atom, s-and p-orbitals were modeled with a single-ζ value while d-orbitals were modeled with a double-ζ basis. Implementing a double-ζ basis for the d-orbitals is a standard practice 75,[77][78][79][80] that enhances the ability of the d-orbital to model contracted nonbondinglike behaviour and more diffuse characteristics emanating from bonding.…”

“…[75][76][77] Most recently, DFT-calibrated Hückel calculations have revealed a strong relationship between the resulting molecular orbital (MO) diagrams of local coordination environments and the extended solids within which they are embedded. [78][79][80] In the following sections, we will take a similar approach to develop σ-bonding MO models to explain the chemistry of octonuclear cluster containing phases. Within our model, we only consider the local coordination of the metal sites as Crystal Hamilton Overlap Populations (COHP) 81 calculations we have performed indicate that M-S interactions far eclipsing the strength of M-M interactions.…”

Bonding Schemes for Polar Intermetallics through Molecular Orbital Models: Ca-Supported Pt–Pt Bonds in Ca10Pt7Si3

“…As has been demonstrated for several transition metal-Si-Al phases [35,36], the bonding in such arrangements can be explored through molecular orbital (MO) models using DFT-calibrated Hückel calculations (other applications and advantages of DFT-calibrated Hückel models are discussed in [33]). To develop such a MO scheme, we used the GGA-DFT electronic structure as the basis for the parameterization of a best-fit Hückel model (see the Computational Procedures section).…”

At the Limits of Isolobal Bonding: π-Based Covalent Magnetism in Mn2Hg5

“…Mn 2 Hg 5 belongs to a family of transition metal–main group (T-E) intermetallics for which an extension of the 18-electron rule of molecular T complexes has proven especially effective for rationalizing structure and bonding. The importance of 18-electron configurations in intermetallic chemistry was perhaps first recognized in the origins of band gaps in half-Heusler alloys. , More recently, we have used density functional theory (DFT)-calibrated simple Hückel calculations and the reversed approximation Molecular Orbital (raMO) method to connect a broad range of structural phenomena to achieving and maintaining such octadecets. − These results are summarized in the 18- n rule, which simply states that each T atom will need 18- n electrons of its own for a closed shell, where n is the number of electrons it gains through the covalent sharing of electrons with neighboring T atoms in multicenter functions isolobal to classical T–T bondshaving the same overall nodal properties and occupancybut with multicenter character arising from contributions from bridging E atoms. Independently, Kitahara and co-workers reached similar conclusions for T–Al and T–Ga phases through an insightful use of maximally localized Wannier functions and demonstrated the applicability of this type of electron counting to T–T′–Al quasicrystal approximants. − …”