Gold Polar Intermetallics: Structural Versatility through Exclusive Bonding Motifs

Abstract

The design of new materials with desired chemical and physical characteristics requires thorough understanding of the underlying composition-structure-property relationships and the experimental possibility of their modification through the controlled involvement of new components. From this point of view, intermetallic phases, a class of compounds formed by two or more metals, present an endless field of combinations that produce several chemical compound classes ranging from simple alloys to true ionic compounds. Polar intermetallics (PICs) belong to the class that is electronically situated in the middle, between Hume-Rothery phases and Zintl compounds and possessing e/a (valence electron per atom) values around 2. In contrast to the latter, where logical rules of formation and classification systems were developed decades ago, polar intermetallics remain a dark horse with a huge diversity of crystal structures but unclear mechanisms of their formation. Partial incorporation of structural and bonding features from both nonpolar and Zintl compounds is commonly observed here. A decent number of PICs can be described in terms of complex metallic alloys (CMAs) following the Hume-Rothery electron-counting schemes but exhibit electronic structure changes that cannot be explained by the latter. Our research is aimed at the discovery and synthesis of new polar intermetallic compounds, their structural characterization, and investigation of their properties in line with the analysis of the principles connecting all of these components. Understanding of the basic structural tendencies is one of the most anticipated outcomes of this analysis, and systematization of the available knowledge is the initial and most important step. In this Account, we focus on a well-represented but rather small section of PICs: ternary intermetallic compounds of gold with electropositive and post-transition metals of groups 12 to 15. The strong influence of relativistic effects in its chemical bonding results in special, frequently unique structural motifs, while at the same time gold participates in common structure types as an ordinary transition element. Enhanced bonding strength leads to the formation and stabilization of complex homo- and heteroatomic clusters and networks that are compositionally restricted to just a few options throughout the periodic table. Because it has the highest absolute electronegativity among metals, comparable to those of some halogens, gold usually plays the role of an anion, even being able to form true salts with the most electropositive metals. We discuss the occurrence of the structure types and show the place of gold intermetallics in the general picture. Among the structures considered are ones as common as AlB or BaAl types, in line with the recently discovered diamond-like homoatomic metal networks, formation of local fivefold symmetry, different types of tunneled structures, and more complex intergrown multicomponent structures.

An Obscured or Nonexistent Binary Intermetallic, Co7Pr17, Its Existent Neighbor Co2Pr5, and Two New Ternaries in the System Co/Sn/Pr, CoSn3Pr1–x, and Co2–xSn7Pr3

“…The DOS curve for CoSn3Pr exhibits a maximum in the range of −0.5 to −2.5 eV which is significantly closer to the Fermi level than in corresponding palladides, platinides or aurides. 27,29,57 The close vicinity of the Co 3d orbitals to the Fermi level suggests that their role must be more important in valence electron count while the transition metals are usually assumed to not provide valence electrons. Such a narrow distribution of the d orbitals is typical for some ionic platinides where relativistic effects play a crucial role, 58,59 while the p-element containing compounds usually show considerably higher dispersion.…”

Overlooked Binary Compounds Uncovered in the Reinspection of the La–Au System: Synthesis, Crystal Structures, and Electronic Properties of La7Au3, La3Au2, and La3Au4

“…The location of gold in the sixth period of the periodic table lends this element physical and chemical characteristics that are often strikingly different from those of the lighter congeners in group 11. 1 To a large extent, these differences are associated with strong relativistic effects impacting the electronic properties of the gold valence states. 1 , 2 In particular, relativistic stabilization of the 6s 2 electronic configuration results in a high electronegativity of gold, comparable to that of iodine on the Pauling scale, 3 although some recently developed electronegativity scales suggest a somewhat lower value for Au in comparison to I.…”

“… 1 To a large extent, these differences are associated with strong relativistic effects impacting the electronic properties of the gold valence states. 1 , 2 In particular, relativistic stabilization of the 6s 2 electronic configuration results in a high electronegativity of gold, comparable to that of iodine on the Pauling scale, 3 although some recently developed electronegativity scales suggest a somewhat lower value for Au in comparison to I. 4 , 5 For this reason, compounds of Au with other metals frequently reveal a significant transfer of the electron density to the Au atoms, which allows their description as aurides, i.e., phases with anionic Au species.…”

Uncovering new transition metal Zintl phases by cation substitution: the crystal chemistry of Ca3CuGe3 and Ca2+nMnxAg2−x+zGe2+n−z (n = 3, 4)

“…Polar intermetallic compounds (PICs) form a large class of extended solids, combining electropositive 'active' metals (s-or f-block metals mainly) with electronegative post-transition pblock elements and/or late transition (noble) metals. [1][2] Interest in polar intermetallic compounds were originally driven by their structural diversity and exotic bonding features. [2][3][4][5] In fact, they represent intermediates between semiconductors and typical metallic systems.…”

“…[1][2] Interest in polar intermetallic compounds were originally driven by their structural diversity and exotic bonding features. [2][3][4][5] In fact, they represent intermediates between semiconductors and typical metallic systems. [5][6] Hence, their bonding pictures are often very complex and, it is virtually impossible to apply simple heuristic concepts to reliably deduce their compositions, structural features or the nature of the bonding.…”