Hexagonal-Diamond-like Gold Lattices, Ba and (Au,T)₃ Interstitials, and Delocalized Bonding in a Family of Intermetallic Phases Ba₂Au₆(Au,T)₃ (T = Zn, Cd, Ga, In, or Sn)

Abstract

Au-rich polar intermetallics exhibit a wide variety of structural motifs, and this hexagonal-diamond-like gold host is unprecedented. The series Ba 2 Au 6 (Au,T) 3 (T = Zn, Cd, Ga, In, or Sn), synthesized through fusion of the elements at 700−800°C followed by annealing at 400−500°C, occur in space group R3̅ c (a ≈ 8.6−8.9 Å, c ≈ 21.9−22.6 Å, and Z = 6). Their remarkable structure, generated by just three independent atoms, features a hexagonal-diamond-like gold superstructure in which tunnels along the 3-fold axes are systematically filled by interstitial Ba atoms (blue) and triangles of disordered (Au,T) 3 atoms (green) in 2:1 proportions. The Au/Zn mixing in the latter spans ∼34 to 87% Zn, whereas the Au/Sn result is virtually invariant compositionally. Complementary bonding between the gold lattice and the disordered (Au,T) 3 units is substantial and very regular. Bonding and charge density analyses indicate delocalized bonding within the gold host and the (Au,T) 3 triangular units, and moderately polarized bonding between Ba and the electronegative framework. The new structure can also be viewed empirically as the result of an atom-by-triad [i.e., Ba by (Au,T) 3 triangle] topological substitution in a BaAu 2 (AlB 2 -type) superstructure.

Gold Network Structures in Rhombohedral and Monoclinic Sr2Au6(Au,T)3 (T = Zn, Ga). A Transition via Relaxation

“…The e/a datum for the galliumricher R3̅ c analogue analyzed is 1.71, larger than that for Ba parallel compound. 14 Structural Descriptions. In contrast, the bottom large ring around Sr contains three longest (3.082(1) Å) and three shortest (2.852(1) Å) Au−Au distances, alternating each other as it accommodates to the larger triangular insert; thus there are two groups of internal bond angles in this ring (Figure 3a and Supporting Information, Figure S4).…”

“…7−13 The recently reported rhombohedral Ba 2 Au 6 (Au,T) 3 (T = Zn, Cd, Ga, In, Sn) series exhibit an unprecedented hexagonaldiamond-like gold host lattice in which tunnels extending along the c axis are filled by ordered Ba atoms and (Au,T) 3 triangles. 14 The structure can be viewed as a topological atom-by-triad substitution of the AlB 2 -type BaAu 2 (P6/mmm), together with the formation of delocalized bonding within the triangles and with the gold host lattice. It is of great interest to examine how the structure and bonding evolve when the Ba is replaced by smaller Sr atoms and how electronic factors matter when Zn is replaced by trivalent Ga. One germane fact here is that the corresponding binary aurides containing the smaller alkaline-earth metals Sr and Ca have CeCu 2 -type structures (Imma), in contrast to the AlB 2 -type BaAu 2 (P6/mmm).…”

“…Tight binding calculations were performed according to the linear-muffin-tin-orbital (LMTO) method in the atomic sphere approximation (ASA) 23,24 to gain better understanding regarding the bonding in the new monoclinic phase, the rhombohedral structure having been so considered earlier for the Ba analogues. 14 An idealized monoclinic C2/c model for Sr 2 Au 7 Zn 2 is described later. No additional empty spheres were necessary beyond those described automatically, subject to a 16% overlap restriction between atom-centered spheres.…”

The Crystal Orbital Hamilton Population (COHP) Method as a Tool to Visualize and Analyze Chemical Bonding in Intermetallic Compounds

“…Notably, a survey of the average ICOHP/bond values and their percentages to the net bonding capabilities for the Au/post-transitionmetal−Au/post-transition-metal interactions in polar intermetallics with one-dimensional polyanionic tunnels in their crystal structures brings to light that the largest ICOHP/bond values and percentage contributions typically stem for the heteroatomic contacts (Table 2). The second of the three aforementioned classes of polar intermetallic compounds contains those materials composed of hexagonal diamond-like gold networks, which have so far been identified to be present in the crystal structures of four different types of polar intermetallics with diverse combinations of post-transition-elements (Zn, Cd, Al, Ga, In, or Sn) and active-metals (Sr, Ba, Eu) The second of the three aforementioned classes of polar intermetallic compounds contains those materials composed of hexagonal diamond-like gold networks, which have so far been identified to be present in the crystal structures of four different types of polar intermetallics with diverse combinations of post-transition-elements (Zn, Cd, Al, Ga, In, or Sn) and active-metals (Sr, Ba, Eu) [93,94,[103][104][105][106][107]. The cavities within the hexagonal-diamond-like gold networks encompass the active-metal atoms, or triangles assembled by extra gold and post-transition-metal atoms ( Figure 5).…”

Revealing Tendencies in the Electronic Structures of Polar Intermetallic Compounds

“…The second group of the active-metal-poor ternary intermetallic compounds, which are composed of an active-metal, gold and a post-transition-element and were taken into consideration for the survey of the previously reported −ICOHP/bond values and their respective percentages to the net bonding capabilities, contains those materials whose crystal structures comprise hexagonal-diamond-fashioned gold networks. In particular, the voids in the hexagonal-diamond-fashioned gold networks, which have to date been observed for four different types of structure and diverse combinations of gold with active metals (Sr, Ba, or Eu) and post-transition-metals (Zn, Cd, Al, Ga, In, or Sn) [132][133][134][135][136][137][138], enclose the active-metal atoms or triangles composed of the gold and the post-transition-metal atoms. Because the crystal structures of some of the inspected active-metal-poor intermetallic compounds comprise atomic sites with positional and/or occupational disorders (Table 4), the electronic structures of these materials were examined for models which approximate the actual crystal structures and often correspond to the lowest total energies.…”