Crystal Structure and Bonding in BaAu₅Ga₂ and AeAu_4+xGa_3–x (Ae = Ba and Eu): Hexagonal Diamond-Type Au Frameworks and Remarkable Cation/Anion Partitioning in the Ae–Au–Ga Systems

Abstract

Five new polar intermetallic compounds in the Ae-Ga-Au system (Ae = Ba, Eu), BaAu(5)Ga(2) (I), BaAu(4.3)Ga(2.7) (II), Ba(1.0)Au(4.5)Ga(2.4 )(III), EuAu(4.8)Ga(2.2) (IV), and Eu(1.1)Au(4.4)Ga(2.2) (V), have been synthesized and their crystal structures determined by single-crystal X-ray diffraction. I crystallizes in the orthorhombic crystal system with a large unit cell [Pearson symbol oP64; Pnma, Z = 8, a = 8.8350(5) Å, b = 7.1888(3)Å, c = 20.3880(7) Å], whereas all other compounds are hexagonal [hP24; P6̅2m, Z = 3, a = 8.54-8.77(1) Å, c = 7.19-7.24(1) Å]. Both structures contain mutually orthogonal layers of Au(6) hexagons in chair and boat conformations, resulting in a hexagonal diamond-like network. Ae atoms and additional (Au/Ga)(3) groups are formally encapsulated by (Au(6))(2) distorted hexagonal prisms formed of three edge-sharing hexagons in the boat conformation or, alternatively, lie between two Au(6) hexagons in the chair conformation. The (Au/Ga)(3) groups can be substituted by Ae atoms in some of the hexagonal structures with no change to the structural symmetry. Tight-binding electronic structure calculations using linear-muffin-tin-orbital methods on idealized models "BaAu(5)Ga(2)" and "BaAu(4)Ga(3)" show both compounds to be metallic with evident pseudogaps near the corresponding Fermi levels. The integrated crystal orbital Hamilton populations are dominated by Au-Au and Au-Ga orbital interactions, although Ba-Au and Ba-Ga contributions are significant. Furthermore, Au-Au interactions vary considerably along different directions in the unit cells, with the largest values for the hexagons in the boat conformation and the lowest values for those in the chair conformation. II revealed that partial substitution of Au atoms in the hexagonal diamond net by a post-transition element (Ga) may occur in this family, whereas the sizes of the (Au/Ga)(3) groups and strong Ba-Au covalent interactions allow for their mutual replacement in the voids.

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).…”

“…For instance, an investigation [107] of the COHP curves for BaAu5Ga2 and BaAu4Ga3 bared that the broad majority of the bonding interactions resides between the Au−Au and Au−Ga interactions. In that connection, it is remarkable that the contributions of the homoatomic and heteroatomic contacts within the hexagonal-diamond-type networks and triangles are comparable for some of these compounds, while the largest percentages to the total bonding capabilities in the electron-poorer intermetallics often stem from the heteroatomic gold−post-transition-metal interactions (see Table 2).…”

From the Ternary Eu(Au/In)2 and EuAu4(Au/In)2 with Remarkable Au/In Distributions to a New Structure Type: The Gold-Rich Eu5Au16(Au/In)6 Structure

“…13 The voids within the Au host frameworks encompass triangular [Au,M] 3 clusters (M = Al, Zn, Ga, In, or Sn) or alkaline-earth Ae (Sr, Ba) and Eu atoms, 13−18 which are mutually exchanged for particular SrAu 4+x Al 3−x -type structures. All these structures can be derived from the binary AAu 2 (A = Sr, Ba, Eu) 19 15 Another class of Au-rich intermetallic compounds which show certain electronic flexibilities comprises those structures adopting the YbAl 4 Mo 2 -type. 22−26 This type of structure is composed of an aluminum network encapsulating ytterbium and molybdenum atoms, respectively.…”

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.…”