Turning Gold into “Diamond”: A Family of Hexagonal Diamond-Type Au-Frameworks Interconnected by Triangular Clusters in the Sr–Al–Au System

A new homologous series of intermetallic
compounds containing three-dimensional
(3-d) tetrahedral frameworks of gold atoms, akin to hexagonal diamond,
have been discovered in four related Sr–Au–Al systems:
(I) hexagonal SrAl3–xAu4+x (0.06(1) ≤ x ≤ 0.46(1), P6̅2m, Z = 3, a = 8.633(1)–8.664(1) Å, c = 7.083(2)–7.107(1) Å); (II)
orthorhombic SrAl2–yAu5+y (y ≤ 0.05(1); Pnma, Z = 4, a = 8.942(1) Å, b = 7.2320(4) Å, c = 9.918(1) Å);
(III) Sr2Al2–zAu7+z (z = 0.32(2); C2/c, Z = 4, a = 14.956(4) Å, b = 8.564(2) Å, c = 8.682(1) Å, β = 123.86(1)°); and (IV) rhombohedral Sr2Al3–wAu6+w (w ≈
0.18(1); R3̅c, Z = 6, a = 8.448(1) Å, c =
21.735(4) Å). These remarkable compounds were obtained by fusion
of the pure elements and were characterized by X-ray diffraction and
electronic structure calculations. Phase I shows a narrow
phase width and adopts the Ba3Ag14.6Al6.4-type structure; phase IV is isostructural with Ba2Au6Zn3, whereas phases II and III represent new structure types. This novel series
can be formulated as Srx[M3]1–xAu2, in which [M3] (= [Al3] or [Al2Au]) triangles replace
some Sr atoms in the hexagonal prismatic-like cavities of the Au network.
The [M3] triangles are either isolated or interconnected
into zigzag chains or nets. According to tight-binding electronic
structure calculations, the greatest overlap populations belong to
the Al–Au bonds, whereas Au–Au interactions have a substantial
nonbonding region surrounding the calculated Fermi levels. QTAIM analysis
of the electron density reveals charge transfer from Sr to the Al–Au
framework in all four systems. A study of chemical bonding by means
of the electron-localizability indicator indicates two- and three-center
interactions within the anionic Al–Au framework.