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-x,Au4+x (0.06(1) <= x <= 0.46(1), P (6) over bar 2m, Z = 3, a = 8.633(1)-8.664(1) angstrom, c = 7.083(2)-7.107(1) angstrom); (II) orthorhombic SrAl2-yAu5+y (y <= 0.05(1); Pnma, Z = 4, a = 8.942(1) angstrom, b = 7.2320(4) angstrom, c = 9.918(1) angstrom); (III) Sr2Al2-x,Au7+z (z = 0.32(2); C2/c, Z = 4, a = 14.956(4) angstrom, b 8.564(2) angstrom, c = 8.682(1) angstrom, beta = 123.86(1)degrees); and (IV) rhombohedral Sr2Al3-wAu6+w (w approximate to 0.18(1); R (3) over barc, Z = 6, a = 8.448(1) angstrom, c = 21.735(4) angstrom). 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 Sr-x[M-3](1-x)Au-2, in which [M-3] (= [Al-3] or [Al2Au]) triangles replace some Sr atoms in the hexagonal prismatic-like cavities of the Au network. The [M-3] 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