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

摘要

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 SrAl_(3-x)Au_(4+x) (0.06(1) ≤ x ≤ 0.46(1), P62m, Z = 3, a = 8.633(1)-8.664(1) A, c = 7.083(2)-7.107(1) A); (Ⅱ) orthorhombic SrAl_2_yAu_(s+y) (y ≤ 0.05(1); Pnma, Z = 4, a = 8.942(1) A, b = 7.2320(4) A, c = 9.918(1) A); (Ⅲ) Sr_2Al_(2-x)Au_(7+z) (z = 0.32(2); C2/c, Z = 4, a = 14.956(4) A, b = 8.564(2) A, c = 8.682(1) A, β = 123.86(1)°); and (Ⅳ) rhombohedral Sr_2Al_(3-w)Au_(6+w)(w≈ 0.18(1); R3c, Z = 6, a = 8.448(1) A, c = 21.735(4)A). 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 Ba_3Ag_(146)Al_(64)-type structure; phase Ⅳ is isostructural with Ba_2Au_6Zn_3, 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 [Al_2Au]) 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.