Relativistic Effects and Gold Site Distributions: Synthesis, Structure, and Bonding in a Polar Intermetallic Na6Cd16Au7

Na6Cd16Au7 has been synthesized via typical high-temperature reactions, and its structure refined by single crystal X-ray diffraction as cubic, Fm3̅m, a = 13.589(1) Å, Z = 4. The structure consists of Cd8 tetrahedral star (TS) building blocks that are face capped by six shared gold (Au2) vertexes and further diagonally bridged via Au1 to generate an orthogonal, three-dimensional framework [Cd8(Au2)6/2(Au1)4/8], an ordered ternary derivative of Mn6Th23. Linear muffin-tin-orbital (LMTO)-atomic sphere approximation (ASA) electronic structure calculations indicate that Na6Cd16Au7 is metallic and that ∼76% of the total crystal orbital Hamilton populations (−ICOHP) originate from polar Cd–Au bonding with 18% more from fewer Cd–Cd contacts. Na6Cd16Au7 (45 valence electron count (vec)) is isotypic with the older electron-richer Mg6Cu16Si7 (56 vec) in which the atom types are switched and bonding characteristics among the network elements are altered considerably (Si for Au, Cu for Cd, Mg for Na). The earlier and more electronegative element Au now occupies the Si site, in accord with the larger relativistic bonding contributions from polar Cd–Au versus Cu–Si bonds with the neighboring Cd in the former Cu positions. Substantial electronic differences in partial densities-of-states (PDOS) and COHP data for all atoms emphasize these. Strong contributions of nearby Au 5d10 to bonding states without altering the formal vec are the likely origin of these effects.

Na₆Cd₁₆Au₇采用常规高温合成法制备，并通过单晶X射线衍射测定并解析其晶体结构：该化合物结晶于立方晶系，空间群为Fm-3m，晶胞参数a = 13.589(1) Å，晶胞内化学式单元数Z=4。其结构以Cd₈四面体星型（TS）结构基元为构建单元，该基元被6个共享的金（Au₂）顶点进行面盖帽，并进一步通过Au1进行对角桥连，由此构筑出正交型三维骨架[Cd₈(Au₂)₆/₂(Au₁)₄/₈]，该骨架是Mn₆Th₂₃的有序三元衍生结构。线性 muffin-tin轨道（Linear muffin-tin-orbital, LMTO）-原子球近似（Atomic sphere approximation, ASA）电子结构计算结果表明，Na₆Cd₁₆Au₇具有金属性，其总晶体轨道哈密顿布居（-ICOHP）中约76%来源于极性Cd-Au键，另有18%来源于相对较少的Cd-Cd接触。Na₆Cd₁₆Au₇的价电子数（valence electron count, vec）为45，与早期报道的富电子同构化合物Mg₆Cu₁₆Si₇（价电子数56）互为同结构类型，二者原子类型完全互换，且骨架原子间的成键特征发生显著改变：Si取代Au的位点、Cu取代Cd的位点、Mg取代Na的位点。电负性更高的Au占据了Si的位点，这与Cd-Au极性键相较于Cu-Si键具有更强的相对论成键贡献相符，且相邻的Cd处于原Cu的位点。各原子的分态密度（Partial densities-of-states, PDOS）与晶体轨道哈密顿布居（COHP）数据均凸显了上述电子结构差异，而Au的5d¹⁰轨道对成键态具有强贡献且未改变形式价电子数，这大概率是上述差异的成因。