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

From Laves Phases to Quasicrystal Approximants in the Na-Au-Cd System

The exploratory synthesis of gold-based polar intermetallic phases has revealed many new compounds with unprecedented crystal structures, unique bonding arrangements, and interesting electronic features. Here, we further understand the complexity of gold's crystal chemistry by studying the Na-Au-Cd ternary composition space. A nearly continuous structure transformation is observed between the seemingly simple binary NaAu₂-NaCd₂ phases, yielding three new intermetallic compounds with the compositions Na(Au_0.89(5)Cd_0.11(5))₂, Na(Au_0.51(4)Cd_0.49(4))₂, and Na₈Au_3.53(1)Cd_13.47(1). Two compounds adopt different Laves phases, while the third crystallizes in a complex decagonal quasicrystal approximant. All three compounds are related through Friauf-Laves polyhedral building units with the gold/cadmium ratio found to control the transition among the unique crystal structures. Electronic structure calculations subsequently revealed the metallic nature of all three compounds with a combination of polar covalent Na-(Au/Cd) interactions and covalent (Au/Cd)-(Au/Cd) bonding interactions stabilizing each structure. These results highlight the crystal and electronic structure relationship among Laves phases and quasicrystal approximants enabled by the unique chemistry of gold.

Electron-Poor Polar Intermetallics: Complex Structures, Novel Clusters, and Intriguing Bonding with Pronounced Electron Delocalization

Intermetallic compounds represent an extensive pool of candidates for energy related applications stemming from magnetic, electric, optic, caloric, and catalytic properties. The discovery of novel intermetallic compounds can enhance understanding of the chemical principles that govern structural stability and chemical bonding as well as finding new applications. Valence electron-poor polar intermetallics with valence electron concentrations (VECs) between 2.0 and 3.0 e^-/atom show a plethora of unprecedented and fascinating structural motifs and bonding features. Therefore, establishing simple structure-bonding-property relationships is especially challenging for this compound class because commonly accepted valence electron counting rules are inappropriate. During our efforts to find quasicrystals and crystalline approximants by valence electron tuning near 2.0 e^-/atom, we observed that compositions close to those of quasicrystals are exceptional sources for unprecedented valence electron-poor polar intermetallics, e.g., Ca₄Au₁₀In₃ containing (Au₁₀In₃) wavy layers, Li_14.7Mg_36.8Cu_21.5Ga₆₆ adopting a type IV clathrate framework, and Sc₄Mg_xCu_15-xGa_7.5 that is incommensurately modulated. In particular, exploratory syntheses of AAu₃T (A = Ca, Sr, Ba and T = Ge, Sn) phases led to interesting bonding features for Au, such as columns, layers, and lonsdaleite-type tetrahedral frameworks. Overall, the breadth of Au-rich polar intermetallics originates, in part, from significant relativistics effect on the valence electrons of Au, effects which result in greater 6s/5d orbital mixing, a small effective metallic radius, and an enhanced Mulliken electronegativity, all leading to ultimate enhanced binding with nearly all metals including itself. Two other successful strategies to mine electron-poor polar intermetallics include lithiation and "cation-rich" phases. Along these lines, we have studied lithiated Zn-rich compounds in which structural complexity can be realized by small amounts of Li replacing Zn atoms in the parent binary compounds CaZn₂, CaZn₃, and CaZn₅; their phase formation and bonding schemes can be rationalized by Fermi surface-Brillouin zone interactions between nearly free-electron states. "Cation-rich", electron-poor polar intermetallics have emerged using rare earth metals as the electropositive ("cationic") component together metal/metalloid clusters that mimic the backbones of aromatic hydrocarbon molecules, which give evidence of extensive electronic delocalization and multicenter bonding. Thus, we can identify three distinct, valence electron-poor, polar intermetallic systems that have yielded unprecedented phases adopting novel structures containing complex clusters and intriguing bonding characteristics. In this Account, we summarize our recent specific progress in the developments of novel Au-rich BaAl₄-type related structures, shown in the "gold-rich grid", lithiation-modulated Ca-Li-Zn phases stabilized by different bonding characteristics, and rare earth-rich polar intermetallics containing unprecedented hydrocarbon-like planar Co-Ge metal clusters and pronounced delocalized multicenter bonding. We will focus mainly on novel structural motifs, bonding analyses, and the role of valence electrons for phase stability.

Ternary aurides RE 4Mg3Au10 (RE = La, Ce, Pr) and RE 4Cd3Au10 (RE = Y, La–Nd, Sm, Gd–Dy) – ordering variants of the Zr7Ni10 type

The intermetallic gold compounds RE 4Mg3Au10 (RE = La, Ce, Pr) and RE 4Cd3Au10 (RE = Y, La–Nd, Sm, Gd–Dy) were obtained from the elements through high-frequency melting in sealed niobium tubes and subsequent annealing in a muffle furnace. The new aurides crystallize with the Ca4In3Au10-type structure. They were characterized through Guinier powder patterns. The structures of Pr4.46Cd2.54Au10 and Tb4.38Cd2.62Au10 were refined from single crystal X-ray diffractometer data: Cmce, a = 1396.73(6), b = 1009.38(3), c = 1019.51(3) pm, wR2 = 0.0423, 1281 F 2 values, 47 variables for Pr4.46Cd2.54Au10 and a = 1362.68(3), b = 995.52(4), c = 1003.79(3) pm, wR2 = 0.0381, 1594 F 2 values, F 2 47 variables for Tb4.38Cd2.62Au10. The 8e sites of both crystals show substantial Cd/Pr respectively Cd/Tb mixing, indicating small homogeneity ranges for all RE 4+x Mg3–x Au10 and RE 4+x Cd3–x Au10 aurides. The gold atoms in these aurides form a pronounced two-dimensional substructure (275–327 pm Au–Au in Pr4.46Cd2.54Au10) which encages the Mg1/Cd1 (coordination number 8) and RE2 (coordination number 11) atoms. These blocks are separated by the Mg2/Cd2 and RE1 atoms with an intergrowth of Mg2/Cd2@Au8 and RE1@Au10 polyhedra. Temperature dependent magnetic susceptibility and specific heat measurements of Tb4Cd3Au10 have shown antiferromagnetic ordering at a Néel temperature of 12(1) K.