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

Abstract

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(Au0.89(5)Cd0.11(5))2, Na­(Au0.51(4)Cd0.49(4))2, and Na8Au3.53(1)Cd13.47(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.

High-Temperature Oxidation-Resistance of Rare-Earth Transition Metal Silicides: Crystal Chemistry Insights from the Sc–Os–Si System

“…These foundational structure types serve as a framework for analyzing Sc­(Os 2– x Si x ) and ScOs 2 Si 2 , thereby enhancing our understanding of their structural characteristics. The MgZn 2 -type structure is a classic representation of the C14 Laves phase (Figure a) with two distinct structural units: trigonal-bipyramidal Zn units that share vertices extending into the c -axis, and Mg centering a face-sharing Friauf truncated tetrahedron. , The literature indicates that Sc can occupy the Mg site and Os can occupy the Zn sites forming ScOs 2 . Prior research has also demonstrated that the Zn sites can accommodate mixed occupancy of transition metal atoms and silicon atoms, resulting in the ordered variant Mg 2 Cu 3 Si-type structure. , During the synthesis here, two compounds, ScOs 1.62 Si 0.38 and ScOs 1.39 Si 0.61 were found to crystallize in the Mg 2 Cu 3 Si-type structure (space group P 6 3 / mmc ), as illustrated in Figure b,c.…”