R₃Au₉Pn (R = Y, Gd–Tm; Pn = Sb, Bi): A Link between Cu₁₀Sn₃ and Gd₁₄Ag₅₁

A new series of intermetallic compounds R₃Au₉Pn (R = Y, Gd–Tm; Pn = Sb, Bi) has been discovered during the explorations of the Au-rich parts of rare-earth-containing ternary systems with p-block elements. The existence of the series is strongly restricted by both geometric and electronic factors. R₃Au₉Pn compounds crystallize in the hexagonal crystal system with space group P6₃/m (a = 8.08–8.24 Å, c = 8.98–9.08 Å). All compounds feature Au-Pn, formally anionic, networks built up by layers of alternating edge-sharing Au@Au₆ and Sb@Au₆ trigonal antiprisms of overall composition Au_6/2Pn connected through additional Au atoms and separated by a triangular cationic substructure formed by R atoms. From a first look, the series appears to be isostructural with recently reported R₃Au₇Sn₃ (a ternary ordered derivative of the Cu₁₀Sn₃-structure type), but no example of R₃Au₉M is known when M is a triel or tetrel element. R₃Au₉Pn also contains Au@Au₆Au₂R₃ fully capped trigonal prisms, which are found to be isostructural with those found in the well-researched R₁₄Au₅₁ series. This structural motif, not present in R₃Au₇Sn₃, represents a previously unrecognized link between Cu₁₀Sn₃ and Gd₁₄Ag₅₁ parent structure types. Magnetic property measurements carried out for Ho₃Au₉Sb reveal a complex magnetic structure characterized by antiferromagnetic interactions at low temperature (T_N = 10 K). Two metamagnetic transitions occur at high field with a change from antiferromagnetic toward ferromagnetic ordering. Density functional theory based computations were performed to understand the materials’ properties and to shed some light on the stability ranges. This allowed a better understanding of the bonding pattern, especially of the Au-containing substructure, and elucidation of the role of the third element in the stability of the structure type.