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 Au6/2Pn 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 (TN = 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.