<i>R</i><sub>3</sub>Au<sub>9</sub><i>Pn</i> (<i>R</i> = Y, Gd–Tm; <i>Pn</i> = Sb, Bi): A Link between Cu<sub>10</sub>Sn<sub>3</sub> and Gd<sub>14</sub>Ag<sub>51</sub>

A new series of intermetallic compounds <i>R</i><sub>3</sub>Au<sub>9</sub><i>Pn</i> (<i>R</i> = Y, Gd–Tm; <i>Pn</i> = Sb, Bi) has been discovered during the explorations of the Au-rich parts of rare-earth-containing ternary systems with p<i>-</i>block elements. The existence of the series is strongly restricted by both geometric and electronic factors. <i>R</i><sub>3</sub>Au<sub>9</sub><i>Pn</i> compounds crystallize in the hexagonal crystal system with space group <i>P</i>6<sub>3</sub>/<i>m</i> (<i>a</i> = 8.08–8.24 Å, <i>c</i> = 8.98–9.08 Å). All compounds feature Au-<i>Pn</i>, formally anionic, networks built up by layers of alternating edge-sharing Au@Au<sub>6</sub> and Sb@Au<sub>6</sub> trigonal antiprisms of overall composition Au<sub>6/2</sub><i>Pn</i> connected through additional Au atoms and separated by a triangular cationic substructure formed by <i>R</i> atoms. From a first look, the series appears to be isostructural with recently reported <i>R</i><sub>3</sub>Au<sub>7</sub>Sn<sub>3</sub> (a ternary ordered derivative of the Cu<sub>10</sub>Sn<sub>3</sub>-structure type), but no example of <i>R</i><sub>3</sub>Au<sub>9</sub><i>M</i> is known when <i>M</i> is a triel or tetrel element. <i>R</i><sub>3</sub>Au<sub>9</sub><i>Pn</i> also contains Au@Au<sub>6</sub>Au<sub>2</sub><i>R</i><sub>3</sub> fully capped trigonal prisms, which are found to be isostructural with those found in the well-researched <i>R</i><sub>14</sub>Au<sub>51</sub> series. This structural motif, not present in <i>R</i><sub>3</sub>Au<sub>7</sub>Sn<sub>3</sub>, represents a previously unrecognized link between Cu<sub>10</sub>Sn<sub>3</sub> and Gd<sub>14</sub>Ag<sub>51</sub> parent structure types. Magnetic property measurements carried out for Ho<sub>3</sub>Au<sub>9</sub>Sb reveal a complex magnetic structure characterized by antiferromagnetic interactions at low temperature (<i>T</i><sub>N</sub> = 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.