Dynamic Bismuth‐Clusters in an Ionic Conducting Copper Iodide Matrix

Black, apparently air‐stable crystals of (Bi9)[Cu8I13] were grown by reacting Bi, Cu and BiI3 in an evacuated, sealed silica ampoule. At room temperature, the structure exhibits eight formula units of largely disordered (Bi9)5+ polycations, which appear as hollow spheres of inhomogeneously distributed electron density on a temporal and spatial average. The clusters are embedded in a disordered three‐dimensional framework of iodidocuprate(I) tetrahedra. The structure adopts the centrosymmetric space group Fm[[EQUATION]]c and can be described as a filling variant of the NaZn13 structure type with Cu+ cations distributed across the numerous tetrahedral voids. At 100 K, the crystal structure is fully ordered and adopts the non‐centrosymmetric, monoclinic space group Cc with pseudo‐cubic metrics. The (Bi9)5+ cluster has the rarely observed shape of a capped square antiprism. The disorder in the [Cu8I13]5– part indicates mobility of the copper(I) cations. Impedance measurements reveal poor electronic conductivity and a weak ionic conductivity of 5·10–5 S·cm–1 at room temperature. The electronic band structure shows a wide gap between the bonding and antibonding states of the [Cu8I13]5– framework, in which molecular states of the bismuth polycations are located, reducing the bandgap to about 0.80 eV.