Dynamic Bismuth Clusters in an Ionic Conducting Copper Iodide Matrix

Black, apparently air‐stable crystals of (Bi 9 )[Cu 8 I 13 ], is grown by reacting Bi, Cu, and BiI 3 in an evacuated, sealed silica ampoule. At room temperature, the structure exhibits eight formula units of largely disordered (Bi 9 ) 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 3D framework of iodidocuprate(I) tetrahedra. The structure adopts the centrosymmetric space group Fm c and can be described as a filling variant of the NaZn 13 structure type with Cu + cations distributed across the numerous tetrahedral voids. At 100 K, the crystal structure is fully ordered and adopts the noncentrosymmetric, monoclinic space group Cc with pseudocubic metrics. The (Bi 9 ) 5+ cluster has the rarely observed shape of a capped square antiprism. The disorder in the [Cu 8 I 13 ] 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 [Cu 8 I 13 ] 5− framework, in which molecular states of the bismuth polycations are located, reducing the bandgap to about 0.80 eV.