Bismuth-Rich Intermetallic Rods with a Note of Zintl-Phase

Black needle-shaped crystals of the bismuth-rich mixed halogenide Bi21Rh4Cl6I7 were obtained by slow cooling of a melt of Bi, RhI3 and BiCl3. Single-crystal X-ray diffraction revealed an orthorhombic structure that consists of infinite intermetallic rods infinity(1)[Bi9Rh2](3+) and discrete anionic groups [(Bi2Cl2I5)-Cl-II](3-) and [(BiCl4I2)-Cl-III](3-). The rods consist of Rh-centered [RhBi8] polyhedra that alternately share triangular and rectangular faces. Using traditional electron counting rules, the rod can be interpreted as a covalent polymer with Rh-2 dumbbells bonded to molecular Bi-2 and Bi-5 units. However, a quantum-chemical bonding analysis shows that the bonds involving Rh atoms are largely diffuse, while two-center bonds dominate in the bismuth units. The below 240 K observed semiconducting behavior of Bi21Rh4Cl6I7 is consistent with a complete localization of the valence electrons. Above 240 K, the resistance along the needle axis is largely independent of temperature.