The cluster salts Bi14Si2MI12 (M = Rh, Ir): [Bi8Si2] and [MBi6I12] building groups in CsCl-like structure

The reaction of bismuth and iridium with iodine in evacuated quartz ampoule s at 1320 K yields black, air insensitive crystals of Bi14Si2IrI12. The sil icon therein is abstracted from the ampoule material whereby the oxygen is gettered in BiOI. The synthesis of Bi14Si2RhI12 requires the addition of ni obium, which gives NbOI2 with the oxygen originating from the SiO2. X-ray d iffraction on single crystals showed that the two isotypic compounds crysta llize in the space groups P 4/mcc with a = 1018.3(1), c = 2020.1(3)pm for M = Ir, and a = 1019.0(1), c = 2018.7(4) pm for M = Rh. The crystal structur es consist of two types of isolated clusters, which form a CsCl-like packin g. In the [MBi6I12] cuboctahedron the central transition metal atom is octa hedrally surrounded by bismuth atoms, and the iodine atoms bridge the edges of the octahedron. The [Bi8Si2] polyhedron is a tetragonal antiprism of bi smuth atoms of which square faces are capped by silicon atoms. Based on cry stal chemistry and band structure calculations the compounds may be formula ted as cluster salts [Bi8Si2](3+)[MBi6I12](3-). Measurements of the electri cal conductivity showed that Bi14Si2IrI12 is a semiconductor with a band ga p of about 0.1 eV. A single unpaired electron out of 1903 electrons per for mula causes paramagnetic behaviour that is superposed by strong diamagnetic contributions.