BaRh2Si9--a new clathrate with a rhodium-silicon framework.

The semiconducting compound BaRh2Si9 is a new kind of intermetallic clathrate. It was obtained by reacting BaSi, Rh and Si at 950 °C. The crystal structure (space group C2/c; Pearson symbol mC48, a = 6.2221(5) Å, b = 21.399(2) Å, c = 6.2272(5) Å, β = 90.306(7)°) displays a covalently bonded Rh-Si framework, in which four-connected Si atoms partly show unusually small bond angles. The Ba atoms are encapsulated in large polyhedral cages formed by 18 Si and 4 Rh atoms. The compound is a diamagnetic p-type semiconductor, which is in agreement with band structure calculations resulting in a band gap of 0.12 eV. Quantum chemical calculations reveal positively charged Ba atoms (Ba(+1.3)) and negatively charged Rh atoms (Rh(-1)). Si atoms with neighboring Rh atoms are positively charged, while those connected only with Si atoms are negatively charged.