The new clathrate-I phase Ba8RhxSi46−x−yy (2.36 < x < 2.74; y = 0.65 for x = 2.74), Pearson symbol cP54, space group Pmn, was prepared as a single phase and characterized. For these compositions, the lattice parameter does not change significantly and was found to be a = 10.347(1) Å. Phase relations were investigated by means of metallographic and thermal analyses. The incongruently melting phase Ba8RhxSi46−x−yy was obtained by annealing a solidified melt of the stoichiometric composition at 1000 °C for five days. Below 850 °C, the clathrate phase slowly decomposes to BaSi2, Si and the new compound BaRh2Si9. Crystal structure and vacancy concentration were investigated by means of single crystal and powder X-ray diffraction and wavelength dispersive X-ray spectroscopy analyses. Quantum chemical calculations show that the Rh 4d states hybridize with Si 3p states so that the DOS structure cannot be simply deduced in a rigid band approach from that of the binary Ba8Si46. Analysis of the chemical bonding applying the ELI-D approach yielded covalent two-center two-electron bonds between Rh and Si atoms. For the composition Ba8Rh2.74Si42.610.65, a temperature dependent diamagnetic susceptibility and an almost temperature independent electrical resistivity (ρ ≈ 5 μΩm) were observed. The magnitude of the negative Seebeck coefficient is increasing linearly with temperature from 0 μV K−1 at 2 K to −35 μV K−1 at 850 K indicating n-type electrical conductivity.
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