Synthesis, crystal structure and physical properties of the clathrate-I phase Ba(8)Rh(x)Si(46-x-y□y)

The new clathrate-I phase Ba(8)Rh(x)Si(46-x-y□y) (2.36 < x < 2.74; y = 0.65 for x = 2.74), Pearson symbol cP54, space group Pm3[combining macron]n, 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 Ba(8)Rh(x)Si(46-x-y□y) 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 BaSi(2), Si and the new compound BaRh(2)Si(9). 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 Ba(8)Si(46). 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 Ba(8)Rh(2.74)Si(42.61□0.65), 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.