Synthesis, Crystal Structure, and Physical Properties of the Type-I Clathrate Ba8−δNix□ySi46–x–y

Type-I clathrate phase Ba8Nix□ySi46–x–y (□ = vacancy) was obtained from the elements at 1000 °C with the homogeneity range 2.4 ≤ x ≤ 3.8 and 0 ≤ y ≤ 0.9. In addition, samples with low Ni content (x = 1.4 and 1.6; y = 0) and small Ba deficiency were prepared from the melt by steel-quenching. Compositions were established by microprobe analysis and crystal structure determination. Ba8−δNix□ySi46–x–y crystallizes in the space group Pm3̅n (No. 223) with lattice parameter ranging from a = 10.3088(1) Å for Ba7.9(1)Ni1.4(1)Si44.6(1) to a = 10.2896(1) Å for Ba8.00(3)Ni3.82(4)Si41.33(6). Single-crystal X-ray diffraction data together with microprobe analysis indicate an increasing number of framework vacancies toward compositions with higher Ni content. For all compositions investigated, Ni K-edge X-ray absorption spectroscopy measurements showed an electronic state close to that of elemental Ni. All samples exhibit metallic-like behavior with moderate thermopower and low thermal conductivity in the temperature range 300–773 K. Samples with compositions Ba7.9(1)Ni1.4(1)Si44.6(1) and Ba7.9(1)Ni1.6(1)Si44.4(1) are superconducting with Tc values of 6.0 and 5.5 K, respectively.