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

Type-I clathrate phase Ba8Nix square Si-y(46-x-y) (square = vacancy) was obtained from the elements at 1000 degrees 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-delta Nix square Si-y(46-x-y) crystallizes in the space group Pm (3) over barn (No. 223) with lattice parameter ranging from a = 10.3088(1) angstrom for Ba7.9(1)Ni1.4(1)Si44.6(1) to a = 10.2896(1) angstrom 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 Xray 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 T-c values of 6.0 and 5.5 K, respectively.