Synthesis, Crystal Structure, and Physical Properties of the Type-I Clathrate Ba8−δNix□ySi46–x–y
Résumé
Type-I clathrate phase Ba8−δNix□ySi46–x–y has been synthesized for (δ = 0, 0.1; 1.6 ≤ x ≤ 3.8; 0 ≤ y ≤ 0.9; □ = vacancy) from the elements. The homogeneity range was revealed by microprobe analyses and powder X-ray diffraction (PXRD) data. The phase crystallizes in the space group (no. 223). Analyses of single-crystal X-ray diffraction data together with microprobe analysis indicate the presence of framework defects for compositions with higher Ni content. Within the homogeneity range, the lattice parameter decreases from a = 10.3088(1) Å for Ba7.9Ni1.6Si44.4 to a = 10.2896(1) Å for Ba8Ni3.8Si41.3. Chemical bonding was analyzed based on the electron localizability indicator (ELI). Ni K-edge X-ray absorption spectroscopy measurements on investigated compositions indicate oxidation state close to elemental Ni. Low-temperature electrical resistivity measurements revealed superconductivity onsets at 6.0 K and 5.5 K for the compositions Ba7.9Ni1.6Si44.4 and Ba7.9Ni1.7Si44.3, respectively. Thermoelectric properties including electrical resistivity, thermopower and thermal conductivity were further investigated in the temperature range 300 - 773 K to assess the thermoelectric potential of this phase. Regardless of the Ni content, the samples exhibit metallic-like behavior resulting in moderate thermopower values. The thermal conductivity values are low ranging from 4 to 7 W m–1 K–1 in the investigated temperature range.
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