We report on the crystal structure and superconducting properties of a novel iridium-silicide, namely Li 2 IrSi 2 . It has a Ag 2 NiO 2 -type structure (space group R-3m) with the lattice parameters a=4.028 30 (6) Å and c=13.161 80(15) Å. The crystal structure comprises IrSi 2 and double Li layers stacked alternately along the c-axis. The IrSi 2 layer includes a two-dimensional Ir equilateral-triangular lattice. Electrical resistivity and static magnetic measurements revealed that Li 2 IrSi 2 is a type-II superconductor with critical temperature (T c ) of 3.3 K. We estimated the following superconducting parameters: lower critical field H c1 (0)∼42 Oe, upper critical field H c2 (0)∼1.7 kOe, penetration depth λ 0 ∼265 nm, coherence length ξ 0 ∼44 nm, and Ginzburg-Landau parameter κ GL ∼6.02. The specific-heat data suggested that superconductivity in Li 2 IrSi 2 could be attributed to weakcoupling Cooper pairs.
We successfully synthesized Na included pnictides of Na 2 Ni 12 P 7 and Na 5 Rh 19 P 12 with tetrahedral TMP 4-unit (TM = Ni, Rh) in their crystal structures using a closed system technique. From powder X-ray diffraction patterns of polycrystalline samples, we determined their lattice constants. Each TMP 4-units in these compounds connect their corner or edge and formed the 3-dimensional network. The electrical resistivity data of both new compounds show the metallic behavior. There is no phase transition down to 2 K. From the results of specific heat measurements, we concluded that the Na 2 Ni 12 P 7 belongs to the Fermi liquid material with relatively large Sommerfeld coefficient value (effective mass).
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