Discovery of superconductivity in Nb 4 SiSb 2 with a V 4 SiSb 2 -type structure and implications of interstitial doping on its physical properties.

We report on the discovery, structural analysis, and the physical properties of Nb 4 SiSb 2 - a hitherto unknown compound crystallizing in the V 4 SiSb 2 -type structure with the tetragonal space group I 4/ mcm and unit cell parameters a = 10.3638(2) Å and c = 4.9151(2) Å. We find Nb 4 SiSb 2 to be a metal undergoing a transition to a superconducting state at a critical temperature of T c ≈ 1.6 K. The bulk nature of the superconductivity in this material is confirmed by the observation of a well defined discontinuity in specific heat with a normalized specific heat jump of Δ C ( T c )/ γT c = 1.33 mJ mol -1 K -2 . We find that for Nb 4 SiSb 2 , the unoccupied sites on the 4b Wyckoff position can be partially occupied with Cu, Pd, or Pt. Low-temperature resistivity measurements show transitions to superconductivity for all three compounds at T c ≈ 1.2 K for Nb 4 Cu 0.2 SiSb 2 , and T c ≈ 0.8 K for Nb 4 Pd 0.2 SiSb 2 as well as for Nb 4 Pt 0.14 SiSb 2 . The addition of electron-donor atoms into these void positions, henceforth, lowers the superconducting transition temperature in comparison to the parent compound.