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 Nb₄SiSb₂ - a hitherto unknown compound crystallizing in the V₄SiSb₂-type structure with the tetragonal space group I4/mcm and unit cell parameters a = 10.3638(2) Å and c = 4.9151(2) Å. We find Nb₄SiSb₂ 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 ΔC(T _c)/γT _c = 1.33 mJ mol^-1 K^-2. We find that for Nb₄SiSb₂, 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 Nb₄Cu_0.2SiSb₂, and T _c ≈ 0.8 K for Nb₄Pd_0.2SiSb₂ as well as for Nb₄Pt_0.14SiSb₂. The addition of electron-donor atoms into these void positions, henceforth, lowers the superconducting transition temperature in comparison to the parent compound.