Discovery of superconductivity in Nb4SiSb2 with a V4SiSb2-type structure and implications of interstitial doping on its physical properties

Abstract

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.

Fig. 1. Schematic representation of crystal structure obtained from SXRD refinement of Nb₄SiSb₂ along (a) the c-direction and (b) the b-direction. (c) Si–Si chains along the c-direction. (d) PXRD pattern of the polycrystalline sample with the respective Rietveld refinement. Green dots: data points, yellow line: calculated peaks, vertical dark red lines: Bragg peak positions, and the blue pattern on the bottom is the difference plot.

Fig. 2. (a) Normalized, low-temperature resistivity of Nb₄SiSb₂ in a temperature range between T = 500 mK and 5 K measured in zero field μ₀H = 0 T. Inset: Temperature-dependant resistivity of Nb₄SiSb₂ in zero field μ₀H = 0 T as ρ(T) between T = 500 mK and 300 K (b) Specific heat capacity of Nb₄SiSb₂ in a temperature range between T = 580 mK and 2 K. The black line corresponds to a fit using the α-model.

Fig. 3. Schematic representation of the structures of Nb₄M_xSiSb₂ from (a) the c-direction and (b) a high symmetry-direction. (c) illustrates the intercalated transition metal M (Cu, Pd, or Pt) in its environment within the _∞[Sb_4/2M_x] channels. The crystal structures were obtained from SXRD refinements.

Fig. 4. (a) Temperature-dependent resistivity of Nb₄Cu_0.2SiSb₂, Nb₄Pd_0.2SiSb₂ and Nb₄Pt_0.14SiSb₂ (a) over the whole temperature range between T = 400 mK and 300 K, and (b) in the vicinity of the superconducting transitions at low temperatures. All measurements were performed in zero field μ₀H = 0 T.