Superconductivity in Nb21S8, a phase with metal cluster chains

Phase pure samples of Nb21S8 are obtained by chemical transport and molten flux reactions in sealed niobium containers at 850-950 degrees C. In the te mperature range from 5 to 290 K the electrical conductivity is found to be moderate metallic with a specific resistivity of 3.90 m Omega cm at 273 K. Magnetic susceptibility measurements give weak, almost temperature independ ent paramagnetism above similar to 40 K. These metallic properties are comp ared with the structure of Nb21S8, which contains linear single and double chains of fused body centered niobium cubes, separated by S and additional Nb atoms. Both physical measurements consistently show a transition into th e superconducting state below 4.1(5) and 3.7(2) K, respectively. Recently i deas were developed that superconductivity is favored by the tendency of pa irwise attraction of conducting electrons at the Fermi level. To see whethe r these ideas can be applied to Nb21S8, electronic band structure calculati ons (TB-LMTO-ASA-method) have been performed. In fact at the X symmetry poi nt the band structure shows two saddle points near the Fermi level giving r ise to electron pairing. Other bands cut the Fermi level with large dispers ion. These two band structure features are discussed as chemical requiremen ts (fingerprints) for the occurrence of superconductivity in this three-dim ensionally tightly bonded phase. Thereby, the applicability of this chemica l picture of superconductivity is demonstrated on this system.