Superconductivity in the ${\mathrm{Zr}}_9$${\mathrm{Nb}}_4$${\mathrm{S}}_4$ κ phase

Superconductivity is reported for a Zr-Nb-S κ-phase material, a compound with highly anisotropic chemical bonding. It is composed of transition-metal icosahedral clusters that are face sharing along the c axis and give metal-metal bonding in that direction. In the a-b plane, the icosahedral clusters are joined by covalent sulfur bonds. The fundamental thermodynamic properties of these superconducting ${\mathrm{Zr}}_9$ ${\mathrm{Nb}}_4$ ${\mathrm{S}}_4$ compounds have been studied in order to determine whether the anisotropic chemical bonding of this structure would lead to unusual superconducting properties. The thermodynamic critical field ${\mathit{H}}_{\mathit{c}}$ roughly follows the BCS prediction with a transition temperature of 10.3 K, a value of the critical field at T=0 of ${\mathrm{\mu }}_0$ ${\mathit{H}}_0$=250 mT, and the Ginzburg-Landau parameter of about κ=30. The thermodynamic properties are very similar to transition-metal superconductors like vanadium.