Rare-Earth Halides as Fluxes for the Synthesis of Tantalum and Niobium Carbide

While investigating the reported superconducting phase of Lu2Cl2C2-x, we discovered a novel synthetic route for making TaC and NbC. TaC and NbC are superconducting refractory carbides with a Tc of 10 and 12 K, respectively. TaC or NbC was initially found as impurity in reactions designed to produce Lu2Cl2C2-x. TaC and NbC can be prepared as single crystals by reacting Ta or Nb and C in a LnCl3 (Ln = Y, Lu) flux at as low of a temperature as 850 °C for 4 days. The products are characterized by powder X-ray diffraction and magnetic susceptibility. The various conditions for the preparation of the refractory carbides in rare-earth fluxes and their properties will be presented and discussed.