Vacuum encapsulated synthesis of 11.5 K NbC superconductor
  Bulk polycrystalline NbC samples are synthesized through solid state reaction route in an evacuated sealed quartz tube. Studied NbC samples are crystallized in NaCl-type cubic structure with space group Fm-3m. To control cell parameters and minute un-reacted phases, different samples are synthesized with various heat treatments. Finally phase pure NbC is achieved. The grain size of the as systemized material being seen from SEM (scanning electron microscopy) is non-uniform of around 3-10\mu m size. Crystal structure and lattice parameters of samples have been calculated by Rietveld analysis of room temperature X-ray powder diffraction data. The lattice parameter increases with synthesis temperature and scales with superconducting transition temperature (Tc). Both AC and DC magnetization exhibited highest Tc at around 11.5 K for an NbC sample with lattice parameter a = 4.471 {\AA}. The lower critical field (Hc1) and irreversibility field (Hirr) measured at 3 K are around 250 Oe and 4.5 kOe respectively. The upper critical field (Hc2) being determined from in-field AC susceptibility measurements is 7.8 kOe and 11.7 kOe with 50% and 90% diamagnetism criteria, respectively. 
Rajveer Jha and V. P. S. Awana
cond-mat.supr-con