Superconductivity of non-stoichiometric intermetallic compound NbB2

We report the synthesis, magnetic susceptibility and crystal structure analysis for NbB2+x (X = 0.0-1.0) samples. The study helps find a correlation among the lattice parameters, chemical composition and the superconducting transition temperature T-c. Rietveld analysis is done on the X-ray diffraction patterns of all synthesized samples to determine the lattice parameters. The a parameter decreases slightly and has a random variation with increasing x, while c parameter increases from 3.26 for pure NbB2 to 3.32 for x = 0.4 i.e. NbB2.4. With higher Boron content (x > 0.4) the c parameter decreases slightly. The stretching of lattice in c direction induces superconductivity in the non-stoichiometric niobium boride. Pure NbB2 is non-superconductor while the other NbB2+x (x > 0.0) samples show diamagnetic signal in the temperature range 8.9-11 K. Magnetization measurements (M-H) at a fixed temperature of 5 K are also carried out in both increasing and decreasing directions of field. The estimated lower and upper critical fields (H-c1 & H-c2) as viewed from M-H plots are around 590 and 2000 Oe, respectively, for NbB2.6 samples. In our case, superconductivity is achieved in NbB2 by varying the Nb/B ratios, rather than changing the processing conditions as reported by others