Structural phase evolution and superconductivity in the non-stoichiometric intermetallic compound niobium diboride

The structural changes and superconductivity in the layered non-stoichiometric niobium diboride compound were systematically studied for both of niobium-deficient and boron-deficient phases by X-ray diffraction analysis and magnetic susceptibility measurements. The niobium and boron vacancies were found to have different effects on the crystal lattice parameters and superconducting properties; superconductivity only exists in niobium-deficient phases with a clear enhancement of inner-plane B-B bonding and the increase of inter-plane distance, while boron vacancies have the opposite effect of enhancing the Nb-B bonding and breaking the superconductivity. A main superconducting phase with Nb-0.83 B-2 Composition was obtained with T-c similar to 9.3-9.9 K, and a = 3.102(1) angstrom and c = 3.322(2) angstrom under the optimum sintering conditions. The low temperature specific heat was measured for three high-pressure synthesized superconducting samples to reveal the origin the increase of Tc with the niobium deficiencies. (C) 2008 Elsevier B.V. All rights reserved.