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 Nb0.83B2 composition was obtained with Tc ∼ 9.3–9.9 K, and a = 3.102(1) Å and c = 3.322(2) Å 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.