Relationship between superconductivity and crystal structure in NbB2+x

We have performed magnetic susceptibility measurements and precise crystal structure analysis of the nonstoichiometric NbB2+x superconductor to reveal the relationship between Tc and crystal structure. From magnetic susceptibility measurements, it was found that Tc increases from 2.2 to 5.8 K with x. The crystal structures including both chemical compositions and lattice constants were precisely determined by Rietveld refinement using synchrotron X-ray powder diffraction data with three different wavelengths. We found that interatomic Nb-B distance increases with Tc. The charge density of NbB2+x was revealed by the maximum entropy method based on Rietveld refinement. We find the difference in the number of electrons in the Nb site and B layer for NbB2+xwith different compositions and Tc.