Superconductivity and crystal growth of NbB2

The niobium diboride NbB2 has the same crystal structure as MgB2 (space group: P6/mmm). Since the transition temperature Tc of NbB2+x depends on the chemical composition x or the lattice parameter, we examined a NbB2+x (x = 1) nominal sample, which was prepared by melting the Nb and B chunks by a tri-arc furnace. The X-ray diffraction (XRD) studies showed that our sample was single phase and had no impurities. However, magnetization measurements with a SQUID magnetometer revealed that the superconducting transition occurred twice at different temperatures. We consider that two-step superconducting transitions correspond to a phase separation in NbB2+x (x = 1). Attempts to obtain a single-phase sample by slow cooling from melt yielded the reduction of the B-rich domain in the sample.